read-cache: load cache extensions on a worker thread

[git.git] / read-cache.c

/*
 * GIT - The information manager from hell
 *
 * Copyright (C) Linus Torvalds, 2005
 */
#define NO_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "config.h"
#include "diff.h"
#include "diffcore.h"
#include "tempfile.h"
#include "lockfile.h"
#include "cache-tree.h"
#include "refs.h"
#include "dir.h"
#include "object-store.h"
#include "tree.h"
#include "commit.h"
#include "blob.h"
#include "resolve-undo.h"
#include "strbuf.h"
#include "varint.h"
#include "split-index.h"
#include "utf8.h"
#include "fsmonitor.h"
#include "thread-utils.h"

/* Mask for the name length in ce_flags in the on-disk index */

#define CE_NAMEMASK  (0x0fff)

/* Index extensions.
 *
 * The first letter should be 'A'..'Z' for extensions that are not
 * necessary for a correct operation (i.e. optimization data).
 * When new extensions are added that _needs_ to be understood in
 * order to correctly interpret the index file, pick character that
 * is outside the range, to cause the reader to abort.
 */

#define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
#define CACHE_EXT_TREE 0x54524545       /* "TREE" */
#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
#define CACHE_EXT_LINK 0x6c696e6b         /* "link" */
#define CACHE_EXT_UNTRACKED 0x554E5452    /* "UNTR" */
#define CACHE_EXT_FSMONITOR 0x46534D4E    /* "FSMN" */
#define CACHE_EXT_ENDOFINDEXENTRIES 0x454F4945  /* "EOIE" */

/* changes that can be kept in $GIT_DIR/index (basically all extensions) */
#define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
                 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
                 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)


/*
 * This is an estimate of the pathname length in the index.  We use
 * this for V4 index files to guess the un-deltafied size of the index
 * in memory because of pathname deltafication.  This is not required
 * for V2/V3 index formats because their pathnames are not compressed.
 * If the initial amount of memory set aside is not sufficient, the
 * mem pool will allocate extra memory.
 */
#define CACHE_ENTRY_PATH_LENGTH 80

static inline struct cache_entry *mem_pool__ce_alloc(struct mem_pool *mem_pool, size_t len)
{
        struct cache_entry *ce;
        ce = mem_pool_alloc(mem_pool, cache_entry_size(len));
        ce->mem_pool_allocated = 1;
        return ce;
}

static inline struct cache_entry *mem_pool__ce_calloc(struct mem_pool *mem_pool, size_t len)
{
        struct cache_entry * ce;
        ce = mem_pool_calloc(mem_pool, 1, cache_entry_size(len));
        ce->mem_pool_allocated = 1;
        return ce;
}

static struct mem_pool *find_mem_pool(struct index_state *istate)
{
        struct mem_pool **pool_ptr;

        if (istate->split_index && istate->split_index->base)
                pool_ptr = &istate->split_index->base->ce_mem_pool;
        else
                pool_ptr = &istate->ce_mem_pool;

        if (!*pool_ptr)
                mem_pool_init(pool_ptr, 0);

        return *pool_ptr;
}

struct index_state the_index;
static const char *alternate_index_output;

static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
{
        istate->cache[nr] = ce;
        add_name_hash(istate, ce);
}

static void replace_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
{
        struct cache_entry *old = istate->cache[nr];

        replace_index_entry_in_base(istate, old, ce);
        remove_name_hash(istate, old);
        discard_cache_entry(old);
        ce->ce_flags &= ~CE_HASHED;
        set_index_entry(istate, nr, ce);
        ce->ce_flags |= CE_UPDATE_IN_BASE;
        mark_fsmonitor_invalid(istate, ce);
        istate->cache_changed |= CE_ENTRY_CHANGED;
}

void rename_index_entry_at(struct index_state *istate, int nr, const char *new_name)
{
        struct cache_entry *old_entry = istate->cache[nr], *new_entry;
        int namelen = strlen(new_name);

        new_entry = make_empty_cache_entry(istate, namelen);
        copy_cache_entry(new_entry, old_entry);
        new_entry->ce_flags &= ~CE_HASHED;
        new_entry->ce_namelen = namelen;
        new_entry->index = 0;
        memcpy(new_entry->name, new_name, namelen + 1);

        cache_tree_invalidate_path(istate, old_entry->name);
        untracked_cache_remove_from_index(istate, old_entry->name);
        remove_index_entry_at(istate, nr);
        add_index_entry(istate, new_entry, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
}

void fill_stat_data(struct stat_data *sd, struct stat *st)
{
        sd->sd_ctime.sec = (unsigned int)st->st_ctime;
        sd->sd_mtime.sec = (unsigned int)st->st_mtime;
        sd->sd_ctime.nsec = ST_CTIME_NSEC(*st);
        sd->sd_mtime.nsec = ST_MTIME_NSEC(*st);
        sd->sd_dev = st->st_dev;
        sd->sd_ino = st->st_ino;
        sd->sd_uid = st->st_uid;
        sd->sd_gid = st->st_gid;
        sd->sd_size = st->st_size;
}

int match_stat_data(const struct stat_data *sd, struct stat *st)
{
        int changed = 0;

        if (sd->sd_mtime.sec != (unsigned int)st->st_mtime)
                changed |= MTIME_CHANGED;
        if (trust_ctime && check_stat &&
            sd->sd_ctime.sec != (unsigned int)st->st_ctime)
                changed |= CTIME_CHANGED;

#ifdef USE_NSEC
        if (check_stat && sd->sd_mtime.nsec != ST_MTIME_NSEC(*st))
                changed |= MTIME_CHANGED;
        if (trust_ctime && check_stat &&
            sd->sd_ctime.nsec != ST_CTIME_NSEC(*st))
                changed |= CTIME_CHANGED;
#endif

        if (check_stat) {
                if (sd->sd_uid != (unsigned int) st->st_uid ||
                        sd->sd_gid != (unsigned int) st->st_gid)
                        changed |= OWNER_CHANGED;
                if (sd->sd_ino != (unsigned int) st->st_ino)
                        changed |= INODE_CHANGED;
        }

#ifdef USE_STDEV
        /*
         * st_dev breaks on network filesystems where different
         * clients will have different views of what "device"
         * the filesystem is on
         */
        if (check_stat && sd->sd_dev != (unsigned int) st->st_dev)
                        changed |= INODE_CHANGED;
#endif

        if (sd->sd_size != (unsigned int) st->st_size)
                changed |= DATA_CHANGED;

        return changed;
}

/*
 * This only updates the "non-critical" parts of the directory
 * cache, ie the parts that aren't tracked by GIT, and only used
 * to validate the cache.
 */
void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
{
        fill_stat_data(&ce->ce_stat_data, st);

        if (assume_unchanged)
                ce->ce_flags |= CE_VALID;

        if (S_ISREG(st->st_mode)) {
                ce_mark_uptodate(ce);
                mark_fsmonitor_valid(ce);
        }
}

static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
{
        int match = -1;
        int fd = git_open_cloexec(ce->name, O_RDONLY);

        if (fd >= 0) {
                struct object_id oid;
                if (!index_fd(&oid, fd, st, OBJ_BLOB, ce->name, 0))
                        match = !oideq(&oid, &ce->oid);
                /* index_fd() closed the file descriptor already */
        }
        return match;
}

static int ce_compare_link(const struct cache_entry *ce, size_t expected_size)
{
        int match = -1;
        void *buffer;
        unsigned long size;
        enum object_type type;
        struct strbuf sb = STRBUF_INIT;

        if (strbuf_readlink(&sb, ce->name, expected_size))
                return -1;

        buffer = read_object_file(&ce->oid, &type, &size);
        if (buffer) {
                if (size == sb.len)
                        match = memcmp(buffer, sb.buf, size);
                free(buffer);
        }
        strbuf_release(&sb);
        return match;
}

static int ce_compare_gitlink(const struct cache_entry *ce)
{
        struct object_id oid;

        /*
         * We don't actually require that the .git directory
         * under GITLINK directory be a valid git directory. It
         * might even be missing (in case nobody populated that
         * sub-project).
         *
         * If so, we consider it always to match.
         */
        if (resolve_gitlink_ref(ce->name, "HEAD", &oid) < 0)
                return 0;
        return !oideq(&oid, &ce->oid);
}

static int ce_modified_check_fs(const struct cache_entry *ce, struct stat *st)
{
        switch (st->st_mode & S_IFMT) {
        case S_IFREG:
                if (ce_compare_data(ce, st))
                        return DATA_CHANGED;
                break;
        case S_IFLNK:
                if (ce_compare_link(ce, xsize_t(st->st_size)))
                        return DATA_CHANGED;
                break;
        case S_IFDIR:
                if (S_ISGITLINK(ce->ce_mode))
                        return ce_compare_gitlink(ce) ? DATA_CHANGED : 0;
                /* else fallthrough */
        default:
                return TYPE_CHANGED;
        }
        return 0;
}

static int ce_match_stat_basic(const struct cache_entry *ce, struct stat *st)
{
        unsigned int changed = 0;

        if (ce->ce_flags & CE_REMOVE)
                return MODE_CHANGED | DATA_CHANGED | TYPE_CHANGED;

        switch (ce->ce_mode & S_IFMT) {
        case S_IFREG:
                changed |= !S_ISREG(st->st_mode) ? TYPE_CHANGED : 0;
                /* We consider only the owner x bit to be relevant for
                 * "mode changes"
                 */
                if (trust_executable_bit &&
                    (0100 & (ce->ce_mode ^ st->st_mode)))
                        changed |= MODE_CHANGED;
                break;
        case S_IFLNK:
                if (!S_ISLNK(st->st_mode) &&
                    (has_symlinks || !S_ISREG(st->st_mode)))
                        changed |= TYPE_CHANGED;
                break;
        case S_IFGITLINK:
                /* We ignore most of the st_xxx fields for gitlinks */
                if (!S_ISDIR(st->st_mode))
                        changed |= TYPE_CHANGED;
                else if (ce_compare_gitlink(ce))
                        changed |= DATA_CHANGED;
                return changed;
        default:
                die("internal error: ce_mode is %o", ce->ce_mode);
        }

        changed |= match_stat_data(&ce->ce_stat_data, st);

        /* Racily smudged entry? */
        if (!ce->ce_stat_data.sd_size) {
                if (!is_empty_blob_sha1(ce->oid.hash))
                        changed |= DATA_CHANGED;
        }

        return changed;
}

static int is_racy_stat(const struct index_state *istate,
                        const struct stat_data *sd)
{
        return (istate->timestamp.sec &&
#ifdef USE_NSEC
                 /* nanosecond timestamped files can also be racy! */
                (istate->timestamp.sec < sd->sd_mtime.sec ||
                 (istate->timestamp.sec == sd->sd_mtime.sec &&
                  istate->timestamp.nsec <= sd->sd_mtime.nsec))
#else
                istate->timestamp.sec <= sd->sd_mtime.sec
#endif
                );
}

static int is_racy_timestamp(const struct index_state *istate,
                             const struct cache_entry *ce)
{
        return (!S_ISGITLINK(ce->ce_mode) &&
                is_racy_stat(istate, &ce->ce_stat_data));
}

int match_stat_data_racy(const struct index_state *istate,
                         const struct stat_data *sd, struct stat *st)
{
        if (is_racy_stat(istate, sd))
                return MTIME_CHANGED;
        return match_stat_data(sd, st);
}

int ie_match_stat(struct index_state *istate,
                  const struct cache_entry *ce, struct stat *st,
                  unsigned int options)
{
        unsigned int changed;
        int ignore_valid = options & CE_MATCH_IGNORE_VALID;
        int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
        int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
        int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;

        if (!ignore_fsmonitor)
                refresh_fsmonitor(istate);
        /*
         * If it's marked as always valid in the index, it's
         * valid whatever the checked-out copy says.
         *
         * skip-worktree has the same effect with higher precedence
         */
        if (!ignore_skip_worktree && ce_skip_worktree(ce))
                return 0;
        if (!ignore_valid && (ce->ce_flags & CE_VALID))
                return 0;
        if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
                return 0;

        /*
         * Intent-to-add entries have not been added, so the index entry
         * by definition never matches what is in the work tree until it
         * actually gets added.
         */
        if (ce_intent_to_add(ce))
                return DATA_CHANGED | TYPE_CHANGED | MODE_CHANGED;

        changed = ce_match_stat_basic(ce, st);

