fsck: support referenced promisor objects

[alt-git.git] / packfile.c

#include "cache.h"
#include "mru.h"
#include "pack.h"
#include "dir.h"
#include "mergesort.h"
#include "packfile.h"
#include "delta.h"
#include "list.h"
#include "streaming.h"
#include "sha1-lookup.h"
#include "commit.h"
#include "object.h"
#include "tag.h"
#include "tree-walk.h"
#include "tree.h"

char *odb_pack_name(struct strbuf *buf,
                    const unsigned char *sha1,
                    const char *ext)
{
        strbuf_reset(buf);
        strbuf_addf(buf, "%s/pack/pack-%s.%s", get_object_directory(),
                    sha1_to_hex(sha1), ext);
        return buf->buf;
}

char *sha1_pack_name(const unsigned char *sha1)
{
        static struct strbuf buf = STRBUF_INIT;
        return odb_pack_name(&buf, sha1, "pack");
}

char *sha1_pack_index_name(const unsigned char *sha1)
{
        static struct strbuf buf = STRBUF_INIT;
        return odb_pack_name(&buf, sha1, "idx");
}

static unsigned int pack_used_ctr;
static unsigned int pack_mmap_calls;
static unsigned int peak_pack_open_windows;
static unsigned int pack_open_windows;
static unsigned int pack_open_fds;
static unsigned int pack_max_fds;
static size_t peak_pack_mapped;
static size_t pack_mapped;
struct packed_git *packed_git;
struct mru packed_git_mru;

#define SZ_FMT PRIuMAX
static inline uintmax_t sz_fmt(size_t s) { return s; }

void pack_report(void)
{
        fprintf(stderr,
                "pack_report: getpagesize()            = %10" SZ_FMT "\n"
                "pack_report: core.packedGitWindowSize = %10" SZ_FMT "\n"
                "pack_report: core.packedGitLimit      = %10" SZ_FMT "\n",
                sz_fmt(getpagesize()),
                sz_fmt(packed_git_window_size),
                sz_fmt(packed_git_limit));
        fprintf(stderr,
                "pack_report: pack_used_ctr            = %10u\n"
                "pack_report: pack_mmap_calls          = %10u\n"
                "pack_report: pack_open_windows        = %10u / %10u\n"
                "pack_report: pack_mapped              = "
                        "%10" SZ_FMT " / %10" SZ_FMT "\n",
                pack_used_ctr,
                pack_mmap_calls,
                pack_open_windows, peak_pack_open_windows,
                sz_fmt(pack_mapped), sz_fmt(peak_pack_mapped));
}

/*
 * Open and mmap the index file at path, perform a couple of
 * consistency checks, then record its information to p.  Return 0 on
 * success.
 */
static int check_packed_git_idx(const char *path, struct packed_git *p)
{
        void *idx_map;
        struct pack_idx_header *hdr;
        size_t idx_size;
        uint32_t version, nr, i, *index;
        int fd = git_open(path);
        struct stat st;

        if (fd < 0)
                return -1;
        if (fstat(fd, &st)) {
                close(fd);
                return -1;
        }
        idx_size = xsize_t(st.st_size);
        if (idx_size < 4 * 256 + 20 + 20) {
                close(fd);
                return error("index file %s is too small", path);
        }
        idx_map = xmmap(NULL, idx_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        hdr = idx_map;
        if (hdr->idx_signature == htonl(PACK_IDX_SIGNATURE)) {
                version = ntohl(hdr->idx_version);
                if (version < 2 || version > 2) {
                        munmap(idx_map, idx_size);
                        return error("index file %s is version %"PRIu32
                                     " and is not supported by this binary"
                                     " (try upgrading GIT to a newer version)",
                                     path, version);
                }
        } else
                version = 1;

        nr = 0;
        index = idx_map;
        if (version > 1)
                index += 2;  /* skip index header */
        for (i = 0; i < 256; i++) {
                uint32_t n = ntohl(index[i]);
                if (n < nr) {
                        munmap(idx_map, idx_size);
                        return error("non-monotonic index %s", path);
                }
                nr = n;
        }

        if (version == 1) {
                /*
                 * Total size:
                 *  - 256 index entries 4 bytes each
                 *  - 24-byte entries * nr (20-byte sha1 + 4-byte offset)
                 *  - 20-byte SHA1 of the packfile
                 *  - 20-byte SHA1 file checksum
                 */
                if (idx_size != 4*256 + nr * 24 + 20 + 20) {
                        munmap(idx_map, idx_size);
                        return error("wrong index v1 file size in %s", path);
                }
        } else if (version == 2) {
                /*
                 * Minimum size:
                 *  - 8 bytes of header
                 *  - 256 index entries 4 bytes each
                 *  - 20-byte sha1 entry * nr
                 *  - 4-byte crc entry * nr
                 *  - 4-byte offset entry * nr
                 *  - 20-byte SHA1 of the packfile
                 *  - 20-byte SHA1 file checksum
                 * And after the 4-byte offset table might be a
                 * variable sized table containing 8-byte entries
                 * for offsets larger than 2^31.
                 */
                unsigned long min_size = 8 + 4*256 + nr*(20 + 4 + 4) + 20 + 20;
                unsigned long max_size = min_size;
                if (nr)
                        max_size += (nr - 1)*8;
                if (idx_size < min_size || idx_size > max_size) {
                        munmap(idx_map, idx_size);
                        return error("wrong index v2 file size in %s", path);
                }
                if (idx_size != min_size &&
                    /*
                     * make sure we can deal with large pack offsets.
                     * 31-bit signed offset won't be enough, neither
                     * 32-bit unsigned one will be.
                     */
                    (sizeof(off_t) <= 4)) {
                        munmap(idx_map, idx_size);
                        return error("pack too large for current definition of off_t in %s", path);
                }
        }

        p->index_version = version;
        p->index_data = idx_map;
        p->index_size = idx_size;
        p->num_objects = nr;
        return 0;
}

int open_pack_index(struct packed_git *p)
{
        char *idx_name;
        size_t len;
        int ret;

        if (p->index_data)
                return 0;

        if (!strip_suffix(p->pack_name, ".pack", &len))
                die("BUG: pack_name does not end in .pack");
        idx_name = xstrfmt("%.*s.idx", (int)len, p->pack_name);
        ret = check_packed_git_idx(idx_name, p);
        free(idx_name);
        return ret;
}

static struct packed_git *alloc_packed_git(int extra)
{
        struct packed_git *p = xmalloc(st_add(sizeof(*p), extra));
        memset(p, 0, sizeof(*p));
        p->pack_fd = -1;
        return p;
}

struct packed_git *parse_pack_index(unsigned char *sha1, const char *idx_path)
{
        const char *path = sha1_pack_name(sha1);
        size_t alloc = st_add(strlen(path), 1);
        struct packed_git *p = alloc_packed_git(alloc);

        memcpy(p->pack_name, path, alloc); /* includes NUL */
        hashcpy(p->sha1, sha1);
        if (check_packed_git_idx(idx_path, p)) {
                free(p);
                return NULL;
        }

        return p;
}

static void scan_windows(struct packed_git *p,
        struct packed_git **lru_p,
        struct pack_window **lru_w,
        struct pack_window **lru_l)
{
        struct pack_window *w, *w_l;

        for (w_l = NULL, w = p->windows; w; w = w->next) {
                if (!w->inuse_cnt) {
                        if (!*lru_w || w->last_used < (*lru_w)->last_used) {
                                *lru_p = p;
                                *lru_w = w;
                                *lru_l = w_l;
                        }
                }
                w_l = w;
        }
}