        /*
         * Within 1 second of this sequence:
         *      echo xyzzy >file && git-update-index --add file
         * running this command:
         *      echo frotz >file
         * would give a falsely clean cache entry.  The mtime and
         * length match the cache, and other stat fields do not change.
         *
         * We could detect this at update-index time (the cache entry
         * being registered/updated records the same time as "now")
         * and delay the return from git-update-index, but that would
         * effectively mean we can make at most one commit per second,
         * which is not acceptable.  Instead, we check cache entries
         * whose mtime are the same as the index file timestamp more
         * carefully than others.
         */
        if (!changed && is_racy_timestamp(istate, ce)) {
                if (assume_racy_is_modified)
                        changed |= DATA_CHANGED;
                else
                        changed |= ce_modified_check_fs(ce, st);
        }

        return changed;
}

int ie_modified(struct index_state *istate,
                const struct cache_entry *ce,
                struct stat *st, unsigned int options)
{
        int changed, changed_fs;

        changed = ie_match_stat(istate, ce, st, options);
        if (!changed)
                return 0;
        /*
         * If the mode or type has changed, there's no point in trying
         * to refresh the entry - it's not going to match
         */
        if (changed & (MODE_CHANGED | TYPE_CHANGED))
                return changed;

        /*
         * Immediately after read-tree or update-index --cacheinfo,
         * the length field is zero, as we have never even read the
         * lstat(2) information once, and we cannot trust DATA_CHANGED
         * returned by ie_match_stat() which in turn was returned by
         * ce_match_stat_basic() to signal that the filesize of the
         * blob changed.  We have to actually go to the filesystem to
         * see if the contents match, and if so, should answer "unchanged".
         *
         * The logic does not apply to gitlinks, as ce_match_stat_basic()
         * already has checked the actual HEAD from the filesystem in the
         * subproject.  If ie_match_stat() already said it is different,
         * then we know it is.
         */
        if ((changed & DATA_CHANGED) &&
            (S_ISGITLINK(ce->ce_mode) || ce->ce_stat_data.sd_size != 0))
                return changed;

        changed_fs = ce_modified_check_fs(ce, st);
        if (changed_fs)
                return changed | changed_fs;
        return 0;
}

int base_name_compare(const char *name1, int len1, int mode1,
                      const char *name2, int len2, int mode2)
{
        unsigned char c1, c2;
        int len = len1 < len2 ? len1 : len2;
        int cmp;

        cmp = memcmp(name1, name2, len);
        if (cmp)
                return cmp;
        c1 = name1[len];
        c2 = name2[len];
        if (!c1 && S_ISDIR(mode1))
                c1 = '/';
        if (!c2 && S_ISDIR(mode2))
                c2 = '/';
        return (c1 < c2) ? -1 : (c1 > c2) ? 1 : 0;
}

/*
 * df_name_compare() is identical to base_name_compare(), except it
 * compares conflicting directory/file entries as equal. Note that
 * while a directory name compares as equal to a regular file, they
 * then individually compare _differently_ to a filename that has
 * a dot after the basename (because '\0' < '.' < '/').
 *
 * This is used by routines that want to traverse the git namespace
 * but then handle conflicting entries together when possible.
 */
int df_name_compare(const char *name1, int len1, int mode1,
                    const char *name2, int len2, int mode2)
{
        int len = len1 < len2 ? len1 : len2, cmp;
        unsigned char c1, c2;

        cmp = memcmp(name1, name2, len);
        if (cmp)
                return cmp;
        /* Directories and files compare equal (same length, same name) */
        if (len1 == len2)
                return 0;
        c1 = name1[len];
        if (!c1 && S_ISDIR(mode1))
                c1 = '/';
        c2 = name2[len];
        if (!c2 && S_ISDIR(mode2))
                c2 = '/';
        if (c1 == '/' && !c2)
                return 0;
        if (c2 == '/' && !c1)
                return 0;
        return c1 - c2;
}

int name_compare(const char *name1, size_t len1, const char *name2, size_t len2)
{
        size_t min_len = (len1 < len2) ? len1 : len2;
        int cmp = memcmp(name1, name2, min_len);
        if (cmp)
                return cmp;
        if (len1 < len2)
                return -1;
        if (len1 > len2)
                return 1;
        return 0;
}

int cache_name_stage_compare(const char *name1, int len1, int stage1, const char *name2, int len2, int stage2)
{
        int cmp;

        cmp = name_compare(name1, len1, name2, len2);
        if (cmp)
                return cmp;

        if (stage1 < stage2)
                return -1;
        if (stage1 > stage2)
                return 1;
        return 0;
}

static int index_name_stage_pos(const struct index_state *istate, const char *name, int namelen, int stage)
{
        int first, last;

        first = 0;
        last = istate->cache_nr;
        while (last > first) {
                int next = (last + first) >> 1;
                struct cache_entry *ce = istate->cache[next];
                int cmp = cache_name_stage_compare(name, namelen, stage, ce->name, ce_namelen(ce), ce_stage(ce));
                if (!cmp)
                        return next;
                if (cmp < 0) {
                        last = next;
                        continue;
                }
                first = next+1;
        }
        return -first-1;
}

int index_name_pos(const struct index_state *istate, const char *name, int namelen)
{
        return index_name_stage_pos(istate, name, namelen, 0);
}

int remove_index_entry_at(struct index_state *istate, int pos)
{
        struct cache_entry *ce = istate->cache[pos];

        record_resolve_undo(istate, ce);
        remove_name_hash(istate, ce);
        save_or_free_index_entry(istate, ce);
        istate->cache_changed |= CE_ENTRY_REMOVED;
        istate->cache_nr--;
        if (pos >= istate->cache_nr)
                return 0;
        MOVE_ARRAY(istate->cache + pos, istate->cache + pos + 1,
                   istate->cache_nr - pos);
        return 1;
}

/*
 * Remove all cache entries marked for removal, that is where
 * CE_REMOVE is set in ce_flags.  This is much more effective than
 * calling remove_index_entry_at() for each entry to be removed.
 */
void remove_marked_cache_entries(struct index_state *istate)
{
        struct cache_entry **ce_array = istate->cache;
        unsigned int i, j;

        for (i = j = 0; i < istate->cache_nr; i++) {
                if (ce_array[i]->ce_flags & CE_REMOVE) {
                        remove_name_hash(istate, ce_array[i]);
                        save_or_free_index_entry(istate, ce_array[i]);
                }
                else
                        ce_array[j++] = ce_array[i];
        }
        if (j == istate->cache_nr)
                return;
        istate->cache_changed |= CE_ENTRY_REMOVED;
        istate->cache_nr = j;
}

int remove_file_from_index(struct index_state *istate, const char *path)
{
        int pos = index_name_pos(istate, path, strlen(path));
        if (pos < 0)
                pos = -pos-1;
        cache_tree_invalidate_path(istate, path);
        untracked_cache_remove_from_index(istate, path);
        while (pos < istate->cache_nr && !strcmp(istate->cache[pos]->name, path))
                remove_index_entry_at(istate, pos);
        return 0;
}

static int compare_name(struct cache_entry *ce, const char *path, int namelen)
{
        return namelen != ce_namelen(ce) || memcmp(path, ce->name, namelen);
}

static int index_name_pos_also_unmerged(struct index_state *istate,
        const char *path, int namelen)
{
        int pos = index_name_pos(istate, path, namelen);
        struct cache_entry *ce;

        if (pos >= 0)
                return pos;

        /* maybe unmerged? */
        pos = -1 - pos;
        if (pos >= istate->cache_nr ||
                        compare_name((ce = istate->cache[pos]), path, namelen))
                return -1;

        /* order of preference: stage 2, 1, 3 */
        if (ce_stage(ce) == 1 && pos + 1 < istate->cache_nr &&
                        ce_stage((ce = istate->cache[pos + 1])) == 2 &&
                        !compare_name(ce, path, namelen))
                pos++;
        return pos;
}

static int different_name(struct cache_entry *ce, struct cache_entry *alias)
{
        int len = ce_namelen(ce);
        return ce_namelen(alias) != len || memcmp(ce->name, alias->name, len);
}

/*
 * If we add a filename that aliases in the cache, we will use the
 * name that we already have - but we don't want to update the same
 * alias twice, because that implies that there were actually two
 * different files with aliasing names!
 *
 * So we use the CE_ADDED flag to verify that the alias was an old
 * one before we accept it as
 */
static struct cache_entry *create_alias_ce(struct index_state *istate,
                                           struct cache_entry *ce,
                                           struct cache_entry *alias)
{
        int len;
        struct cache_entry *new_entry;

        if (alias->ce_flags & CE_ADDED)
                die("Will not add file alias '%s' ('%s' already exists in index)", ce->name, alias->name);

        /* Ok, create the new entry using the name of the existing alias */
        len = ce_namelen(alias);
        new_entry = make_empty_cache_entry(istate, len);
        memcpy(new_entry->name, alias->name, len);
        copy_cache_entry(new_entry, ce);
        save_or_free_index_entry(istate, ce);
        return new_entry;
}

void set_object_name_for_intent_to_add_entry(struct cache_entry *ce)
{
        struct object_id oid;
        if (write_object_file("", 0, blob_type, &oid))
                die("cannot create an empty blob in the object database");
        oidcpy(&ce->oid, &oid);
}

int add_to_index(struct index_state *istate, const char *path, struct stat *st, int flags)
{
        int namelen, was_same;
        mode_t st_mode = st->st_mode;
        struct cache_entry *ce, *alias = NULL;
        unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY;
        int verbose = flags & (ADD_CACHE_VERBOSE | ADD_CACHE_PRETEND);
        int pretend = flags & ADD_CACHE_PRETEND;
        int intent_only = flags & ADD_CACHE_INTENT;
        int add_option = (ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE|
                          (intent_only ? ADD_CACHE_NEW_ONLY : 0));
        int newflags = HASH_WRITE_OBJECT;

        if (flags & HASH_RENORMALIZE)
                newflags |= HASH_RENORMALIZE;

        if (!S_ISREG(st_mode) && !S_ISLNK(st_mode) && !S_ISDIR(st_mode))
                return error("%s: can only add regular files, symbolic links or git-directories", path);

        namelen = strlen(path);
        if (S_ISDIR(st_mode)) {
                while (namelen && path[namelen-1] == '/')
                        namelen--;
        }
        ce = make_empty_cache_entry(istate, namelen);
        memcpy(ce->name, path, namelen);
        ce->ce_namelen = namelen;
        if (!intent_only)
                fill_stat_cache_info(ce, st);
        else
                ce->ce_flags |= CE_INTENT_TO_ADD;


        if (trust_executable_bit && has_symlinks) {
                ce->ce_mode = create_ce_mode(st_mode);
        } else {
                /* If there is an existing entry, pick the mode bits and type
                 * from it, otherwise assume unexecutable regular file.
                 */
                struct cache_entry *ent;
                int pos = index_name_pos_also_unmerged(istate, path, namelen);

                ent = (0 <= pos) ? istate->cache[pos] : NULL;
                ce->ce_mode = ce_mode_from_stat(ent, st_mode);
        }

        /* When core.ignorecase=true, determine if a directory of the same name but differing
         * case already exists within the Git repository.  If it does, ensure the directory
         * case of the file being added to the repository matches (is folded into) the existing
         * entry's directory case.
         */
        if (ignore_case) {
                adjust_dirname_case(istate, ce->name);
        }
        if (!(flags & HASH_RENORMALIZE)) {
                alias = index_file_exists(istate, ce->name,
                                          ce_namelen(ce), ignore_case);
                if (alias &&
                    !ce_stage(alias) &&
                    !ie_match_stat(istate, alias, st, ce_option)) {
                        /* Nothing changed, really */
                        if (!S_ISGITLINK(alias->ce_mode))
                                ce_mark_uptodate(alias);
                        alias->ce_flags |= CE_ADDED;

                        discard_cache_entry(ce);
                        return 0;
                }
        }
        if (!intent_only) {
                if (index_path(&ce->oid, path, st, newflags)) {
                        discard_cache_entry(ce);
                        return error("unable to index file %s", path);
                }
        } else
                set_object_name_for_intent_to_add_entry(ce);

        if (ignore_case && alias && different_name(ce, alias))
                ce = create_alias_ce(istate, ce, alias);
        ce->ce_flags |= CE_ADDED;

        /* It was suspected to be racily clean, but it turns out to be Ok */
        was_same = (alias &&
                    !ce_stage(alias) &&
                    oideq(&alias->oid, &ce->oid) &&
                    ce->ce_mode == alias->ce_mode);

        if (pretend)
                discard_cache_entry(ce);
        else if (add_index_entry(istate, ce, add_option)) {
                discard_cache_entry(ce);
                return error("unable to add %s to index", path);
        }
        if (verbose && !was_same)
                printf("add '%s'\n", path);
        return 0;
}

int add_file_to_index(struct index_state *istate, const char *path, int flags)
{
        struct stat st;
        if (lstat(path, &st))
                die_errno("unable to stat '%s'", path);
        return add_to_index(istate, path, &st, flags);
}

struct cache_entry *make_empty_cache_entry(struct index_state *istate, size_t len)
{
        return mem_pool__ce_calloc(find_mem_pool(istate), len);
}

struct cache_entry *make_empty_transient_cache_entry(size_t len)
{
        return xcalloc(1, cache_entry_size(len));
}

struct cache_entry *make_cache_entry(struct index_state *istate,
                                     unsigned int mode,
                                     const struct object_id *oid,
                                     const char *path,
                                     int stage,
                                     unsigned int refresh_options)
{
        struct cache_entry *ce, *ret;
        int len;

        if (!verify_path(path, mode)) {
                error("Invalid path '%s'", path);
                return NULL;
        }

        len = strlen(path);
        ce = make_empty_cache_entry(istate, len);

        oidcpy(&ce->oid, oid);
        memcpy(ce->name, path, len);
        ce->ce_flags = create_ce_flags(stage);
        ce->ce_namelen = len;
        ce->ce_mode = create_ce_mode(mode);

        ret = refresh_cache_entry(&the_index, ce, refresh_options);
        if (ret != ce)
                discard_cache_entry(ce);
        return ret;
}

struct cache_entry *make_transient_cache_entry(unsigned int mode, const struct object_id *oid,
                                               const char *path, int stage)
{
        struct cache_entry *ce;
        int len;

        if (!verify_path(path, mode)) {
                error("Invalid path '%s'", path);
                return NULL;
        }

        len = strlen(path);
        ce = make_empty_transient_cache_entry(len);

        oidcpy(&ce->oid, oid);
        memcpy(ce->name, path, len);
        ce->ce_flags = create_ce_flags(stage);
        ce->ce_namelen = len;
        ce->ce_mode = create_ce_mode(mode);

        return ce;
}

/*
 * Chmod an index entry with either +x or -x.
 *
 * Returns -1 if the chmod for the particular cache entry failed (if it's
 * not a regular file), -2 if an invalid flip argument is passed in, 0
 * otherwise.
 */
int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
                      char flip)
{
        if (!S_ISREG(ce->ce_mode))
                return -1;
        switch (flip) {
        case '+':
                ce->ce_mode |= 0111;
                break;
        case '-':
                ce->ce_mode &= ~0111;
                break;
        default:
                return -2;
        }
        cache_tree_invalidate_path(istate, ce->name);
        ce->ce_flags |= CE_UPDATE_IN_BASE;
        mark_fsmonitor_invalid(istate, ce);
        istate->cache_changed |= CE_ENTRY_CHANGED;

        return 0;
}

int ce_same_name(const struct cache_entry *a, const struct cache_entry *b)
{
        int len = ce_namelen(a);
        return ce_namelen(b) == len && !memcmp(a->name, b->name, len);
}

/*
 * We fundamentally don't like some paths: we don't want
 * dot or dot-dot anywhere, and for obvious reasons don't
 * want to recurse into ".git" either.
 *
 * Also, we don't want double slashes or slashes at the
 * end that can make pathnames ambiguous.
 */
static int verify_dotfile(const char *rest, unsigned mode)
{
        /*
         * The first character was '.', but that
         * has already been discarded, we now test
         * the rest.
         */

        /* "." is not allowed */
        if (*rest == '\0' || is_dir_sep(*rest))
                return 0;

        switch (*rest) {
        /*
         * ".git" followed by NUL or slash is bad. Note that we match
         * case-insensitively here, even if ignore_case is not set.
         * This outlaws ".GIT" everywhere out of an abundance of caution,
         * since there's really no good reason to allow it.
         *
         * Once we've seen ".git", we can also find ".gitmodules", etc (also
         * case-insensitively).
         */
        case 'g':
        case 'G':
                if (rest[1] != 'i' && rest[1] != 'I')
                        break;
                if (rest[2] != 't' && rest[2] != 'T')
                        break;
                if (rest[3] == '\0' || is_dir_sep(rest[3]))
                        return 0;
                if (S_ISLNK(mode)) {
                        rest += 3;
                        if (skip_iprefix(rest, "modules", &rest) &&
                            (*rest == '\0' || is_dir_sep(*rest)))
                                return 0;
                }
                break;
        case '.':
                if (rest[1] == '\0' || is_dir_sep(rest[1]))
                        return 0;
        }
        return 1;
}

int verify_path(const char *path, unsigned mode)
{
        char c;

        if (has_dos_drive_prefix(path))
                return 0;

        goto inside;
        for (;;) {
                if (!c)
                        return 1;
                if (is_dir_sep(c)) {
inside:
                        if (protect_hfs) {
                                if (is_hfs_dotgit(path))
                                        return 0;
                                if (S_ISLNK(mode)) {
                                        if (is_hfs_dotgitmodules(path))
                                                return 0;
                                }
                        }
                        if (protect_ntfs) {
                                if (is_ntfs_dotgit(path))
                                        return 0;
                                if (S_ISLNK(mode)) {
                                        if (is_ntfs_dotgitmodules(path))
                                                return 0;
                                }
                        }

                        c = *path++;
                        if ((c == '.' && !verify_dotfile(path, mode)) ||
                            is_dir_sep(c) || c == '\0')
                                return 0;
                }
                c = *path++;
        }
}

/*
 * Do we have another file that has the beginning components being a
 * proper superset of the name we're trying to add?
 */
static int has_file_name(struct index_state *istate,
                         const struct cache_entry *ce, int pos, int ok_to_replace)
{
        int retval = 0;
        int len = ce_namelen(ce);
        int stage = ce_stage(ce);
        const char *name = ce->name;

        while (pos < istate->cache_nr) {
                struct cache_entry *p = istate->cache[pos++];

                if (len >= ce_namelen(p))
                        break;
                if (memcmp(name, p->name, len))
                        break;
                if (ce_stage(p) != stage)
                        continue;
                if (p->name[len] != '/')
                        continue;
                if (p->ce_flags & CE_REMOVE)
                        continue;
                retval = -1;
                if (!ok_to_replace)
                        break;
                remove_index_entry_at(istate, --pos);
        }
        return retval;
}


/*
 * Like strcmp(), but also return the offset of the first change.
 * If strings are equal, return the length.
 */
int strcmp_offset(const char *s1, const char *s2, size_t *first_change)
{
        size_t k;

        if (!first_change)
                return strcmp(s1, s2);

        for (k = 0; s1[k] == s2[k]; k++)
                if (s1[k] == '\0')
                        break;

        *first_change = k;
        return (unsigned char)s1[k] - (unsigned char)s2[k];
}

/*
 * Do we have another file with a pathname that is a proper
 * subset of the name we're trying to add?
 *
 * That is, is there another file in the index with a path
 * that matches a sub-directory in the given entry?
 */
static int has_dir_name(struct index_state *istate,
                        const struct cache_entry *ce, int pos, int ok_to_replace)
{
        int retval = 0;
        int stage = ce_stage(ce);
        const char *name = ce->name;
        const char *slash = name + ce_namelen(ce);
        size_t len_eq_last;
        int cmp_last = 0;

        /*
         * We are frequently called during an iteration on a sorted
         * list of pathnames and while building a new index.  Therefore,
         * there is a high probability that this entry will eventually
         * be appended to the index, rather than inserted in the middle.
         * If we can confirm that, we can avoid binary searches on the
         * components of the pathname.
         *
         * Compare the entry's full path with the last path in the index.
         */
        if (istate->cache_nr > 0) {
                cmp_last = strcmp_offset(name,
                        istate->cache[istate->cache_nr - 1]->name,
                        &len_eq_last);
                if (cmp_last > 0) {
                        if (len_eq_last == 0) {
                                /*
                                 * The entry sorts AFTER the last one in the
                                 * index and their paths have no common prefix,
                                 * so there cannot be a F/D conflict.
                                 */
                                return retval;
                        } else {
                                /*
                                 * The entry sorts AFTER the last one in the
                                 * index, but has a common prefix.  Fall through
                                 * to the loop below to disect the entry's path
                                 * and see where the difference is.
                                 */
                        }
                } else if (cmp_last == 0) {
                        /*
                         * The entry exactly matches the last one in the
                         * index, but because of multiple stage and CE_REMOVE
                         * items, we fall through and let the regular search
                         * code handle it.
                         */
                }
        }

        for (;;) {
                size_t len;

                for (;;) {
                        if (*--slash == '/')
                                break;
                        if (slash <= ce->name)
                                return retval;
                }
                len = slash - name;

                if (cmp_last > 0) {
                        /*
                         * (len + 1) is a directory boundary (including
                         * the trailing slash).  And since the loop is
                         * decrementing "slash", the first iteration is
                         * the longest directory prefix; subsequent
                         * iterations consider parent directories.
                         */

                        if (len + 1 <= len_eq_last) {
                                /*
                                 * The directory prefix (including the trailing
                                 * slash) also appears as a prefix in the last
                                 * entry, so the remainder cannot collide (because
                                 * strcmp said the whole path was greater).
                                 *
                                 * EQ: last: xxx/A
                                 *     this: xxx/B
                                 *
                                 * LT: last: xxx/file_A
                                 *     this: xxx/file_B
                                 */
                                return retval;
                        }

                        if (len > len_eq_last) {
                                /*
                                 * This part of the directory prefix (excluding
                                 * the trailing slash) is longer than the known
                                 * equal portions, so this sub-directory cannot
                                 * collide with a file.
                                 *
                                 * GT: last: xxxA
                                 *     this: xxxB/file
                                 */
                                return retval;
                        }

                        if (istate->cache_nr > 0 &&
                                ce_namelen(istate->cache[istate->cache_nr - 1]) > len) {
                                /*
                                 * The directory prefix lines up with part of
                                 * a longer file or directory name, but sorts
                                 * after it, so this sub-directory cannot
                                 * collide with a file.
                                 *
                                 * last: xxx/yy-file (because '-' sorts before '/')
                                 * this: xxx/yy/abc
                                 */
                                return retval;
                        }

                        /*
                         * This is a possible collision. Fall through and
                         * let the regular search code handle it.
                         *
                         * last: xxx
                         * this: xxx/file
                         */
                }

                pos = index_name_stage_pos(istate, name, len, stage);
                if (pos >= 0) {
                        /*
                         * Found one, but not so fast.  This could
                         * be a marker that says "I was here, but
                         * I am being removed".  Such an entry is
                         * not a part of the resulting tree, and
                         * it is Ok to have a directory at the same
                         * path.
                         */
                        if (!(istate->cache[pos]->ce_flags & CE_REMOVE)) {
                                retval = -1;
                                if (!ok_to_replace)
                                        break;
                                remove_index_entry_at(istate, pos);
                                continue;
                        }
                }
                else
                        pos = -pos-1;

                /*
                 * Trivial optimization: if we find an entry that
                 * already matches the sub-directory, then we know
                 * we're ok, and we can exit.
                 */
                while (pos < istate->cache_nr) {
                        struct cache_entry *p = istate->cache[pos];
                        if ((ce_namelen(p) <= len) ||
                            (p->name[len] != '/') ||
                            memcmp(p->name, name, len))
                                break; /* not our subdirectory */
                        if (ce_stage(p) == stage && !(p->ce_flags & CE_REMOVE))
                                /*
                                 * p is at the same stage as our entry, and
                                 * is a subdirectory of what we are looking
                                 * at, so we cannot have conflicts at our
                                 * level or anything shorter.
                                 */
                                return retval;
                        pos++;
                }
        }
        return retval;
}