static int unuse_one_window(struct packed_git *current)
{
        struct packed_git *p, *lru_p = NULL;
        struct pack_window *lru_w = NULL, *lru_l = NULL;

        if (current)
                scan_windows(current, &lru_p, &lru_w, &lru_l);
        for (p = packed_git; p; p = p->next)
                scan_windows(p, &lru_p, &lru_w, &lru_l);
        if (lru_p) {
                munmap(lru_w->base, lru_w->len);
                pack_mapped -= lru_w->len;
                if (lru_l)
                        lru_l->next = lru_w->next;
                else
                        lru_p->windows = lru_w->next;
                free(lru_w);
                pack_open_windows--;
                return 1;
        }
        return 0;
}

void release_pack_memory(size_t need)
{
        size_t cur = pack_mapped;
        while (need >= (cur - pack_mapped) && unuse_one_window(NULL))
                ; /* nothing */
}

void close_pack_windows(struct packed_git *p)
{
        while (p->windows) {
                struct pack_window *w = p->windows;

                if (w->inuse_cnt)
                        die("pack '%s' still has open windows to it",
                            p->pack_name);
                munmap(w->base, w->len);
                pack_mapped -= w->len;
                pack_open_windows--;
                p->windows = w->next;
                free(w);
        }
}

static int close_pack_fd(struct packed_git *p)
{
        if (p->pack_fd < 0)
                return 0;

        close(p->pack_fd);
        pack_open_fds--;
        p->pack_fd = -1;

        return 1;
}

void close_pack_index(struct packed_git *p)
{
        if (p->index_data) {
                munmap((void *)p->index_data, p->index_size);
                p->index_data = NULL;
        }
}

static void close_pack(struct packed_git *p)
{
        close_pack_windows(p);
        close_pack_fd(p);
        close_pack_index(p);
}

void close_all_packs(void)
{
        struct packed_git *p;

        for (p = packed_git; p; p = p->next)
                if (p->do_not_close)
                        die("BUG: want to close pack marked 'do-not-close'");
                else
                        close_pack(p);
}

/*
 * The LRU pack is the one with the oldest MRU window, preferring packs
 * with no used windows, or the oldest mtime if it has no windows allocated.
 */
static void find_lru_pack(struct packed_git *p, struct packed_git **lru_p, struct pack_window **mru_w, int *accept_windows_inuse)
{
        struct pack_window *w, *this_mru_w;
        int has_windows_inuse = 0;

        /*
         * Reject this pack if it has windows and the previously selected
         * one does not.  If this pack does not have windows, reject
         * it if the pack file is newer than the previously selected one.
         */
        if (*lru_p && !*mru_w && (p->windows || p->mtime > (*lru_p)->mtime))
                return;

        for (w = this_mru_w = p->windows; w; w = w->next) {
                /*
                 * Reject this pack if any of its windows are in use,
                 * but the previously selected pack did not have any
                 * inuse windows.  Otherwise, record that this pack
                 * has windows in use.
                 */
                if (w->inuse_cnt) {
                        if (*accept_windows_inuse)
                                has_windows_inuse = 1;
                        else
                                return;
                }

                if (w->last_used > this_mru_w->last_used)
                        this_mru_w = w;

                /*
                 * Reject this pack if it has windows that have been
                 * used more recently than the previously selected pack.
                 * If the previously selected pack had windows inuse and
                 * we have not encountered a window in this pack that is
                 * inuse, skip this check since we prefer a pack with no
                 * inuse windows to one that has inuse windows.
                 */
                if (*mru_w && *accept_windows_inuse == has_windows_inuse &&
                    this_mru_w->last_used > (*mru_w)->last_used)
                        return;
        }

        /*
         * Select this pack.
         */
        *mru_w = this_mru_w;
        *lru_p = p;
        *accept_windows_inuse = has_windows_inuse;
}

static int close_one_pack(void)
{
        struct packed_git *p, *lru_p = NULL;
        struct pack_window *mru_w = NULL;
        int accept_windows_inuse = 1;

        for (p = packed_git; p; p = p->next) {
                if (p->pack_fd == -1)
                        continue;
                find_lru_pack(p, &lru_p, &mru_w, &accept_windows_inuse);
        }

        if (lru_p)
                return close_pack_fd(lru_p);

        return 0;
}

static unsigned int get_max_fd_limit(void)
{
#ifdef RLIMIT_NOFILE
        {
                struct rlimit lim;

                if (!getrlimit(RLIMIT_NOFILE, &lim))
                        return lim.rlim_cur;
        }
#endif

#ifdef _SC_OPEN_MAX
        {
                long open_max = sysconf(_SC_OPEN_MAX);
                if (0 < open_max)
                        return open_max;
                /*
                 * Otherwise, we got -1 for one of the two
                 * reasons:
                 *
                 * (1) sysconf() did not understand _SC_OPEN_MAX
                 *     and signaled an error with -1; or
                 * (2) sysconf() said there is no limit.
                 *
                 * We _could_ clear errno before calling sysconf() to
                 * tell these two cases apart and return a huge number
                 * in the latter case to let the caller cap it to a
                 * value that is not so selfish, but letting the
                 * fallback OPEN_MAX codepath take care of these cases
                 * is a lot simpler.
                 */
        }
#endif

#ifdef OPEN_MAX
        return OPEN_MAX;
#else
        return 1; /* see the caller ;-) */
#endif
}

/*
 * Do not call this directly as this leaks p->pack_fd on error return;
 * call open_packed_git() instead.
 */
static int open_packed_git_1(struct packed_git *p)
{
        struct stat st;
        struct pack_header hdr;
        unsigned char sha1[20];
        unsigned char *idx_sha1;
        long fd_flag;
        ssize_t read_result;

        if (!p->index_data && open_pack_index(p))
                return error("packfile %s index unavailable", p->pack_name);

        if (!pack_max_fds) {
                unsigned int max_fds = get_max_fd_limit();

                /* Save 3 for stdin/stdout/stderr, 22 for work */
                if (25 < max_fds)
                        pack_max_fds = max_fds - 25;
                else
                        pack_max_fds = 1;
        }

        while (pack_max_fds <= pack_open_fds && close_one_pack())
                ; /* nothing */

        p->pack_fd = git_open(p->pack_name);
        if (p->pack_fd < 0 || fstat(p->pack_fd, &st))
                return -1;
        pack_open_fds++;

        /* If we created the struct before we had the pack we lack size. */
        if (!p->pack_size) {
                if (!S_ISREG(st.st_mode))
                        return error("packfile %s not a regular file", p->pack_name);
                p->pack_size = st.st_size;
        } else if (p->pack_size != st.st_size)
                return error("packfile %s size changed", p->pack_name);

        /* We leave these file descriptors open with sliding mmap;
         * there is no point keeping them open across exec(), though.
         */
        fd_flag = fcntl(p->pack_fd, F_GETFD, 0);
        if (fd_flag < 0)
                return error("cannot determine file descriptor flags");
        fd_flag |= FD_CLOEXEC;
        if (fcntl(p->pack_fd, F_SETFD, fd_flag) == -1)
                return error("cannot set FD_CLOEXEC");