/* We may be in a situation where we already have path/file and path
 * is being added, or we already have path and path/file is being
 * added.  Either one would result in a nonsense tree that has path
 * twice when git-write-tree tries to write it out.  Prevent it.
 *
 * If ok-to-replace is specified, we remove the conflicting entries
 * from the cache so the caller should recompute the insert position.
 * When this happens, we return non-zero.
 */
static int check_file_directory_conflict(struct index_state *istate,
                                         const struct cache_entry *ce,
                                         int pos, int ok_to_replace)
{
        int retval;

        /*
         * When ce is an "I am going away" entry, we allow it to be added
         */
        if (ce->ce_flags & CE_REMOVE)
                return 0;

        /*
         * We check if the path is a sub-path of a subsequent pathname
         * first, since removing those will not change the position
         * in the array.
         */
        retval = has_file_name(istate, ce, pos, ok_to_replace);

        /*
         * Then check if the path might have a clashing sub-directory
         * before it.
         */
        return retval + has_dir_name(istate, ce, pos, ok_to_replace);
}

static int add_index_entry_with_check(struct index_state *istate, struct cache_entry *ce, int option)
{
        int pos;
        int ok_to_add = option & ADD_CACHE_OK_TO_ADD;
        int ok_to_replace = option & ADD_CACHE_OK_TO_REPLACE;
        int skip_df_check = option & ADD_CACHE_SKIP_DFCHECK;
        int new_only = option & ADD_CACHE_NEW_ONLY;

        if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
                cache_tree_invalidate_path(istate, ce->name);

        /*
         * If this entry's path sorts after the last entry in the index,
         * we can avoid searching for it.
         */
        if (istate->cache_nr > 0 &&
                strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
                pos = -istate->cache_nr - 1;
        else
                pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));

        /* existing match? Just replace it. */
        if (pos >= 0) {
                if (!new_only)
                        replace_index_entry(istate, pos, ce);
                return 0;
        }
        pos = -pos-1;

        if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
                untracked_cache_add_to_index(istate, ce->name);

        /*
         * Inserting a merged entry ("stage 0") into the index
         * will always replace all non-merged entries..
         */
        if (pos < istate->cache_nr && ce_stage(ce) == 0) {
                while (ce_same_name(istate->cache[pos], ce)) {
                        ok_to_add = 1;
                        if (!remove_index_entry_at(istate, pos))
                                break;
                }
        }

        if (!ok_to_add)
                return -1;
        if (!verify_path(ce->name, ce->ce_mode))
                return error("Invalid path '%s'", ce->name);

        if (!skip_df_check &&
            check_file_directory_conflict(istate, ce, pos, ok_to_replace)) {
                if (!ok_to_replace)
                        return error("'%s' appears as both a file and as a directory",
                                     ce->name);
                pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
                pos = -pos-1;
        }
        return pos + 1;
}

int add_index_entry(struct index_state *istate, struct cache_entry *ce, int option)
{
        int pos;

        if (option & ADD_CACHE_JUST_APPEND)
                pos = istate->cache_nr;
        else {
                int ret;
                ret = add_index_entry_with_check(istate, ce, option);
                if (ret <= 0)
                        return ret;
                pos = ret - 1;
        }

        /* Make sure the array is big enough .. */
        ALLOC_GROW(istate->cache, istate->cache_nr + 1, istate->cache_alloc);

        /* Add it in.. */
        istate->cache_nr++;
        if (istate->cache_nr > pos + 1)
                MOVE_ARRAY(istate->cache + pos + 1, istate->cache + pos,
                           istate->cache_nr - pos - 1);
        set_index_entry(istate, pos, ce);
        istate->cache_changed |= CE_ENTRY_ADDED;
        return 0;
}

/*
 * "refresh" does not calculate a new sha1 file or bring the
 * cache up-to-date for mode/content changes. But what it
 * _does_ do is to "re-match" the stat information of a file
 * with the cache, so that you can refresh the cache for a
 * file that hasn't been changed but where the stat entry is
 * out of date.
 *
 * For example, you'd want to do this after doing a "git-read-tree",
 * to link up the stat cache details with the proper files.
 */
static struct cache_entry *refresh_cache_ent(struct index_state *istate,
                                             struct cache_entry *ce,
                                             unsigned int options, int *err,
                                             int *changed_ret)
{
        struct stat st;
        struct cache_entry *updated;
        int changed;
        int refresh = options & CE_MATCH_REFRESH;
        int ignore_valid = options & CE_MATCH_IGNORE_VALID;
        int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
        int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
        int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;

        if (!refresh || ce_uptodate(ce))
                return ce;

        if (!ignore_fsmonitor)
                refresh_fsmonitor(istate);
        /*
         * CE_VALID or CE_SKIP_WORKTREE means the user promised us
         * that the change to the work tree does not matter and told
         * us not to worry.
         */
        if (!ignore_skip_worktree && ce_skip_worktree(ce)) {
                ce_mark_uptodate(ce);
                return ce;
        }
        if (!ignore_valid && (ce->ce_flags & CE_VALID)) {
                ce_mark_uptodate(ce);
                return ce;
        }
        if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
                ce_mark_uptodate(ce);
                return ce;
        }

        if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
                if (ignore_missing)
                        return ce;
                if (err)
                        *err = ENOENT;
                return NULL;
        }

        if (lstat(ce->name, &st) < 0) {
                if (ignore_missing && errno == ENOENT)
                        return ce;
                if (err)
                        *err = errno;
                return NULL;
        }

        changed = ie_match_stat(istate, ce, &st, options);
        if (changed_ret)
                *changed_ret = changed;
        if (!changed) {
                /*
                 * The path is unchanged.  If we were told to ignore
                 * valid bit, then we did the actual stat check and
                 * found that the entry is unmodified.  If the entry
                 * is not marked VALID, this is the place to mark it
                 * valid again, under "assume unchanged" mode.
                 */
                if (ignore_valid && assume_unchanged &&
                    !(ce->ce_flags & CE_VALID))
                        ; /* mark this one VALID again */
                else {
                        /*
                         * We do not mark the index itself "modified"
                         * because CE_UPTODATE flag is in-core only;
                         * we are not going to write this change out.
                         */
                        if (!S_ISGITLINK(ce->ce_mode)) {
                                ce_mark_uptodate(ce);
                                mark_fsmonitor_valid(ce);
                        }
                        return ce;
                }
        }

        if (ie_modified(istate, ce, &st, options)) {
                if (err)
                        *err = EINVAL;
                return NULL;
        }

        updated = make_empty_cache_entry(istate, ce_namelen(ce));
        copy_cache_entry(updated, ce);
        memcpy(updated->name, ce->name, ce->ce_namelen + 1);
        fill_stat_cache_info(updated, &st);
        /*
         * If ignore_valid is not set, we should leave CE_VALID bit
         * alone.  Otherwise, paths marked with --no-assume-unchanged
         * (i.e. things to be edited) will reacquire CE_VALID bit
         * automatically, which is not really what we want.
         */
        if (!ignore_valid && assume_unchanged &&
            !(ce->ce_flags & CE_VALID))
                updated->ce_flags &= ~CE_VALID;

        /* istate->cache_changed is updated in the caller */
        return updated;
}

static void show_file(const char * fmt, const char * name, int in_porcelain,
                      int * first, const char *header_msg)
{
        if (in_porcelain && *first && header_msg) {
                printf("%s\n", header_msg);
                *first = 0;
        }
        printf(fmt, name);
}

int refresh_index(struct index_state *istate, unsigned int flags,
                  const struct pathspec *pathspec,
                  char *seen, const char *header_msg)
{
        int i;
        int has_errors = 0;
        int really = (flags & REFRESH_REALLY) != 0;
        int allow_unmerged = (flags & REFRESH_UNMERGED) != 0;
        int quiet = (flags & REFRESH_QUIET) != 0;
        int not_new = (flags & REFRESH_IGNORE_MISSING) != 0;
        int ignore_submodules = (flags & REFRESH_IGNORE_SUBMODULES) != 0;
        int first = 1;
        int in_porcelain = (flags & REFRESH_IN_PORCELAIN);
        unsigned int options = (CE_MATCH_REFRESH |
                                (really ? CE_MATCH_IGNORE_VALID : 0) |
                                (not_new ? CE_MATCH_IGNORE_MISSING : 0));
        const char *modified_fmt;
        const char *deleted_fmt;
        const char *typechange_fmt;
        const char *added_fmt;
        const char *unmerged_fmt;

        trace_performance_enter();
        modified_fmt = (in_porcelain ? "M\t%s\n" : "%s: needs update\n");
        deleted_fmt = (in_porcelain ? "D\t%s\n" : "%s: needs update\n");
        typechange_fmt = (in_porcelain ? "T\t%s\n" : "%s needs update\n");
        added_fmt = (in_porcelain ? "A\t%s\n" : "%s needs update\n");
        unmerged_fmt = (in_porcelain ? "U\t%s\n" : "%s: needs merge\n");
        for (i = 0; i < istate->cache_nr; i++) {
                struct cache_entry *ce, *new_entry;
                int cache_errno = 0;
                int changed = 0;
                int filtered = 0;

                ce = istate->cache[i];
                if (ignore_submodules && S_ISGITLINK(ce->ce_mode))
                        continue;

                if (pathspec && !ce_path_match(&the_index, ce, pathspec, seen))
                        filtered = 1;

                if (ce_stage(ce)) {
                        while ((i < istate->cache_nr) &&
                               ! strcmp(istate->cache[i]->name, ce->name))
                                i++;
                        i--;
                        if (allow_unmerged)
                                continue;
                        if (!filtered)
                                show_file(unmerged_fmt, ce->name, in_porcelain,
                                          &first, header_msg);
                        has_errors = 1;
                        continue;
                }

                if (filtered)
                        continue;

                new_entry = refresh_cache_ent(istate, ce, options, &cache_errno, &changed);
                if (new_entry == ce)
                        continue;
                if (!new_entry) {
                        const char *fmt;

                        if (really && cache_errno == EINVAL) {
                                /* If we are doing --really-refresh that
                                 * means the index is not valid anymore.
                                 */
                                ce->ce_flags &= ~CE_VALID;
                                ce->ce_flags |= CE_UPDATE_IN_BASE;
                                mark_fsmonitor_invalid(istate, ce);
                                istate->cache_changed |= CE_ENTRY_CHANGED;
                        }
                        if (quiet)
                                continue;

                        if (cache_errno == ENOENT)
                                fmt = deleted_fmt;
                        else if (ce_intent_to_add(ce))
                                fmt = added_fmt; /* must be before other checks */
                        else if (changed & TYPE_CHANGED)
                                fmt = typechange_fmt;
                        else
                                fmt = modified_fmt;
                        show_file(fmt,
                                  ce->name, in_porcelain, &first, header_msg);
                        has_errors = 1;
                        continue;
                }

                replace_index_entry(istate, i, new_entry);
        }
        trace_performance_leave("refresh index");
        return has_errors;
}

struct cache_entry *refresh_cache_entry(struct index_state *istate,
                                        struct cache_entry *ce,
                                        unsigned int options)
{
        return refresh_cache_ent(istate, ce, options, NULL, NULL);
}


/*****************************************************************
 * Index File I/O
 *****************************************************************/

#define INDEX_FORMAT_DEFAULT 3

static unsigned int get_index_format_default(void)
{
        char *envversion = getenv("GIT_INDEX_VERSION");
        char *endp;
        int value;
        unsigned int version = INDEX_FORMAT_DEFAULT;

        if (!envversion) {
                if (!git_config_get_int("index.version", &value))
                        version = value;
                if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
                        warning(_("index.version set, but the value is invalid.\n"
                                  "Using version %i"), INDEX_FORMAT_DEFAULT);
                        return INDEX_FORMAT_DEFAULT;
                }
                return version;
        }

        version = strtoul(envversion, &endp, 10);
        if (*endp ||
            version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
                warning(_("GIT_INDEX_VERSION set, but the value is invalid.\n"
                          "Using version %i"), INDEX_FORMAT_DEFAULT);
                version = INDEX_FORMAT_DEFAULT;
        }
        return version;
}

/*
 * dev/ino/uid/gid/size are also just tracked to the low 32 bits
 * Again - this is just a (very strong in practice) heuristic that
 * the inode hasn't changed.
 *
 * We save the fields in big-endian order to allow using the
 * index file over NFS transparently.
 */
struct ondisk_cache_entry {
        struct cache_time ctime;
        struct cache_time mtime;
        uint32_t dev;
        uint32_t ino;
        uint32_t mode;
        uint32_t uid;
        uint32_t gid;
        uint32_t size;
        unsigned char sha1[20];
        uint16_t flags;
        char name[FLEX_ARRAY]; /* more */
};

/*
 * This struct is used when CE_EXTENDED bit is 1
 * The struct must match ondisk_cache_entry exactly from
 * ctime till flags
 */
struct ondisk_cache_entry_extended {
        struct cache_time ctime;
        struct cache_time mtime;
        uint32_t dev;
        uint32_t ino;
        uint32_t mode;
        uint32_t uid;
        uint32_t gid;
        uint32_t size;
        unsigned char sha1[20];
        uint16_t flags;
        uint16_t flags2;
        char name[FLEX_ARRAY]; /* more */
};

/* These are only used for v3 or lower */
#define align_padding_size(size, len) ((size + (len) + 8) & ~7) - (size + len)
#define align_flex_name(STRUCT,len) ((offsetof(struct STRUCT,name) + (len) + 8) & ~7)
#define ondisk_cache_entry_size(len) align_flex_name(ondisk_cache_entry,len)
#define ondisk_cache_entry_extended_size(len) align_flex_name(ondisk_cache_entry_extended,len)
#define ondisk_ce_size(ce) (((ce)->ce_flags & CE_EXTENDED) ? \
                            ondisk_cache_entry_extended_size(ce_namelen(ce)) : \
                            ondisk_cache_entry_size(ce_namelen(ce)))

/* Allow fsck to force verification of the index checksum. */
int verify_index_checksum;

/* Allow fsck to force verification of the cache entry order. */
int verify_ce_order;

static int verify_hdr(const struct cache_header *hdr, unsigned long size)
{
        git_hash_ctx c;
        unsigned char hash[GIT_MAX_RAWSZ];
        int hdr_version;

        if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
                return error("bad signature");
        hdr_version = ntohl(hdr->hdr_version);
        if (hdr_version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < hdr_version)
                return error("bad index version %d", hdr_version);

        if (!verify_index_checksum)
                return 0;

        the_hash_algo->init_fn(&c);
        the_hash_algo->update_fn(&c, hdr, size - the_hash_algo->rawsz);
        the_hash_algo->final_fn(hash, &c);
        if (!hasheq(hash, (unsigned char *)hdr + size - the_hash_algo->rawsz))
                return error("bad index file sha1 signature");
        return 0;
}

static int read_index_extension(struct index_state *istate,
                                const char *ext, const char *data, unsigned long sz)
{
        switch (CACHE_EXT(ext)) {
        case CACHE_EXT_TREE:
                istate->cache_tree = cache_tree_read(data, sz);
                break;
        case CACHE_EXT_RESOLVE_UNDO:
                istate->resolve_undo = resolve_undo_read(data, sz);
                break;
        case CACHE_EXT_LINK:
                if (read_link_extension(istate, data, sz))
                        return -1;
                break;
        case CACHE_EXT_UNTRACKED:
                istate->untracked = read_untracked_extension(data, sz);
                break;
        case CACHE_EXT_FSMONITOR:
                read_fsmonitor_extension(istate, data, sz);
                break;
        case CACHE_EXT_ENDOFINDEXENTRIES:
                /* already handled in do_read_index() */
                break;
        default:
                if (*ext < 'A' || 'Z' < *ext)
                        return error("index uses %.4s extension, which we do not understand",
                                     ext);
                fprintf(stderr, "ignoring %.4s extension\n", ext);
                break;
        }
        return 0;
}

int hold_locked_index(struct lock_file *lk, int lock_flags)
{
        return hold_lock_file_for_update(lk, get_index_file(), lock_flags);
}

int read_index(struct index_state *istate)
{
        return read_index_from(istate, get_index_file(), get_git_dir());
}

static struct cache_entry *create_from_disk(struct index_state *istate,
                                            struct ondisk_cache_entry *ondisk,
                                            unsigned long *ent_size,
                                            const struct cache_entry *previous_ce)
{
        struct cache_entry *ce;
        size_t len;
        const char *name;
        unsigned int flags;
        size_t copy_len;
        /*
         * Adjacent cache entries tend to share the leading paths, so it makes
         * sense to only store the differences in later entries.  In the v4
         * on-disk format of the index, each on-disk cache entry stores the
         * number of bytes to be stripped from the end of the previous name,
         * and the bytes to append to the result, to come up with its name.
         */
        int expand_name_field = istate->version == 4;

        /* On-disk flags are just 16 bits */
        flags = get_be16(&ondisk->flags);
        len = flags & CE_NAMEMASK;

        if (flags & CE_EXTENDED) {
                struct ondisk_cache_entry_extended *ondisk2;
                int extended_flags;
                ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
                extended_flags = get_be16(&ondisk2->flags2) << 16;
                /* We do not yet understand any bit out of CE_EXTENDED_FLAGS */
                if (extended_flags & ~CE_EXTENDED_FLAGS)
                        die("Unknown index entry format %08x", extended_flags);
                flags |= extended_flags;
                name = ondisk2->name;
        }
        else
                name = ondisk->name;

        if (expand_name_field) {
                const unsigned char *cp = (const unsigned char *)name;
                size_t strip_len, previous_len;

                previous_len = previous_ce ? previous_ce->ce_namelen : 0;
                strip_len = decode_varint(&cp);
                if (previous_len < strip_len) {
                        if (previous_ce)
                                die(_("malformed name field in the index, near path '%s'"),
                                    previous_ce->name);
                        else
                                die(_("malformed name field in the index in the first path"));
                }
                copy_len = previous_len - strip_len;
                name = (const char *)cp;
        }

        if (len == CE_NAMEMASK) {
                len = strlen(name);
                if (expand_name_field)
                        len += copy_len;
        }

        ce = mem_pool__ce_alloc(istate->ce_mem_pool, len);

        ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
        ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
        ce->ce_stat_data.sd_ctime.nsec = get_be32(&ondisk->ctime.nsec);
        ce->ce_stat_data.sd_mtime.nsec = get_be32(&ondisk->mtime.nsec);
        ce->ce_stat_data.sd_dev   = get_be32(&ondisk->dev);
        ce->ce_stat_data.sd_ino   = get_be32(&ondisk->ino);
        ce->ce_mode  = get_be32(&ondisk->mode);
        ce->ce_stat_data.sd_uid   = get_be32(&ondisk->uid);
        ce->ce_stat_data.sd_gid   = get_be32(&ondisk->gid);
        ce->ce_stat_data.sd_size  = get_be32(&ondisk->size);
        ce->ce_flags = flags & ~CE_NAMEMASK;
        ce->ce_namelen = len;
        ce->index = 0;
        hashcpy(ce->oid.hash, ondisk->sha1);

        if (expand_name_field) {
                if (copy_len)
                        memcpy(ce->name, previous_ce->name, copy_len);
                memcpy(ce->name + copy_len, name, len + 1 - copy_len);
                *ent_size = (name - ((char *)ondisk)) + len + 1 - copy_len;
        } else {
                memcpy(ce->name, name, len + 1);
                *ent_size = ondisk_ce_size(ce);
        }
        return ce;
}

static void check_ce_order(struct index_state *istate)
{
        unsigned int i;

        if (!verify_ce_order)
                return;

        for (i = 1; i < istate->cache_nr; i++) {
                struct cache_entry *ce = istate->cache[i - 1];
                struct cache_entry *next_ce = istate->cache[i];
                int name_compare = strcmp(ce->name, next_ce->name);

                if (0 < name_compare)
                        die("unordered stage entries in index");
                if (!name_compare) {
                        if (!ce_stage(ce))
                                die("multiple stage entries for merged file '%s'",
                                    ce->name);
                        if (ce_stage(ce) > ce_stage(next_ce))
                                die("unordered stage entries for '%s'",
                                    ce->name);
                }
        }
}

static void tweak_untracked_cache(struct index_state *istate)
{
        switch (git_config_get_untracked_cache()) {
        case -1: /* keep: do nothing */
                break;
        case 0: /* false */
                remove_untracked_cache(istate);
                break;
        case 1: /* true */
                add_untracked_cache(istate);
                break;
        default: /* unknown value: do nothing */
                break;
        }
}

static void tweak_split_index(struct index_state *istate)
{
        switch (git_config_get_split_index()) {
        case -1: /* unset: do nothing */
                break;
        case 0: /* false */
                remove_split_index(istate);
                break;
        case 1: /* true */
                add_split_index(istate);
                break;
        default: /* unknown value: do nothing */
                break;
        }
}

static void post_read_index_from(struct index_state *istate)
{
        check_ce_order(istate);
        tweak_untracked_cache(istate);
        tweak_split_index(istate);
        tweak_fsmonitor(istate);
}

static size_t estimate_cache_size_from_compressed(unsigned int entries)
{
        return entries * (sizeof(struct cache_entry) + CACHE_ENTRY_PATH_LENGTH);
}

static size_t estimate_cache_size(size_t ondisk_size, unsigned int entries)
{
        long per_entry = sizeof(struct cache_entry) - sizeof(struct ondisk_cache_entry);

        /*
         * Account for potential alignment differences.
         */
        per_entry += align_padding_size(sizeof(struct cache_entry), -sizeof(struct ondisk_cache_entry));
        return ondisk_size + entries * per_entry;
}

static size_t read_eoie_extension(const char *mmap, size_t mmap_size);
static void write_eoie_extension(struct strbuf *sb, git_hash_ctx *eoie_context, size_t offset);

struct load_index_extensions
{
#ifndef NO_PTHREADS
        pthread_t pthread;
#endif
        struct index_state *istate;
        const char *mmap;
        size_t mmap_size;
        unsigned long src_offset;
};

static void *load_index_extensions(void *_data)
{
        struct load_index_extensions *p = _data;
        unsigned long src_offset = p->src_offset;

        while (src_offset <= p->mmap_size - the_hash_algo->rawsz - 8) {
                /* After an array of active_nr index entries,
                 * there can be arbitrary number of extended
                 * sections, each of which is prefixed with
                 * extension name (4-byte) and section length
                 * in 4-byte network byte order.
                 */
                uint32_t extsize = get_be32(p->mmap + src_offset + 4);
                if (read_index_extension(p->istate,
                                         p->mmap + src_offset,
                                         p->mmap + src_offset + 8,
                                         extsize) < 0) {
                        munmap((void *)p->mmap, p->mmap_size);
                        die(_("index file corrupt"));
                }
                src_offset += 8;
                src_offset += extsize;
        }

        return NULL;
}

/* remember to discard_cache() before reading a different cache! */
int do_read_index(struct index_state *istate, const char *path, int must_exist)
{
        int fd, i;
        struct stat st;
        unsigned long src_offset;
        const struct cache_header *hdr;
        const char *mmap;
        size_t mmap_size;
        const struct cache_entry *previous_ce = NULL;
        struct load_index_extensions p;
        size_t extension_offset = 0;
#ifndef NO_PTHREADS
        int nr_threads;
#endif

        if (istate->initialized)
                return istate->cache_nr;

        istate->timestamp.sec = 0;
        istate->timestamp.nsec = 0;
        fd = open(path, O_RDONLY);
        if (fd < 0) {
                if (!must_exist && errno == ENOENT)
                        return 0;
                die_errno("%s: index file open failed", path);
        }

        if (fstat(fd, &st))
                die_errno("cannot stat the open index");

        mmap_size = xsize_t(st.st_size);
        if (mmap_size < sizeof(struct cache_header) + the_hash_algo->rawsz)
                die("index file smaller than expected");

        mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
        if (mmap == MAP_FAILED)
                die_errno("unable to map index file");
        close(fd);

        hdr = (const struct cache_header *)mmap;
        if (verify_hdr(hdr, mmap_size) < 0)
                goto unmap;

        hashcpy(istate->oid.hash, (const unsigned char *)hdr + mmap_size - the_hash_algo->rawsz);
        istate->version = ntohl(hdr->hdr_version);
        istate->cache_nr = ntohl(hdr->hdr_entries);
        istate->cache_alloc = alloc_nr(istate->cache_nr);
        istate->cache = xcalloc(istate->cache_alloc, sizeof(*istate->cache));
        istate->initialized = 1;

        p.istate = istate;
        p.mmap = mmap;
        p.mmap_size = mmap_size;