        /* Verify we recognize this pack file format. */
        read_result = read_in_full(p->pack_fd, &hdr, sizeof(hdr));
        if (read_result < 0)
                return error_errno("error reading from %s", p->pack_name);
        if (read_result != sizeof(hdr))
                return error("file %s is far too short to be a packfile", p->pack_name);
        if (hdr.hdr_signature != htonl(PACK_SIGNATURE))
                return error("file %s is not a GIT packfile", p->pack_name);
        if (!pack_version_ok(hdr.hdr_version))
                return error("packfile %s is version %"PRIu32" and not"
                        " supported (try upgrading GIT to a newer version)",
                        p->pack_name, ntohl(hdr.hdr_version));

        /* Verify the pack matches its index. */
        if (p->num_objects != ntohl(hdr.hdr_entries))
                return error("packfile %s claims to have %"PRIu32" objects"
                             " while index indicates %"PRIu32" objects",
                             p->pack_name, ntohl(hdr.hdr_entries),
                             p->num_objects);
        if (lseek(p->pack_fd, p->pack_size - sizeof(sha1), SEEK_SET) == -1)
                return error("end of packfile %s is unavailable", p->pack_name);
        read_result = read_in_full(p->pack_fd, sha1, sizeof(sha1));
        if (read_result < 0)
                return error_errno("error reading from %s", p->pack_name);
        if (read_result != sizeof(sha1))
                return error("packfile %s signature is unavailable", p->pack_name);
        idx_sha1 = ((unsigned char *)p->index_data) + p->index_size - 40;
        if (hashcmp(sha1, idx_sha1))
                return error("packfile %s does not match index", p->pack_name);
        return 0;
}

static int open_packed_git(struct packed_git *p)
{
        if (!open_packed_git_1(p))
                return 0;
        close_pack_fd(p);
        return -1;
}

static int in_window(struct pack_window *win, off_t offset)
{
        /* We must promise at least 20 bytes (one hash) after the
         * offset is available from this window, otherwise the offset
         * is not actually in this window and a different window (which
         * has that one hash excess) must be used.  This is to support
         * the object header and delta base parsing routines below.
         */
        off_t win_off = win->offset;
        return win_off <= offset
                && (offset + 20) <= (win_off + win->len);
}

unsigned char *use_pack(struct packed_git *p,
                struct pack_window **w_cursor,
                off_t offset,
                unsigned long *left)
{
        struct pack_window *win = *w_cursor;

        /* Since packfiles end in a hash of their content and it's
         * pointless to ask for an offset into the middle of that
         * hash, and the in_window function above wouldn't match
         * don't allow an offset too close to the end of the file.
         */
        if (!p->pack_size && p->pack_fd == -1 && open_packed_git(p))
                die("packfile %s cannot be accessed", p->pack_name);
        if (offset > (p->pack_size - 20))
                die("offset beyond end of packfile (truncated pack?)");
        if (offset < 0)
                die(_("offset before end of packfile (broken .idx?)"));

        if (!win || !in_window(win, offset)) {
                if (win)
                        win->inuse_cnt--;
                for (win = p->windows; win; win = win->next) {
                        if (in_window(win, offset))
                                break;
                }
                if (!win) {
                        size_t window_align = packed_git_window_size / 2;
                        off_t len;

                        if (p->pack_fd == -1 && open_packed_git(p))
                                die("packfile %s cannot be accessed", p->pack_name);

                        win = xcalloc(1, sizeof(*win));
                        win->offset = (offset / window_align) * window_align;
                        len = p->pack_size - win->offset;
                        if (len > packed_git_window_size)
                                len = packed_git_window_size;
                        win->len = (size_t)len;
                        pack_mapped += win->len;
                        while (packed_git_limit < pack_mapped
                                && unuse_one_window(p))
                                ; /* nothing */
                        win->base = xmmap(NULL, win->len,
                                PROT_READ, MAP_PRIVATE,
                                p->pack_fd, win->offset);
                        if (win->base == MAP_FAILED)
                                die_errno("packfile %s cannot be mapped",
                                          p->pack_name);
                        if (!win->offset && win->len == p->pack_size
                                && !p->do_not_close)
                                close_pack_fd(p);
                        pack_mmap_calls++;
                        pack_open_windows++;
                        if (pack_mapped > peak_pack_mapped)
                                peak_pack_mapped = pack_mapped;
                        if (pack_open_windows > peak_pack_open_windows)
                                peak_pack_open_windows = pack_open_windows;
                        win->next = p->windows;
                        p->windows = win;
                }
        }
        if (win != *w_cursor) {
                win->last_used = pack_used_ctr++;
                win->inuse_cnt++;
                *w_cursor = win;
        }
        offset -= win->offset;
        if (left)
                *left = win->len - xsize_t(offset);
        return win->base + offset;
}

void unuse_pack(struct pack_window **w_cursor)
{
        struct pack_window *w = *w_cursor;
        if (w) {
                w->inuse_cnt--;
                *w_cursor = NULL;
        }
}

static void try_to_free_pack_memory(size_t size)
{
        release_pack_memory(size);
}

struct packed_git *add_packed_git(const char *path, size_t path_len, int local)
{
        static int have_set_try_to_free_routine;
        struct stat st;
        size_t alloc;
        struct packed_git *p;

        if (!have_set_try_to_free_routine) {
                have_set_try_to_free_routine = 1;
                set_try_to_free_routine(try_to_free_pack_memory);
        }

        /*
         * Make sure a corresponding .pack file exists and that
         * the index looks sane.
         */
        if (!strip_suffix_mem(path, &path_len, ".idx"))
                return NULL;

        /*
         * ".promisor" is long enough to hold any suffix we're adding (and
         * the use xsnprintf double-checks that)
         */
        alloc = st_add3(path_len, strlen(".promisor"), 1);
        p = alloc_packed_git(alloc);
        memcpy(p->pack_name, path, path_len);

        xsnprintf(p->pack_name + path_len, alloc - path_len, ".keep");
        if (!access(p->pack_name, F_OK))
                p->pack_keep = 1;

        xsnprintf(p->pack_name + path_len, alloc - path_len, ".promisor");
        if (!access(p->pack_name, F_OK))
                p->pack_promisor = 1;

        xsnprintf(p->pack_name + path_len, alloc - path_len, ".pack");
        if (stat(p->pack_name, &st) || !S_ISREG(st.st_mode)) {
                free(p);
                return NULL;
        }

        /* ok, it looks sane as far as we can check without
         * actually mapping the pack file.
         */
        p->pack_size = st.st_size;
        p->pack_local = local;
        p->mtime = st.st_mtime;
        if (path_len < 40 || get_sha1_hex(path + path_len - 40, p->sha1))
                hashclr(p->sha1);
        return p;
}

void install_packed_git(struct packed_git *pack)
{
        if (pack->pack_fd != -1)
                pack_open_fds++;

        pack->next = packed_git;
        packed_git = pack;
}

void (*report_garbage)(unsigned seen_bits, const char *path);

static void report_helper(const struct string_list *list,
                          int seen_bits, int first, int last)
{
        if (seen_bits == (PACKDIR_FILE_PACK|PACKDIR_FILE_IDX))
                return;

        for (; first < last; first++)
                report_garbage(seen_bits, list->items[first].string);
}

static void report_pack_garbage(struct string_list *list)
{
        int i, baselen = -1, first = 0, seen_bits = 0;

        if (!report_garbage)
                return;

        string_list_sort(list);

        for (i = 0; i < list->nr; i++) {
                const char *path = list->items[i].string;
                if (baselen != -1 &&
                    strncmp(path, list->items[first].string, baselen)) {
                        report_helper(list, seen_bits, first, i);
                        baselen = -1;
                        seen_bits = 0;
                }
                if (baselen == -1) {
                        const char *dot = strrchr(path, '.');
                        if (!dot) {
                                report_garbage(PACKDIR_FILE_GARBAGE, path);
                                continue;
                        }
                        baselen = dot - path + 1;
                        first = i;
                }
                if (!strcmp(path + baselen, "pack"))
                        seen_bits |= 1;
                else if (!strcmp(path + baselen, "idx"))
                        seen_bits |= 2;
        }
        report_helper(list, seen_bits, first, list->nr);
}

static void prepare_packed_git_one(char *objdir, int local)
{
        struct strbuf path = STRBUF_INIT;
        size_t dirnamelen;
        DIR *dir;
        struct dirent *de;
        struct string_list garbage = STRING_LIST_INIT_DUP;

        strbuf_addstr(&path, objdir);
        strbuf_addstr(&path, "/pack");
        dir = opendir(path.buf);
        if (!dir) {
                if (errno != ENOENT)
                        error_errno("unable to open object pack directory: %s",
                                    path.buf);
                strbuf_release(&path);
                return;
        }
        strbuf_addch(&path, '/');
        dirnamelen = path.len;
        while ((de = readdir(dir)) != NULL) {
                struct packed_git *p;
                size_t base_len;

                if (is_dot_or_dotdot(de->d_name))
                        continue;

                strbuf_setlen(&path, dirnamelen);
                strbuf_addstr(&path, de->d_name);

                base_len = path.len;
                if (strip_suffix_mem(path.buf, &base_len, ".idx")) {
                        /* Don't reopen a pack we already have. */
                        for (p = packed_git; p; p = p->next) {
                                size_t len;
                                if (strip_suffix(p->pack_name, ".pack", &len) &&
                                    len == base_len &&
                                    !memcmp(p->pack_name, path.buf, len))
                                        break;
                        }
                        if (p == NULL &&
                            /*
                             * See if it really is a valid .idx file with
                             * corresponding .pack file that we can map.
                             */
                            (p = add_packed_git(path.buf, path.len, local)) != NULL)
                                install_packed_git(p);
                }

                if (!report_garbage)
                        continue;

                if (ends_with(de->d_name, ".idx") ||
                    ends_with(de->d_name, ".pack") ||
                    ends_with(de->d_name, ".bitmap") ||
                    ends_with(de->d_name, ".keep") ||
                    ends_with(de->d_name, ".promisor"))
                        string_list_append(&garbage, path.buf);
                else
                        report_garbage(PACKDIR_FILE_GARBAGE, path.buf);
        }
        closedir(dir);
        report_pack_garbage(&garbage);
        string_list_clear(&garbage, 0);
        strbuf_release(&path);
}

static int approximate_object_count_valid;

/*
 * Give a fast, rough count of the number of objects in the repository. This
 * ignores loose objects completely. If you have a lot of them, then either
 * you should repack because your performance will be awful, or they are
 * all unreachable objects about to be pruned, in which case they're not really
 * interesting as a measure of repo size in the first place.
 */
unsigned long approximate_object_count(void)
{
        static unsigned long count;
        if (!approximate_object_count_valid) {
                struct packed_git *p;

                prepare_packed_git();
                count = 0;
                for (p = packed_git; p; p = p->next) {
                        if (open_pack_index(p))
                                continue;
                        count += p->num_objects;
                }
        }
        return count;
}

static void *get_next_packed_git(const void *p)
{
        return ((const struct packed_git *)p)->next;
}

static void set_next_packed_git(void *p, void *next)
{
        ((struct packed_git *)p)->next = next;
}

static int sort_pack(const void *a_, const void *b_)
{
        const struct packed_git *a = a_;
        const struct packed_git *b = b_;
        int st;

        /*
         * Local packs tend to contain objects specific to our
         * variant of the project than remote ones.  In addition,
         * remote ones could be on a network mounted filesystem.
         * Favor local ones for these reasons.
         */
        st = a->pack_local - b->pack_local;
        if (st)
                return -st;

        /*
         * Younger packs tend to contain more recent objects,
         * and more recent objects tend to get accessed more
         * often.
         */
        if (a->mtime < b->mtime)
                return 1;
        else if (a->mtime == b->mtime)
                return 0;
        return -1;
}

static void rearrange_packed_git(void)
{
        packed_git = llist_mergesort(packed_git, get_next_packed_git,
                                     set_next_packed_git, sort_pack);
}

static void prepare_packed_git_mru(void)
{
        struct packed_git *p;

        mru_clear(&packed_git_mru);
        for (p = packed_git; p; p = p->next)
                mru_append(&packed_git_mru, p);
}

static int prepare_packed_git_run_once = 0;
void prepare_packed_git(void)
{
        struct alternate_object_database *alt;

        if (prepare_packed_git_run_once)
                return;
        prepare_packed_git_one(get_object_directory(), 1);
        prepare_alt_odb();
        for (alt = alt_odb_list; alt; alt = alt->next)
                prepare_packed_git_one(alt->path, 0);
        rearrange_packed_git();
        prepare_packed_git_mru();
        prepare_packed_git_run_once = 1;
}

void reprepare_packed_git(void)
{
        approximate_object_count_valid = 0;
        prepare_packed_git_run_once = 0;
        prepare_packed_git();
}

unsigned long unpack_object_header_buffer(const unsigned char *buf,
                unsigned long len, enum object_type *type, unsigned long *sizep)
{
        unsigned shift;
        unsigned long size, c;
        unsigned long used = 0;

        c = buf[used++];
        *type = (c >> 4) & 7;
        size = c & 15;
        shift = 4;
        while (c & 0x80) {
                if (len <= used || bitsizeof(long) <= shift) {
                        error("bad object header");
                        size = used = 0;
                        break;
                }
                c = buf[used++];
                size += (c & 0x7f) << shift;
                shift += 7;
        }
        *sizep = size;
        return used;
}

unsigned long get_size_from_delta(struct packed_git *p,
                                  struct pack_window **w_curs,
                                  off_t curpos)
{
        const unsigned char *data;
        unsigned char delta_head[20], *in;
        git_zstream stream;
        int st;

        memset(&stream, 0, sizeof(stream));
        stream.next_out = delta_head;
        stream.avail_out = sizeof(delta_head);

        git_inflate_init(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = git_inflate(&stream, Z_FINISH);
                curpos += stream.next_in - in;
        } while ((st == Z_OK || st == Z_BUF_ERROR) &&
                 stream.total_out < sizeof(delta_head));
        git_inflate_end(&stream);
        if ((st != Z_STREAM_END) && stream.total_out != sizeof(delta_head)) {
                error("delta data unpack-initial failed");
                return 0;
        }

        /* Examine the initial part of the delta to figure out
         * the result size.
         */
        data = delta_head;

        /* ignore base size */
        get_delta_hdr_size(&data, delta_head+sizeof(delta_head));

        /* Read the result size */
        return get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
}

int unpack_object_header(struct packed_git *p,
                         struct pack_window **w_curs,
                         off_t *curpos,
                         unsigned long *sizep)
{
        unsigned char *base;
        unsigned long left;
        unsigned long used;
        enum object_type type;