#ifndef NO_PTHREADS
        nr_threads = git_config_get_index_threads();
        if (!nr_threads)
                nr_threads = online_cpus();

        if (nr_threads > 1) {
                extension_offset = read_eoie_extension(mmap, mmap_size);
                if (extension_offset) {
                        int err;

                        p.src_offset = extension_offset;
                        err = pthread_create(&p.pthread, NULL, load_index_extensions, &p);
                        if (err)
                                die(_("unable to create load_index_extensions thread: %s"), strerror(err));

                        nr_threads--;
                }
        }
#endif

        if (istate->version == 4) {
                mem_pool_init(&istate->ce_mem_pool,
                              estimate_cache_size_from_compressed(istate->cache_nr));
        } else {
                mem_pool_init(&istate->ce_mem_pool,
                              estimate_cache_size(mmap_size, istate->cache_nr));
        }

        src_offset = sizeof(*hdr);
        for (i = 0; i < istate->cache_nr; i++) {
                struct ondisk_cache_entry *disk_ce;
                struct cache_entry *ce;
                unsigned long consumed;

                disk_ce = (struct ondisk_cache_entry *)(mmap + src_offset);
                ce = create_from_disk(istate, disk_ce, &consumed, previous_ce);
                set_index_entry(istate, i, ce);

                src_offset += consumed;
                previous_ce = ce;
        }
        istate->timestamp.sec = st.st_mtime;
        istate->timestamp.nsec = ST_MTIME_NSEC(st);

        /* if we created a thread, join it otherwise load the extensions on the primary thread */
#ifndef NO_PTHREADS
        if (extension_offset) {
                int ret = pthread_join(p.pthread, NULL);
                if (ret)
                        die(_("unable to join load_index_extensions thread: %s"), strerror(ret));
        }
#endif
        if (!extension_offset) {
                p.src_offset = src_offset;
                load_index_extensions(&p);
        }
        munmap((void *)mmap, mmap_size);
        return istate->cache_nr;

unmap:
        munmap((void *)mmap, mmap_size);
        die("index file corrupt");
}

/*
 * Signal that the shared index is used by updating its mtime.
 *
 * This way, shared index can be removed if they have not been used
 * for some time.
 */
static void freshen_shared_index(const char *shared_index, int warn)
{
        if (!check_and_freshen_file(shared_index, 1) && warn)
                warning("could not freshen shared index '%s'", shared_index);
}

int read_index_from(struct index_state *istate, const char *path,
                    const char *gitdir)
{
        struct split_index *split_index;
        int ret;
        char *base_oid_hex;
        char *base_path;

        /* istate->initialized covers both .git/index and .git/sharedindex.xxx */
        if (istate->initialized)
                return istate->cache_nr;

        trace_performance_enter();
        ret = do_read_index(istate, path, 0);
        trace_performance_leave("read cache %s", path);

        split_index = istate->split_index;
        if (!split_index || is_null_oid(&split_index->base_oid)) {
                post_read_index_from(istate);
                return ret;
        }

        trace_performance_enter();
        if (split_index->base)
                discard_index(split_index->base);
        else
                split_index->base = xcalloc(1, sizeof(*split_index->base));

        base_oid_hex = oid_to_hex(&split_index->base_oid);
        base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
        ret = do_read_index(split_index->base, base_path, 1);
        if (!oideq(&split_index->base_oid, &split_index->base->oid))
                die("broken index, expect %s in %s, got %s",
                    base_oid_hex, base_path,
                    oid_to_hex(&split_index->base->oid));

        freshen_shared_index(base_path, 0);
        merge_base_index(istate);
        post_read_index_from(istate);
        free(base_path);
        trace_performance_leave("read cache %s", base_path);
        return ret;
}

int is_index_unborn(struct index_state *istate)
{
        return (!istate->cache_nr && !istate->timestamp.sec);
}

int discard_index(struct index_state *istate)
{
        /*
         * Cache entries in istate->cache[] should have been allocated
         * from the memory pool associated with this index, or from an
         * associated split_index. There is no need to free individual
         * cache entries. validate_cache_entries can detect when this
         * assertion does not hold.
         */
        validate_cache_entries(istate);

        resolve_undo_clear_index(istate);
        istate->cache_nr = 0;
        istate->cache_changed = 0;
        istate->timestamp.sec = 0;
        istate->timestamp.nsec = 0;
        free_name_hash(istate);
        cache_tree_free(&(istate->cache_tree));
        istate->initialized = 0;
        FREE_AND_NULL(istate->cache);
        istate->cache_alloc = 0;
        discard_split_index(istate);
        free_untracked_cache(istate->untracked);
        istate->untracked = NULL;

        if (istate->ce_mem_pool) {
                mem_pool_discard(istate->ce_mem_pool, should_validate_cache_entries());
                istate->ce_mem_pool = NULL;
        }

        return 0;
}

/*
 * Validate the cache entries of this index.
 * All cache entries associated with this index
 * should have been allocated by the memory pool
 * associated with this index, or by a referenced
 * split index.
 */
void validate_cache_entries(const struct index_state *istate)
{
        int i;

        if (!should_validate_cache_entries() ||!istate || !istate->initialized)
                return;

        for (i = 0; i < istate->cache_nr; i++) {
                if (!istate) {
                        die("internal error: cache entry is not allocated from expected memory pool");
                } else if (!istate->ce_mem_pool ||
                        !mem_pool_contains(istate->ce_mem_pool, istate->cache[i])) {
                        if (!istate->split_index ||
                                !istate->split_index->base ||
                                !istate->split_index->base->ce_mem_pool ||
                                !mem_pool_contains(istate->split_index->base->ce_mem_pool, istate->cache[i])) {
                                die("internal error: cache entry is not allocated from expected memory pool");
                        }
                }
        }

        if (istate->split_index)
                validate_cache_entries(istate->split_index->base);
}

int unmerged_index(const struct index_state *istate)
{
        int i;
        for (i = 0; i < istate->cache_nr; i++) {
                if (ce_stage(istate->cache[i]))
                        return 1;
        }
        return 0;
}

int index_has_changes(const struct index_state *istate,
                      struct tree *tree,
                      struct strbuf *sb)
{
        struct object_id cmp;
        int i;

        if (istate != &the_index) {
                BUG("index_has_changes cannot yet accept istate != &the_index; do_diff_cache needs updating first.");
        }
        if (tree)
                cmp = tree->object.oid;
        if (tree || !get_oid_tree("HEAD", &cmp)) {
                struct diff_options opt;

                diff_setup(&opt);
                opt.flags.exit_with_status = 1;
                if (!sb)
                        opt.flags.quick = 1;
                do_diff_cache(&cmp, &opt);
                diffcore_std(&opt);
                for (i = 0; sb && i < diff_queued_diff.nr; i++) {
                        if (i)
                                strbuf_addch(sb, ' ');
                        strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
                }
                diff_flush(&opt);
                return opt.flags.has_changes != 0;
        } else {
                for (i = 0; sb && i < istate->cache_nr; i++) {
                        if (i)
                                strbuf_addch(sb, ' ');
                        strbuf_addstr(sb, istate->cache[i]->name);
                }
                return !!istate->cache_nr;
        }
}

#define WRITE_BUFFER_SIZE 8192
static unsigned char write_buffer[WRITE_BUFFER_SIZE];
static unsigned long write_buffer_len;

static int ce_write_flush(git_hash_ctx *context, int fd)
{
        unsigned int buffered = write_buffer_len;
        if (buffered) {
                the_hash_algo->update_fn(context, write_buffer, buffered);
                if (write_in_full(fd, write_buffer, buffered) < 0)
                        return -1;
                write_buffer_len = 0;
        }
        return 0;
}

static int ce_write(git_hash_ctx *context, int fd, void *data, unsigned int len)
{
        while (len) {
                unsigned int buffered = write_buffer_len;
                unsigned int partial = WRITE_BUFFER_SIZE - buffered;
                if (partial > len)
                        partial = len;
                memcpy(write_buffer + buffered, data, partial);
                buffered += partial;
                if (buffered == WRITE_BUFFER_SIZE) {
                        write_buffer_len = buffered;
                        if (ce_write_flush(context, fd))
                                return -1;
                        buffered = 0;
                }
                write_buffer_len = buffered;
                len -= partial;
                data = (char *) data + partial;
        }
        return 0;
}

static int write_index_ext_header(git_hash_ctx *context, git_hash_ctx *eoie_context,
                                  int fd, unsigned int ext, unsigned int sz)
{
        ext = htonl(ext);
        sz = htonl(sz);
        if (eoie_context) {
                the_hash_algo->update_fn(eoie_context, &ext, 4);
                the_hash_algo->update_fn(eoie_context, &sz, 4);
        }
        return ((ce_write(context, fd, &ext, 4) < 0) ||
                (ce_write(context, fd, &sz, 4) < 0)) ? -1 : 0;
}

static int ce_flush(git_hash_ctx *context, int fd, unsigned char *hash)
{
        unsigned int left = write_buffer_len;

        if (left) {
                write_buffer_len = 0;
                the_hash_algo->update_fn(context, write_buffer, left);
        }

        /* Flush first if not enough space for hash signature */
        if (left + the_hash_algo->rawsz > WRITE_BUFFER_SIZE) {
                if (write_in_full(fd, write_buffer, left) < 0)
                        return -1;
                left = 0;
        }

        /* Append the hash signature at the end */
        the_hash_algo->final_fn(write_buffer + left, context);
        hashcpy(hash, write_buffer + left);
        left += the_hash_algo->rawsz;
        return (write_in_full(fd, write_buffer, left) < 0) ? -1 : 0;
}

static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
{
        /*
         * The only thing we care about in this function is to smudge the
         * falsely clean entry due to touch-update-touch race, so we leave
         * everything else as they are.  We are called for entries whose
         * ce_stat_data.sd_mtime match the index file mtime.
         *
         * Note that this actually does not do much for gitlinks, for
         * which ce_match_stat_basic() always goes to the actual
         * contents.  The caller checks with is_racy_timestamp() which
         * always says "no" for gitlinks, so we are not called for them ;-)
         */
        struct stat st;

        if (lstat(ce->name, &st) < 0)
                return;
        if (ce_match_stat_basic(ce, &st))
                return;
        if (ce_modified_check_fs(ce, &st)) {
                /* This is "racily clean"; smudge it.  Note that this
                 * is a tricky code.  At first glance, it may appear
                 * that it can break with this sequence:
                 *
                 * $ echo xyzzy >frotz
                 * $ git-update-index --add frotz
                 * $ : >frotz
                 * $ sleep 3
                 * $ echo filfre >nitfol
                 * $ git-update-index --add nitfol
                 *
                 * but it does not.  When the second update-index runs,
                 * it notices that the entry "frotz" has the same timestamp
                 * as index, and if we were to smudge it by resetting its
                 * size to zero here, then the object name recorded
                 * in index is the 6-byte file but the cached stat information
                 * becomes zero --- which would then match what we would
                 * obtain from the filesystem next time we stat("frotz").
                 *
                 * However, the second update-index, before calling
                 * this function, notices that the cached size is 6
                 * bytes and what is on the filesystem is an empty
                 * file, and never calls us, so the cached size information
                 * for "frotz" stays 6 which does not match the filesystem.
                 */
                ce->ce_stat_data.sd_size = 0;
        }
}