        /* use_pack() assures us we have [base, base + 20) available
         * as a range that we can look at.  (Its actually the hash
         * size that is assured.)  With our object header encoding
         * the maximum deflated object size is 2^137, which is just
         * insane, so we know won't exceed what we have been given.
         */
        base = use_pack(p, w_curs, *curpos, &left);
        used = unpack_object_header_buffer(base, left, &type, sizep);
        if (!used) {
                type = OBJ_BAD;
        } else
                *curpos += used;

        return type;
}

void mark_bad_packed_object(struct packed_git *p, const unsigned char *sha1)
{
        unsigned i;
        for (i = 0; i < p->num_bad_objects; i++)
                if (!hashcmp(sha1, p->bad_object_sha1 + GIT_SHA1_RAWSZ * i))
                        return;
        p->bad_object_sha1 = xrealloc(p->bad_object_sha1,
                                      st_mult(GIT_MAX_RAWSZ,
                                              st_add(p->num_bad_objects, 1)));
        hashcpy(p->bad_object_sha1 + GIT_SHA1_RAWSZ * p->num_bad_objects, sha1);
        p->num_bad_objects++;
}

const struct packed_git *has_packed_and_bad(const unsigned char *sha1)
{
        struct packed_git *p;
        unsigned i;

        for (p = packed_git; p; p = p->next)
                for (i = 0; i < p->num_bad_objects; i++)
                        if (!hashcmp(sha1, p->bad_object_sha1 + 20 * i))
                                return p;
        return NULL;
}

static off_t get_delta_base(struct packed_git *p,
                                    struct pack_window **w_curs,
                                    off_t *curpos,
                                    enum object_type type,
                                    off_t delta_obj_offset)
{
        unsigned char *base_info = use_pack(p, w_curs, *curpos, NULL);
        off_t base_offset;

        /* use_pack() assured us we have [base_info, base_info + 20)
         * as a range that we can look at without walking off the
         * end of the mapped window.  Its actually the hash size
         * that is assured.  An OFS_DELTA longer than the hash size
         * is stupid, as then a REF_DELTA would be smaller to store.
         */
        if (type == OBJ_OFS_DELTA) {
                unsigned used = 0;
                unsigned char c = base_info[used++];
                base_offset = c & 127;
                while (c & 128) {
                        base_offset += 1;
                        if (!base_offset || MSB(base_offset, 7))
                                return 0;  /* overflow */
                        c = base_info[used++];
                        base_offset = (base_offset << 7) + (c & 127);
                }
                base_offset = delta_obj_offset - base_offset;
                if (base_offset <= 0 || base_offset >= delta_obj_offset)
                        return 0;  /* out of bound */
                *curpos += used;
        } else if (type == OBJ_REF_DELTA) {
                /* The base entry _must_ be in the same pack */
                base_offset = find_pack_entry_one(base_info, p);
                *curpos += 20;
        } else
                die("I am totally screwed");
        return base_offset;
}

/*
 * Like get_delta_base above, but we return the sha1 instead of the pack
 * offset. This means it is cheaper for REF deltas (we do not have to do
 * the final object lookup), but more expensive for OFS deltas (we
 * have to load the revidx to convert the offset back into a sha1).
 */
static const unsigned char *get_delta_base_sha1(struct packed_git *p,
                                                struct pack_window **w_curs,
                                                off_t curpos,
                                                enum object_type type,
                                                off_t delta_obj_offset)
{
        if (type == OBJ_REF_DELTA) {
                unsigned char *base = use_pack(p, w_curs, curpos, NULL);
                return base;
        } else if (type == OBJ_OFS_DELTA) {
                struct revindex_entry *revidx;
                off_t base_offset = get_delta_base(p, w_curs, &curpos,
                                                   type, delta_obj_offset);

                if (!base_offset)
                        return NULL;

                revidx = find_pack_revindex(p, base_offset);
                if (!revidx)
                        return NULL;

                return nth_packed_object_sha1(p, revidx->nr);
        } else
                return NULL;
}

static int retry_bad_packed_offset(struct packed_git *p, off_t obj_offset)
{
        int type;
        struct revindex_entry *revidx;
        const unsigned char *sha1;
        revidx = find_pack_revindex(p, obj_offset);
        if (!revidx)
                return OBJ_BAD;
        sha1 = nth_packed_object_sha1(p, revidx->nr);
        mark_bad_packed_object(p, sha1);
        type = sha1_object_info(sha1, NULL);
        if (type <= OBJ_NONE)
                return OBJ_BAD;
        return type;
}

#define POI_STACK_PREALLOC 64

static enum object_type packed_to_object_type(struct packed_git *p,
                                              off_t obj_offset,
                                              enum object_type type,
                                              struct pack_window **w_curs,
                                              off_t curpos)
{
        off_t small_poi_stack[POI_STACK_PREALLOC];
        off_t *poi_stack = small_poi_stack;
        int poi_stack_nr = 0, poi_stack_alloc = POI_STACK_PREALLOC;

        while (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
                off_t base_offset;
                unsigned long size;
                /* Push the object we're going to leave behind */
                if (poi_stack_nr >= poi_stack_alloc && poi_stack == small_poi_stack) {
                        poi_stack_alloc = alloc_nr(poi_stack_nr);
                        ALLOC_ARRAY(poi_stack, poi_stack_alloc);
                        memcpy(poi_stack, small_poi_stack, sizeof(off_t)*poi_stack_nr);
                } else {
                        ALLOC_GROW(poi_stack, poi_stack_nr+1, poi_stack_alloc);
                }
                poi_stack[poi_stack_nr++] = obj_offset;
                /* If parsing the base offset fails, just unwind */
                base_offset = get_delta_base(p, w_curs, &curpos, type, obj_offset);
                if (!base_offset)
                        goto unwind;
                curpos = obj_offset = base_offset;
                type = unpack_object_header(p, w_curs, &curpos, &size);
                if (type <= OBJ_NONE) {
                        /* If getting the base itself fails, we first
                         * retry the base, otherwise unwind */
                        type = retry_bad_packed_offset(p, base_offset);
                        if (type > OBJ_NONE)
                                goto out;
                        goto unwind;
                }
        }

        switch (type) {
        case OBJ_BAD:
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                break;
        default:
                error("unknown object type %i at offset %"PRIuMAX" in %s",
                      type, (uintmax_t)obj_offset, p->pack_name);
                type = OBJ_BAD;
        }

out:
        if (poi_stack != small_poi_stack)
                free(poi_stack);
        return type;

unwind:
        while (poi_stack_nr) {
                obj_offset = poi_stack[--poi_stack_nr];
                type = retry_bad_packed_offset(p, obj_offset);
                if (type > OBJ_NONE)
                        goto out;
        }
        type = OBJ_BAD;
        goto out;
}

static struct hashmap delta_base_cache;
static size_t delta_base_cached;

static LIST_HEAD(delta_base_cache_lru);

struct delta_base_cache_key {
        struct packed_git *p;
        off_t base_offset;
};

struct delta_base_cache_entry {
        struct hashmap hash;
        struct delta_base_cache_key key;
        struct list_head lru;
        void *data;
        unsigned long size;
        enum object_type type;
};

static unsigned int pack_entry_hash(struct packed_git *p, off_t base_offset)
{
        unsigned int hash;

        hash = (unsigned int)(intptr_t)p + (unsigned int)base_offset;
        hash += (hash >> 8) + (hash >> 16);
        return hash;
}

static struct delta_base_cache_entry *
get_delta_base_cache_entry(struct packed_git *p, off_t base_offset)
{
        struct hashmap_entry entry;
        struct delta_base_cache_key key;

        if (!delta_base_cache.cmpfn)
                return NULL;

        hashmap_entry_init(&entry, pack_entry_hash(p, base_offset));
        key.p = p;
        key.base_offset = base_offset;
        return hashmap_get(&delta_base_cache, &entry, &key);
}

static int delta_base_cache_key_eq(const struct delta_base_cache_key *a,
                                   const struct delta_base_cache_key *b)
{
        return a->p == b->p && a->base_offset == b->base_offset;
}

static int delta_base_cache_hash_cmp(const void *unused_cmp_data,
                                     const void *va, const void *vb,
                                     const void *vkey)
{
        const struct delta_base_cache_entry *a = va, *b = vb;
        const struct delta_base_cache_key *key = vkey;
        if (key)
                return !delta_base_cache_key_eq(&a->key, key);
        else
                return !delta_base_cache_key_eq(&a->key, &b->key);
}

static int in_delta_base_cache(struct packed_git *p, off_t base_offset)
{
        return !!get_delta_base_cache_entry(p, base_offset);
}

/*
 * Remove the entry from the cache, but do _not_ free the associated
 * entry data. The caller takes ownership of the "data" buffer, and
 * should copy out any fields it wants before detaching.
 */
static void detach_delta_base_cache_entry(struct delta_base_cache_entry *ent)
{
        hashmap_remove(&delta_base_cache, ent, &ent->key);
        list_del(&ent->lru);
        delta_base_cached -= ent->size;
        free(ent);
}

static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
        unsigned long *base_size, enum object_type *type)
{
        struct delta_base_cache_entry *ent;

        ent = get_delta_base_cache_entry(p, base_offset);
        if (!ent)
                return unpack_entry(p, base_offset, type, base_size);

        if (type)
                *type = ent->type;
        if (base_size)
                *base_size = ent->size;
        return xmemdupz(ent->data, ent->size);
}

static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
{
        free(ent->data);
        detach_delta_base_cache_entry(ent);
}

void clear_delta_base_cache(void)
{
        struct list_head *lru, *tmp;
        list_for_each_safe(lru, tmp, &delta_base_cache_lru) {
                struct delta_base_cache_entry *entry =
                        list_entry(lru, struct delta_base_cache_entry, lru);
                release_delta_base_cache(entry);
        }
}

static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
        void *base, unsigned long base_size, enum object_type type)
{
        struct delta_base_cache_entry *ent = xmalloc(sizeof(*ent));
        struct list_head *lru, *tmp;

        delta_base_cached += base_size;

        list_for_each_safe(lru, tmp, &delta_base_cache_lru) {
                struct delta_base_cache_entry *f =
                        list_entry(lru, struct delta_base_cache_entry, lru);
                if (delta_base_cached <= delta_base_cache_limit)
                        break;
                release_delta_base_cache(f);
        }

        ent->key.p = p;
        ent->key.base_offset = base_offset;
        ent->type = type;
        ent->data = base;
        ent->size = base_size;
        list_add_tail(&ent->lru, &delta_base_cache_lru);

        if (!delta_base_cache.cmpfn)
                hashmap_init(&delta_base_cache, delta_base_cache_hash_cmp, NULL, 0);
        hashmap_entry_init(ent, pack_entry_hash(p, base_offset));
        hashmap_add(&delta_base_cache, ent);
}

int packed_object_info(struct packed_git *p, off_t obj_offset,
                       struct object_info *oi)
{
        struct pack_window *w_curs = NULL;
        unsigned long size;
        off_t curpos = obj_offset;
        enum object_type type;

        /*
         * We always get the representation type, but only convert it to
         * a "real" type later if the caller is interested.
         */
        if (oi->contentp) {
                *oi->contentp = cache_or_unpack_entry(p, obj_offset, oi->sizep,
                                                      &type);
                if (!*oi->contentp)
                        type = OBJ_BAD;
        } else {
                type = unpack_object_header(p, &w_curs, &curpos, &size);
        }

        if (!oi->contentp && oi->sizep) {
                if (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
                        off_t tmp_pos = curpos;
                        off_t base_offset = get_delta_base(p, &w_curs, &tmp_pos,
                                                           type, obj_offset);
                        if (!base_offset) {
                                type = OBJ_BAD;
                                goto out;
                        }
                        *oi->sizep = get_size_from_delta(p, &w_curs, tmp_pos);
                        if (*oi->sizep == 0) {
                                type = OBJ_BAD;
                                goto out;
                        }
                } else {
                        *oi->sizep = size;
                }
        }

        if (oi->disk_sizep) {
                struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
                *oi->disk_sizep = revidx[1].offset - obj_offset;
        }

        if (oi->typep || oi->typename) {
                enum object_type ptot;
                ptot = packed_to_object_type(p, obj_offset, type, &w_curs,
                                             curpos);
                if (oi->typep)
                        *oi->typep = ptot;
                if (oi->typename) {
                        const char *tn = typename(ptot);
                        if (tn)
                                strbuf_addstr(oi->typename, tn);
                }
                if (ptot < 0) {
                        type = OBJ_BAD;
                        goto out;
                }
        }

        if (oi->delta_base_sha1) {
                if (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
                        const unsigned char *base;

                        base = get_delta_base_sha1(p, &w_curs, curpos,
                                                   type, obj_offset);
                        if (!base) {
                                type = OBJ_BAD;
                                goto out;
                        }

                        hashcpy(oi->delta_base_sha1, base);
                } else
                        hashclr(oi->delta_base_sha1);
        }

        oi->whence = in_delta_base_cache(p, obj_offset) ? OI_DBCACHED :
                                                          OI_PACKED;

out:
        unuse_pack(&w_curs);
        return type;
}

static void *unpack_compressed_entry(struct packed_git *p,
                                    struct pack_window **w_curs,
                                    off_t curpos,
                                    unsigned long size)
{
        int st;
        git_zstream stream;
        unsigned char *buffer, *in;

        buffer = xmallocz_gently(size);
        if (!buffer)
                return NULL;
        memset(&stream, 0, sizeof(stream));
        stream.next_out = buffer;
        stream.avail_out = size + 1;

        git_inflate_init(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = git_inflate(&stream, Z_FINISH);
                if (!stream.avail_out)
                        break; /* the payload is larger than it should be */
                curpos += stream.next_in - in;
        } while (st == Z_OK || st == Z_BUF_ERROR);
        git_inflate_end(&stream);
        if ((st != Z_STREAM_END) || stream.total_out != size) {
                free(buffer);
                return NULL;
        }

        return buffer;
}

static void write_pack_access_log(struct packed_git *p, off_t obj_offset)
{
        static struct trace_key pack_access = TRACE_KEY_INIT(PACK_ACCESS);
        trace_printf_key(&pack_access, "%s %"PRIuMAX"\n",
                         p->pack_name, (uintmax_t)obj_offset);
}

int do_check_packed_object_crc;