/* Copy miscellaneous fields but not the name */
static void copy_cache_entry_to_ondisk(struct ondisk_cache_entry *ondisk,
                                       struct cache_entry *ce)
{
        short flags;

        ondisk->ctime.sec = htonl(ce->ce_stat_data.sd_ctime.sec);
        ondisk->mtime.sec = htonl(ce->ce_stat_data.sd_mtime.sec);
        ondisk->ctime.nsec = htonl(ce->ce_stat_data.sd_ctime.nsec);
        ondisk->mtime.nsec = htonl(ce->ce_stat_data.sd_mtime.nsec);
        ondisk->dev  = htonl(ce->ce_stat_data.sd_dev);
        ondisk->ino  = htonl(ce->ce_stat_data.sd_ino);
        ondisk->mode = htonl(ce->ce_mode);
        ondisk->uid  = htonl(ce->ce_stat_data.sd_uid);
        ondisk->gid  = htonl(ce->ce_stat_data.sd_gid);
        ondisk->size = htonl(ce->ce_stat_data.sd_size);
        hashcpy(ondisk->sha1, ce->oid.hash);

        flags = ce->ce_flags & ~CE_NAMEMASK;
        flags |= (ce_namelen(ce) >= CE_NAMEMASK ? CE_NAMEMASK : ce_namelen(ce));
        ondisk->flags = htons(flags);
        if (ce->ce_flags & CE_EXTENDED) {
                struct ondisk_cache_entry_extended *ondisk2;
                ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
                ondisk2->flags2 = htons((ce->ce_flags & CE_EXTENDED_FLAGS) >> 16);
        }
}

static int ce_write_entry(git_hash_ctx *c, int fd, struct cache_entry *ce,
                          struct strbuf *previous_name, struct ondisk_cache_entry *ondisk)
{
        int size;
        int result;
        unsigned int saved_namelen;
        int stripped_name = 0;
        static unsigned char padding[8] = { 0x00 };

        if (ce->ce_flags & CE_STRIP_NAME) {
                saved_namelen = ce_namelen(ce);
                ce->ce_namelen = 0;
                stripped_name = 1;
        }

        if (ce->ce_flags & CE_EXTENDED)
                size = offsetof(struct ondisk_cache_entry_extended, name);
        else
                size = offsetof(struct ondisk_cache_entry, name);

        if (!previous_name) {
                int len = ce_namelen(ce);
                copy_cache_entry_to_ondisk(ondisk, ce);
                result = ce_write(c, fd, ondisk, size);
                if (!result)
                        result = ce_write(c, fd, ce->name, len);
                if (!result)
                        result = ce_write(c, fd, padding, align_padding_size(size, len));
        } else {
                int common, to_remove, prefix_size;
                unsigned char to_remove_vi[16];
                for (common = 0;
                     (ce->name[common] &&
                      common < previous_name->len &&
                      ce->name[common] == previous_name->buf[common]);
                     common++)
                        ; /* still matching */
                to_remove = previous_name->len - common;
                prefix_size = encode_varint(to_remove, to_remove_vi);

                copy_cache_entry_to_ondisk(ondisk, ce);
                result = ce_write(c, fd, ondisk, size);
                if (!result)
                        result = ce_write(c, fd, to_remove_vi, prefix_size);
                if (!result)
                        result = ce_write(c, fd, ce->name + common, ce_namelen(ce) - common);
                if (!result)
                        result = ce_write(c, fd, padding, 1);

                strbuf_splice(previous_name, common, to_remove,
                              ce->name + common, ce_namelen(ce) - common);
        }
        if (stripped_name) {
                ce->ce_namelen = saved_namelen;
                ce->ce_flags &= ~CE_STRIP_NAME;
        }

        return result;
}

/*
 * This function verifies if index_state has the correct sha1 of the
 * index file.  Don't die if we have any other failure, just return 0.
 */
static int verify_index_from(const struct index_state *istate, const char *path)
{
        int fd;
        ssize_t n;
        struct stat st;
        unsigned char hash[GIT_MAX_RAWSZ];

        if (!istate->initialized)
                return 0;

        fd = open(path, O_RDONLY);
        if (fd < 0)
                return 0;

        if (fstat(fd, &st))
                goto out;

        if (st.st_size < sizeof(struct cache_header) + the_hash_algo->rawsz)
                goto out;

        n = pread_in_full(fd, hash, the_hash_algo->rawsz, st.st_size - the_hash_algo->rawsz);
        if (n != the_hash_algo->rawsz)
                goto out;

        if (!hasheq(istate->oid.hash, hash))
                goto out;

        close(fd);
        return 1;

out:
        close(fd);
        return 0;
}

static int verify_index(const struct index_state *istate)
{
        return verify_index_from(istate, get_index_file());
}

static int has_racy_timestamp(struct index_state *istate)
{
        int entries = istate->cache_nr;
        int i;

        for (i = 0; i < entries; i++) {
                struct cache_entry *ce = istate->cache[i];
                if (is_racy_timestamp(istate, ce))
                        return 1;
        }
        return 0;
}

void update_index_if_able(struct index_state *istate, struct lock_file *lockfile)
{
        if ((istate->cache_changed || has_racy_timestamp(istate)) &&
            verify_index(istate))
                write_locked_index(istate, lockfile, COMMIT_LOCK);
        else
                rollback_lock_file(lockfile);
}

/*
 * On success, `tempfile` is closed. If it is the temporary file
 * of a `struct lock_file`, we will therefore effectively perform
 * a 'close_lock_file_gently()`. Since that is an implementation
 * detail of lockfiles, callers of `do_write_index()` should not
 * rely on it.
 */
static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
                          int strip_extensions)
{
        uint64_t start = getnanotime();
        int newfd = tempfile->fd;
        git_hash_ctx c, eoie_c;
        struct cache_header hdr;
        int i, err = 0, removed, extended, hdr_version;
        struct cache_entry **cache = istate->cache;
        int entries = istate->cache_nr;
        struct stat st;
        struct ondisk_cache_entry_extended ondisk;
        struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
        int drop_cache_tree = istate->drop_cache_tree;
        off_t offset;

        for (i = removed = extended = 0; i < entries; i++) {
                if (cache[i]->ce_flags & CE_REMOVE)
                        removed++;

                /* reduce extended entries if possible */
                cache[i]->ce_flags &= ~CE_EXTENDED;
                if (cache[i]->ce_flags & CE_EXTENDED_FLAGS) {
                        extended++;
                        cache[i]->ce_flags |= CE_EXTENDED;
                }
        }

        if (!istate->version) {
                istate->version = get_index_format_default();
                if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
                        init_split_index(istate);
        }

        /* demote version 3 to version 2 when the latter suffices */
        if (istate->version == 3 || istate->version == 2)
                istate->version = extended ? 3 : 2;

        hdr_version = istate->version;

        hdr.hdr_signature = htonl(CACHE_SIGNATURE);
        hdr.hdr_version = htonl(hdr_version);
        hdr.hdr_entries = htonl(entries - removed);

        the_hash_algo->init_fn(&c);
        if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
                return -1;

        offset = lseek(newfd, 0, SEEK_CUR);
        if (offset < 0)
                return -1;
        offset += write_buffer_len;
        previous_name = (hdr_version == 4) ? &previous_name_buf : NULL;

        for (i = 0; i < entries; i++) {
                struct cache_entry *ce = cache[i];
                if (ce->ce_flags & CE_REMOVE)
                        continue;
                if (!ce_uptodate(ce) && is_racy_timestamp(istate, ce))
                        ce_smudge_racily_clean_entry(ce);
                if (is_null_oid(&ce->oid)) {
                        static const char msg[] = "cache entry has null sha1: %s";
                        static int allow = -1;

                        if (allow < 0)
                                allow = git_env_bool("GIT_ALLOW_NULL_SHA1", 0);
                        if (allow)
                                warning(msg, ce->name);
                        else
                                err = error(msg, ce->name);

                        drop_cache_tree = 1;
                }
                if (ce_write_entry(&c, newfd, ce, previous_name, (struct ondisk_cache_entry *)&ondisk) < 0)
                        err = -1;

                if (err)
                        break;
        }
        strbuf_release(&previous_name_buf);

        if (err)
                return err;

        /* Write extension data here */
        offset = lseek(newfd, 0, SEEK_CUR);
        if (offset < 0)
                return -1;
        offset += write_buffer_len;
        the_hash_algo->init_fn(&eoie_c);

        if (!strip_extensions && istate->split_index) {
                struct strbuf sb = STRBUF_INIT;

                err = write_link_extension(&sb, istate) < 0 ||
                        write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_LINK,
                                               sb.len) < 0 ||
                        ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }
        if (!strip_extensions && !drop_cache_tree && istate->cache_tree) {
                struct strbuf sb = STRBUF_INIT;

                cache_tree_write(&sb, istate->cache_tree);
                err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_TREE, sb.len) < 0
                        || ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }
        if (!strip_extensions && istate->resolve_undo) {
                struct strbuf sb = STRBUF_INIT;

                resolve_undo_write(&sb, istate->resolve_undo);
                err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_RESOLVE_UNDO,
                                             sb.len) < 0
                        || ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }
        if (!strip_extensions && istate->untracked) {
                struct strbuf sb = STRBUF_INIT;

                write_untracked_extension(&sb, istate->untracked);
                err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_UNTRACKED,
                                             sb.len) < 0 ||
                        ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }
        if (!strip_extensions && istate->fsmonitor_last_update) {
                struct strbuf sb = STRBUF_INIT;

                write_fsmonitor_extension(&sb, istate);
                err = write_index_ext_header(&c, &eoie_c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
                        || ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }

        /*
         * CACHE_EXT_ENDOFINDEXENTRIES must be written as the last entry before the SHA1
         * so that it can be found and processed before all the index entries are
         * read.  Write it out regardless of the strip_extensions parameter as we need it
         * when loading the shared index.
         */
        if (offset) {
                struct strbuf sb = STRBUF_INIT;

                write_eoie_extension(&sb, &eoie_c, offset);
                err = write_index_ext_header(&c, NULL, newfd, CACHE_EXT_ENDOFINDEXENTRIES, sb.len) < 0
                        || ce_write(&c, newfd, sb.buf, sb.len) < 0;
                strbuf_release(&sb);
                if (err)
                        return -1;
        }

        if (ce_flush(&c, newfd, istate->oid.hash))
                return -1;
        if (close_tempfile_gently(tempfile)) {
                error(_("could not close '%s'"), tempfile->filename.buf);
                return -1;
        }
        if (stat(tempfile->filename.buf, &st))
                return -1;
        istate->timestamp.sec = (unsigned int)st.st_mtime;
        istate->timestamp.nsec = ST_MTIME_NSEC(st);
        trace_performance_since(start, "write index, changed mask = %x", istate->cache_changed);
        return 0;
}

void set_alternate_index_output(const char *name)
{
        alternate_index_output = name;
}

static int commit_locked_index(struct lock_file *lk)
{
        if (alternate_index_output)
                return commit_lock_file_to(lk, alternate_index_output);
        else
                return commit_lock_file(lk);
}

static int do_write_locked_index(struct index_state *istate, struct lock_file *lock,
                                 unsigned flags)
{
        int ret = do_write_index(istate, lock->tempfile, 0);
        if (ret)
                return ret;
        if (flags & COMMIT_LOCK)
                return commit_locked_index(lock);
        return close_lock_file_gently(lock);
}

static int write_split_index(struct index_state *istate,
                             struct lock_file *lock,
                             unsigned flags)
{
        int ret;
        prepare_to_write_split_index(istate);
        ret = do_write_locked_index(istate, lock, flags);
        finish_writing_split_index(istate);
        return ret;
}

static const char *shared_index_expire = "2.weeks.ago";

static unsigned long get_shared_index_expire_date(void)
{
        static unsigned long shared_index_expire_date;
        static int shared_index_expire_date_prepared;

        if (!shared_index_expire_date_prepared) {
                git_config_get_expiry("splitindex.sharedindexexpire",
                                      &shared_index_expire);
                shared_index_expire_date = approxidate(shared_index_expire);
                shared_index_expire_date_prepared = 1;
        }

        return shared_index_expire_date;
}

static int should_delete_shared_index(const char *shared_index_path)
{
        struct stat st;
        unsigned long expiration;

        /* Check timestamp */
        expiration = get_shared_index_expire_date();
        if (!expiration)
                return 0;
        if (stat(shared_index_path, &st))
                return error_errno(_("could not stat '%s'"), shared_index_path);
        if (st.st_mtime > expiration)
                return 0;

        return 1;
}

static int clean_shared_index_files(const char *current_hex)
{
        struct dirent *de;
        DIR *dir = opendir(get_git_dir());

        if (!dir)
                return error_errno(_("unable to open git dir: %s"), get_git_dir());

        while ((de = readdir(dir)) != NULL) {
                const char *sha1_hex;
                const char *shared_index_path;
                if (!skip_prefix(de->d_name, "sharedindex.", &sha1_hex))
                        continue;
                if (!strcmp(sha1_hex, current_hex))
                        continue;
                shared_index_path = git_path("%s", de->d_name);
                if (should_delete_shared_index(shared_index_path) > 0 &&
                    unlink(shared_index_path))
                        warning_errno(_("unable to unlink: %s"), shared_index_path);
        }
        closedir(dir);

        return 0;
}

static int write_shared_index(struct index_state *istate,
                              struct tempfile **temp)
{
        struct split_index *si = istate->split_index;
        int ret;

        move_cache_to_base_index(istate);
        ret = do_write_index(si->base, *temp, 1);
        if (ret)
                return ret;
        ret = adjust_shared_perm(get_tempfile_path(*temp));
        if (ret) {
                error("cannot fix permission bits on %s", get_tempfile_path(*temp));
                return ret;
        }
        ret = rename_tempfile(temp,
                              git_path("sharedindex.%s", oid_to_hex(&si->base->oid)));
        if (!ret) {
                oidcpy(&si->base_oid, &si->base->oid);
                clean_shared_index_files(oid_to_hex(&si->base->oid));
        }

        return ret;
}

static const int default_max_percent_split_change = 20;

static int too_many_not_shared_entries(struct index_state *istate)
{
        int i, not_shared = 0;
        int max_split = git_config_get_max_percent_split_change();

        switch (max_split) {
        case -1:
                /* not or badly configured: use the default value */
                max_split = default_max_percent_split_change;
                break;
        case 0:
                return 1; /* 0% means always write a new shared index */
        case 100:
                return 0; /* 100% means never write a new shared index */
        default:
                break; /* just use the configured value */
        }

        /* Count not shared entries */
        for (i = 0; i < istate->cache_nr; i++) {
                struct cache_entry *ce = istate->cache[i];
                if (!ce->index)
                        not_shared++;
        }

        return (int64_t)istate->cache_nr * max_split < (int64_t)not_shared * 100;
}

int write_locked_index(struct index_state *istate, struct lock_file *lock,
                       unsigned flags)
{
        int new_shared_index, ret;
        struct split_index *si = istate->split_index;

        if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
                cache_tree_verify(istate);

        if ((flags & SKIP_IF_UNCHANGED) && !istate->cache_changed) {
                if (flags & COMMIT_LOCK)
                        rollback_lock_file(lock);
                return 0;
        }

        if (istate->fsmonitor_last_update)
                fill_fsmonitor_bitmap(istate);

        if (!si || alternate_index_output ||
            (istate->cache_changed & ~EXTMASK)) {
                if (si)
                        oidclr(&si->base_oid);
                ret = do_write_locked_index(istate, lock, flags);
                goto out;
        }

        if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0)) {
                int v = si->base_oid.hash[0];
                if ((v & 15) < 6)
                        istate->cache_changed |= SPLIT_INDEX_ORDERED;
        }
        if (too_many_not_shared_entries(istate))
                istate->cache_changed |= SPLIT_INDEX_ORDERED;

        new_shared_index = istate->cache_changed & SPLIT_INDEX_ORDERED;

        if (new_shared_index) {
                struct tempfile *temp;
                int saved_errno;

                temp = mks_tempfile(git_path("sharedindex_XXXXXX"));
                if (!temp) {
                        oidclr(&si->base_oid);
                        ret = do_write_locked_index(istate, lock, flags);
                        goto out;
                }
                ret = write_shared_index(istate, &temp);

                saved_errno = errno;
                if (is_tempfile_active(temp))
                        delete_tempfile(&temp);
                errno = saved_errno;

                if (ret)
                        goto out;
        }

        ret = write_split_index(istate, lock, flags);

        /* Freshen the shared index only if the split-index was written */
        if (!ret && !new_shared_index) {
                const char *shared_index = git_path("sharedindex.%s",
                                                    oid_to_hex(&si->base_oid));
                freshen_shared_index(shared_index, 1);
        }

out:
        if (flags & COMMIT_LOCK)
                rollback_lock_file(lock);
        return ret;
}

/*
 * Read the index file that is potentially unmerged into given
 * index_state, dropping any unmerged entries to stage #0 (potentially
 * resulting in a path appearing as both a file and a directory in the
 * index; the caller is responsible to clear out the extra entries
 * before writing the index to a tree).  Returns true if the index is
 * unmerged.  Callers who want to refuse to work from an unmerged
 * state can call this and check its return value, instead of calling
 * read_cache().
 */
int read_index_unmerged(struct index_state *istate)
{
        int i;
        int unmerged = 0;

        read_index(istate);
        for (i = 0; i < istate->cache_nr; i++) {
                struct cache_entry *ce = istate->cache[i];
                struct cache_entry *new_ce;
                int len;

                if (!ce_stage(ce))
                        continue;
                unmerged = 1;
                len = ce_namelen(ce);
                new_ce = make_empty_cache_entry(istate, len);
                memcpy(new_ce->name, ce->name, len);
                new_ce->ce_flags = create_ce_flags(0) | CE_CONFLICTED;
                new_ce->ce_namelen = len;
                new_ce->ce_mode = ce->ce_mode;
                if (add_index_entry(istate, new_ce, ADD_CACHE_SKIP_DFCHECK))
                        return error("%s: cannot drop to stage #0",
                                     new_ce->name);
        }
        return unmerged;
}

/*
 * Returns 1 if the path is an "other" path with respect to
 * the index; that is, the path is not mentioned in the index at all,
 * either as a file, a directory with some files in the index,
 * or as an unmerged entry.
 *
 * We helpfully remove a trailing "/" from directories so that
 * the output of read_directory can be used as-is.
 */
int index_name_is_other(const struct index_state *istate, const char *name,
                int namelen)
{
        int pos;
        if (namelen && name[namelen - 1] == '/')
                namelen--;
        pos = index_name_pos(istate, name, namelen);
        if (0 <= pos)
                return 0;       /* exact match */
        pos = -pos - 1;
        if (pos < istate->cache_nr) {
                struct cache_entry *ce = istate->cache[pos];
                if (ce_namelen(ce) == namelen &&
                    !memcmp(ce->name, name, namelen))
                        return 0; /* Yup, this one exists unmerged */
        }
        return 1;
}

void *read_blob_data_from_index(const struct index_state *istate,
                                const char *path, unsigned long *size)
{
        int pos, len;
        unsigned long sz;
        enum object_type type;
        void *data;

        len = strlen(path);
        pos = index_name_pos(istate, path, len);
        if (pos < 0) {
                /*
                 * We might be in the middle of a merge, in which
                 * case we would read stage #2 (ours).
                 */
                int i;
                for (i = -pos - 1;
                     (pos < 0 && i < istate->cache_nr &&
                      !strcmp(istate->cache[i]->name, path));
                     i++)
                        if (ce_stage(istate->cache[i]) == 2)
                                pos = i;
        }
        if (pos < 0)
                return NULL;
        data = read_object_file(&istate->cache[pos]->oid, &type, &sz);
        if (!data || type != OBJ_BLOB) {
                free(data);
                return NULL;
        }
        if (size)
                *size = sz;
        return data;
}

void stat_validity_clear(struct stat_validity *sv)
{
        FREE_AND_NULL(sv->sd);
}

int stat_validity_check(struct stat_validity *sv, const char *path)
{
        struct stat st;

        if (stat(path, &st) < 0)
                return sv->sd == NULL;
        if (!sv->sd)
                return 0;
        return S_ISREG(st.st_mode) && !match_stat_data(sv->sd, &st);
}

void stat_validity_update(struct stat_validity *sv, int fd)
{
        struct stat st;

        if (fstat(fd, &st) < 0 || !S_ISREG(st.st_mode))
                stat_validity_clear(sv);
        else {
                if (!sv->sd)
                        sv->sd = xcalloc(1, sizeof(struct stat_data));
                fill_stat_data(sv->sd, &st);
        }
}

void move_index_extensions(struct index_state *dst, struct index_state *src)
{
        dst->untracked = src->untracked;
        src->untracked = NULL;
        dst->cache_tree = src->cache_tree;
        src->cache_tree = NULL;
}

struct cache_entry *dup_cache_entry(const struct cache_entry *ce,
                                    struct index_state *istate)
{
        unsigned int size = ce_size(ce);
        int mem_pool_allocated;
        struct cache_entry *new_entry = make_empty_cache_entry(istate, ce_namelen(ce));
        mem_pool_allocated = new_entry->mem_pool_allocated;

        memcpy(new_entry, ce, size);
        new_entry->mem_pool_allocated = mem_pool_allocated;
        return new_entry;
}

void discard_cache_entry(struct cache_entry *ce)
{
        if (ce && should_validate_cache_entries())
                memset(ce, 0xCD, cache_entry_size(ce->ce_namelen));

        if (ce && ce->mem_pool_allocated)
                return;

        free(ce);
}

int should_validate_cache_entries(void)
{
        static int validate_index_cache_entries = -1;

        if (validate_index_cache_entries < 0) {
                if (getenv("GIT_TEST_VALIDATE_INDEX_CACHE_ENTRIES"))
                        validate_index_cache_entries = 1;
                else
                        validate_index_cache_entries = 0;
        }

        return validate_index_cache_entries;
}

#define EOIE_SIZE (4 + GIT_SHA1_RAWSZ) /* <4-byte offset> + <20-byte hash> */
#define EOIE_SIZE_WITH_HEADER (4 + 4 + EOIE_SIZE) /* <4-byte signature> + <4-byte length> + EOIE_SIZE */

static size_t read_eoie_extension(const char *mmap, size_t mmap_size)
{
        /*
         * The end of index entries (EOIE) extension is guaranteed to be last
         * so that it can be found by scanning backwards from the EOF.
         *
         * "EOIE"
         * <4-byte length>
         * <4-byte offset>
         * <20-byte hash>
         */
        const char *index, *eoie;
        uint32_t extsize;
        size_t offset, src_offset;
        unsigned char hash[GIT_MAX_RAWSZ];
        git_hash_ctx c;

        /* ensure we have an index big enough to contain an EOIE extension */
        if (mmap_size < sizeof(struct cache_header) + EOIE_SIZE_WITH_HEADER + the_hash_algo->rawsz)
                return 0;

        /* validate the extension signature */
        index = eoie = mmap + mmap_size - EOIE_SIZE_WITH_HEADER - the_hash_algo->rawsz;
        if (CACHE_EXT(index) != CACHE_EXT_ENDOFINDEXENTRIES)
                return 0;
        index += sizeof(uint32_t);

        /* validate the extension size */
        extsize = get_be32(index);
        if (extsize != EOIE_SIZE)
                return 0;
        index += sizeof(uint32_t);

        /*
         * Validate the offset we're going to look for the first extension
         * signature is after the index header and before the eoie extension.
         */
        offset = get_be32(index);
        if (mmap + offset < mmap + sizeof(struct cache_header))
                return 0;
        if (mmap + offset >= eoie)
                return 0;
        index += sizeof(uint32_t);

        /*
         * The hash is computed over extension types and their sizes (but not
         * their contents).  E.g. if we have "TREE" extension that is N-bytes
         * long, "REUC" extension that is M-bytes long, followed by "EOIE",
         * then the hash would be:
         *
         * SHA-1("TREE" + <binary representation of N> +
         *       "REUC" + <binary representation of M>)
         */
        src_offset = offset;
        the_hash_algo->init_fn(&c);
        while (src_offset < mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER) {
                /* After an array of active_nr index entries,
                 * there can be arbitrary number of extended
                 * sections, each of which is prefixed with
                 * extension name (4-byte) and section length
                 * in 4-byte network byte order.
                 */
                uint32_t extsize;
                memcpy(&extsize, mmap + src_offset + 4, 4);
                extsize = ntohl(extsize);

                /* verify the extension size isn't so large it will wrap around */
                if (src_offset + 8 + extsize < src_offset)
                        return 0;

                the_hash_algo->update_fn(&c, mmap + src_offset, 8);

                src_offset += 8;
                src_offset += extsize;
        }
        the_hash_algo->final_fn(hash, &c);
        if (!hasheq(hash, (const unsigned char *)index))
                return 0;

        /* Validate that the extension offsets returned us back to the eoie extension. */
        if (src_offset != mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER)
                return 0;

        return offset;
}

static void write_eoie_extension(struct strbuf *sb, git_hash_ctx *eoie_context, size_t offset)
{
        uint32_t buffer;
        unsigned char hash[GIT_MAX_RAWSZ];

        /* offset */
        put_be32(&buffer, offset);
        strbuf_add(sb, &buffer, sizeof(uint32_t));

        /* hash */
        the_hash_algo->final_fn(hash, eoie_context);
        strbuf_add(sb, hash, the_hash_algo->rawsz);
}