#define UNPACK_ENTRY_STACK_PREALLOC 64
struct unpack_entry_stack_ent {
        off_t obj_offset;
        off_t curpos;
        unsigned long size;
};

static void *read_object(const unsigned char *sha1, enum object_type *type,
                         unsigned long *size)
{
        struct object_info oi = OBJECT_INFO_INIT;
        void *content;
        oi.typep = type;
        oi.sizep = size;
        oi.contentp = &content;

        if (sha1_object_info_extended(sha1, &oi, 0) < 0)
                return NULL;
        return content;
}

void *unpack_entry(struct packed_git *p, off_t obj_offset,
                   enum object_type *final_type, unsigned long *final_size)
{
        struct pack_window *w_curs = NULL;
        off_t curpos = obj_offset;
        void *data = NULL;
        unsigned long size;
        enum object_type type;
        struct unpack_entry_stack_ent small_delta_stack[UNPACK_ENTRY_STACK_PREALLOC];
        struct unpack_entry_stack_ent *delta_stack = small_delta_stack;
        int delta_stack_nr = 0, delta_stack_alloc = UNPACK_ENTRY_STACK_PREALLOC;
        int base_from_cache = 0;

        write_pack_access_log(p, obj_offset);

        /* PHASE 1: drill down to the innermost base object */
        for (;;) {
                off_t base_offset;
                int i;
                struct delta_base_cache_entry *ent;

                ent = get_delta_base_cache_entry(p, curpos);
                if (ent) {
                        type = ent->type;
                        data = ent->data;
                        size = ent->size;
                        detach_delta_base_cache_entry(ent);
                        base_from_cache = 1;
                        break;
                }

                if (do_check_packed_object_crc && p->index_version > 1) {
                        struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
                        off_t len = revidx[1].offset - obj_offset;
                        if (check_pack_crc(p, &w_curs, obj_offset, len, revidx->nr)) {
                                const unsigned char *sha1 =
                                        nth_packed_object_sha1(p, revidx->nr);
                                error("bad packed object CRC for %s",
                                      sha1_to_hex(sha1));
                                mark_bad_packed_object(p, sha1);
                                data = NULL;
                                goto out;
                        }
                }

                type = unpack_object_header(p, &w_curs, &curpos, &size);
                if (type != OBJ_OFS_DELTA && type != OBJ_REF_DELTA)
                        break;

                base_offset = get_delta_base(p, &w_curs, &curpos, type, obj_offset);
                if (!base_offset) {
                        error("failed to validate delta base reference "
                              "at offset %"PRIuMAX" from %s",
                              (uintmax_t)curpos, p->pack_name);
                        /* bail to phase 2, in hopes of recovery */
                        data = NULL;
                        break;
                }

                /* push object, proceed to base */
                if (delta_stack_nr >= delta_stack_alloc
                    && delta_stack == small_delta_stack) {
                        delta_stack_alloc = alloc_nr(delta_stack_nr);
                        ALLOC_ARRAY(delta_stack, delta_stack_alloc);
                        memcpy(delta_stack, small_delta_stack,
                               sizeof(*delta_stack)*delta_stack_nr);
                } else {
                        ALLOC_GROW(delta_stack, delta_stack_nr+1, delta_stack_alloc);
                }
                i = delta_stack_nr++;
                delta_stack[i].obj_offset = obj_offset;
                delta_stack[i].curpos = curpos;
                delta_stack[i].size = size;

                curpos = obj_offset = base_offset;
        }

        /* PHASE 2: handle the base */
        switch (type) {
        case OBJ_OFS_DELTA:
        case OBJ_REF_DELTA:
                if (data)
                        die("BUG: unpack_entry: left loop at a valid delta");
                break;
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                if (!base_from_cache)
                        data = unpack_compressed_entry(p, &w_curs, curpos, size);
                break;
        default:
                data = NULL;
                error("unknown object type %i at offset %"PRIuMAX" in %s",
                      type, (uintmax_t)obj_offset, p->pack_name);
        }

        /* PHASE 3: apply deltas in order */

        /* invariants:
         *   'data' holds the base data, or NULL if there was corruption
         */
        while (delta_stack_nr) {
                void *delta_data;
                void *base = data;
                void *external_base = NULL;
                unsigned long delta_size, base_size = size;
                int i;

                data = NULL;

                if (base)
                        add_delta_base_cache(p, obj_offset, base, base_size, type);

                if (!base) {
                        /*
                         * We're probably in deep shit, but let's try to fetch
                         * the required base anyway from another pack or loose.
                         * This is costly but should happen only in the presence
                         * of a corrupted pack, and is better than failing outright.
                         */
                        struct revindex_entry *revidx;
                        const unsigned char *base_sha1;
                        revidx = find_pack_revindex(p, obj_offset);
                        if (revidx) {
                                base_sha1 = nth_packed_object_sha1(p, revidx->nr);
                                error("failed to read delta base object %s"
                                      " at offset %"PRIuMAX" from %s",
                                      sha1_to_hex(base_sha1), (uintmax_t)obj_offset,
                                      p->pack_name);
                                mark_bad_packed_object(p, base_sha1);
                                base = read_object(base_sha1, &type, &base_size);
                                external_base = base;
                        }
                }

                i = --delta_stack_nr;
                obj_offset = delta_stack[i].obj_offset;
                curpos = delta_stack[i].curpos;
                delta_size = delta_stack[i].size;

                if (!base)
                        continue;

                delta_data = unpack_compressed_entry(p, &w_curs, curpos, delta_size);

                if (!delta_data) {
                        error("failed to unpack compressed delta "
                              "at offset %"PRIuMAX" from %s",
                              (uintmax_t)curpos, p->pack_name);
                        data = NULL;
                        free(external_base);
                        continue;
                }

                data = patch_delta(base, base_size,
                                   delta_data, delta_size,
                                   &size);

                /*
                 * We could not apply the delta; warn the user, but keep going.
                 * Our failure will be noticed either in the next iteration of
                 * the loop, or if this is the final delta, in the caller when
                 * we return NULL. Those code paths will take care of making
                 * a more explicit warning and retrying with another copy of
                 * the object.
                 */
                if (!data)
                        error("failed to apply delta");

                free(delta_data);
                free(external_base);
        }

        if (final_type)
                *final_type = type;
        if (final_size)
                *final_size = size;

out:
        unuse_pack(&w_curs);

        if (delta_stack != small_delta_stack)
                free(delta_stack);

        return data;
}

const unsigned char *nth_packed_object_sha1(struct packed_git *p,
                                            uint32_t n)
{
        const unsigned char *index = p->index_data;
        if (!index) {
                if (open_pack_index(p))
                        return NULL;
                index = p->index_data;
        }
        if (n >= p->num_objects)
                return NULL;
        index += 4 * 256;
        if (p->index_version == 1) {
                return index + 24 * n + 4;
        } else {
                index += 8;
                return index + 20 * n;
        }
}

const struct object_id *nth_packed_object_oid(struct object_id *oid,
                                              struct packed_git *p,
                                              uint32_t n)
{
        const unsigned char *hash = nth_packed_object_sha1(p, n);
        if (!hash)
                return NULL;
        hashcpy(oid->hash, hash);
        return oid;
}

void check_pack_index_ptr(const struct packed_git *p, const void *vptr)
{
        const unsigned char *ptr = vptr;
        const unsigned char *start = p->index_data;
        const unsigned char *end = start + p->index_size;
        if (ptr < start)
                die(_("offset before start of pack index for %s (corrupt index?)"),
                    p->pack_name);
        /* No need to check for underflow; .idx files must be at least 8 bytes */
        if (ptr >= end - 8)
                die(_("offset beyond end of pack index for %s (truncated index?)"),
                    p->pack_name);
}

off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
{
        const unsigned char *index = p->index_data;
        index += 4 * 256;
        if (p->index_version == 1) {
                return ntohl(*((uint32_t *)(index + 24 * n)));
        } else {
                uint32_t off;
                index += 8 + p->num_objects * (20 + 4);
                off = ntohl(*((uint32_t *)(index + 4 * n)));
                if (!(off & 0x80000000))
                        return off;
                index += p->num_objects * 4 + (off & 0x7fffffff) * 8;
                check_pack_index_ptr(p, index);
                return (((uint64_t)ntohl(*((uint32_t *)(index + 0)))) << 32) |
                                   ntohl(*((uint32_t *)(index + 4)));
        }
}

off_t find_pack_entry_one(const unsigned char *sha1,
                                  struct packed_git *p)
{
        const uint32_t *level1_ofs = p->index_data;
        const unsigned char *index = p->index_data;
        unsigned hi, lo, stride;
        static int debug_lookup = -1;

        if (debug_lookup < 0)
                debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");

        if (!index) {
                if (open_pack_index(p))
                        return 0;
                level1_ofs = p->index_data;
                index = p->index_data;
        }
        if (p->index_version > 1) {
                level1_ofs += 2;
                index += 8;
        }
        index += 4 * 256;
        hi = ntohl(level1_ofs[*sha1]);
        lo = ((*sha1 == 0x0) ? 0 : ntohl(level1_ofs[*sha1 - 1]));
        if (p->index_version > 1) {
                stride = 20;
        } else {
                stride = 24;
                index += 4;
        }

        if (debug_lookup)
                printf("%02x%02x%02x... lo %u hi %u nr %"PRIu32"\n",
                       sha1[0], sha1[1], sha1[2], lo, hi, p->num_objects);

        while (lo < hi) {
                unsigned mi = lo + (hi - lo) / 2;
                int cmp = hashcmp(index + mi * stride, sha1);

                if (debug_lookup)
                        printf("lo %u hi %u rg %u mi %u\n",
                               lo, hi, hi - lo, mi);
                if (!cmp)
                        return nth_packed_object_offset(p, mi);
                if (cmp > 0)
                        hi = mi;
                else
                        lo = mi+1;
        }
        return 0;
}

int is_pack_valid(struct packed_git *p)
{
        /* An already open pack is known to be valid. */
        if (p->pack_fd != -1)
                return 1;

        /* If the pack has one window completely covering the
         * file size, the pack is known to be valid even if
         * the descriptor is not currently open.
         */
        if (p->windows) {
                struct pack_window *w = p->windows;

                if (!w->offset && w->len == p->pack_size)
                        return 1;
        }

        /* Force the pack to open to prove its valid. */
        return !open_packed_git(p);
}

struct packed_git *find_sha1_pack(const unsigned char *sha1,
                                  struct packed_git *packs)
{
        struct packed_git *p;

        for (p = packs; p; p = p->next) {
                if (find_pack_entry_one(sha1, p))
                        return p;
        }
        return NULL;

}

static int fill_pack_entry(const unsigned char *sha1,
                           struct pack_entry *e,
                           struct packed_git *p)
{
        off_t offset;

        if (p->num_bad_objects) {
                unsigned i;
                for (i = 0; i < p->num_bad_objects; i++)
                        if (!hashcmp(sha1, p->bad_object_sha1 + 20 * i))
                                return 0;
        }

        offset = find_pack_entry_one(sha1, p);
        if (!offset)
                return 0;

        /*
         * We are about to tell the caller where they can locate the
         * requested object.  We better make sure the packfile is
         * still here and can be accessed before supplying that
         * answer, as it may have been deleted since the index was
         * loaded!
         */
        if (!is_pack_valid(p))
                return 0;
        e->offset = offset;
        e->p = p;
        hashcpy(e->sha1, sha1);
        return 1;
}

/*
 * Iff a pack file contains the object named by sha1, return true and
 * store its location to e.
 */
int find_pack_entry(const unsigned char *sha1, struct pack_entry *e)
{
        struct mru_entry *p;

        prepare_packed_git();
        if (!packed_git)
                return 0;

        for (p = packed_git_mru.head; p; p = p->next) {
                if (fill_pack_entry(sha1, e, p->item)) {
                        mru_mark(&packed_git_mru, p);
                        return 1;
                }
        }
        return 0;
}

int has_sha1_pack(const unsigned char *sha1)
{
        struct pack_entry e;
        return find_pack_entry(sha1, &e);
}

int has_pack_index(const unsigned char *sha1)
{
        struct stat st;
        if (stat(sha1_pack_index_name(sha1), &st))
                return 0;
        return 1;
}

static int for_each_object_in_pack(struct packed_git *p, each_packed_object_fn cb, void *data)
{
        uint32_t i;
        int r = 0;

        for (i = 0; i < p->num_objects; i++) {
                struct object_id oid;

                if (!nth_packed_object_oid(&oid, p, i))
                        return error("unable to get sha1 of object %u in %s",
                                     i, p->pack_name);

                r = cb(&oid, p, i, data);
                if (r)
                        break;
        }
        return r;
}

int for_each_packed_object(each_packed_object_fn cb, void *data, unsigned flags)
{
        struct packed_git *p;
        int r = 0;
        int pack_errors = 0;

        prepare_packed_git();
        for (p = packed_git; p; p = p->next) {
                if ((flags & FOR_EACH_OBJECT_LOCAL_ONLY) && !p->pack_local)
                        continue;
                if ((flags & FOR_EACH_OBJECT_PROMISOR_ONLY) &&
                    !p->pack_promisor)
                        continue;
                if (open_pack_index(p)) {
                        pack_errors = 1;
                        continue;
                }
                r = for_each_object_in_pack(p, cb, data);
                if (r)
                        break;
        }
        return r ? r : pack_errors;
}

static int add_promisor_object(const struct object_id *oid,
                               struct packed_git *pack,
                               uint32_t pos,
                               void *set_)
{
        struct oidset *set = set_;
        struct object *obj = parse_object(oid);
        if (!obj)
                return 1;

        oidset_insert(set, oid);

        /*
         * If this is a tree, commit, or tag, the objects it refers
         * to are also promisor objects. (Blobs refer to no objects.)
         */
        if (obj->type == OBJ_TREE) {
                struct tree *tree = (struct tree *)obj;
                struct tree_desc desc;
                struct name_entry entry;
                if (init_tree_desc_gently(&desc, tree->buffer, tree->size))
                        /*
                         * Error messages are given when packs are
                         * verified, so do not print any here.
                         */
                        return 0;
                while (tree_entry_gently(&desc, &entry))
                        oidset_insert(set, entry.oid);
        } else if (obj->type == OBJ_COMMIT) {
                struct commit *commit = (struct commit *) obj;
                struct commit_list *parents = commit->parents;

                oidset_insert(set, &commit->tree->object.oid);
                for (; parents; parents = parents->next)
                        oidset_insert(set, &parents->item->object.oid);
        } else if (obj->type == OBJ_TAG) {
                struct tag *tag = (struct tag *) obj;
                oidset_insert(set, &tag->tagged->oid);
        }
        return 0;
}

int is_promisor_object(const struct object_id *oid)
{
        static struct oidset promisor_objects;
        static int promisor_objects_prepared;

        if (!promisor_objects_prepared) {
                if (repository_format_partial_clone) {
                        for_each_packed_object(add_promisor_object,
                                               &promisor_objects,
                                               FOR_EACH_OBJECT_PROMISOR_ONLY);
                }
                promisor_objects_prepared = 1;
        }
        return oidset_contains(&promisor_objects, oid);
}