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[git/trast.git] / sha1_file.c

/*
 * GIT - The information manager from hell
 *
 * Copyright (C) Linus Torvalds, 2005
 *
 * This handles basic git sha1 object files - packing, unpacking,
 * creation etc.
 */
#include "cache.h"
#include "delta.h"
#include "pack.h"
#include "blob.h"
#include "commit.h"
#include "tag.h"
#include "tree.h"
#include "refs.h"
#include "pack-revindex.h"
#include "sha1-lookup.h"

#ifndef O_NOATIME
#if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
#define O_NOATIME 01000000
#else
#define O_NOATIME 0
#endif
#endif

#ifdef NO_C99_FORMAT
#define SZ_FMT "lu"
static unsigned long sz_fmt(size_t s) { return (unsigned long)s; }
#else
#define SZ_FMT "zu"
static size_t sz_fmt(size_t s) { return s; }
#endif

const unsigned char null_sha1[20];

const signed char hexval_table[256] = {
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 00-07 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 08-0f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 10-17 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 18-1f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 20-27 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 28-2f */
          0,  1,  2,  3,  4,  5,  6,  7,                /* 30-37 */
          8,  9, -1, -1, -1, -1, -1, -1,                /* 38-3f */
         -1, 10, 11, 12, 13, 14, 15, -1,                /* 40-47 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 48-4f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 50-57 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 58-5f */
         -1, 10, 11, 12, 13, 14, 15, -1,                /* 60-67 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 68-67 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 70-77 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 78-7f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 80-87 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 88-8f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 90-97 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* 98-9f */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* a0-a7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* a8-af */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* b0-b7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* b8-bf */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* c0-c7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* c8-cf */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* d0-d7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* d8-df */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* e0-e7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* e8-ef */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* f0-f7 */
         -1, -1, -1, -1, -1, -1, -1, -1,                /* f8-ff */
};

int get_sha1_hex(const char *hex, unsigned char *sha1)
{
        int i;
        for (i = 0; i < 20; i++) {
                unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
                if (val & ~0xff)
                        return -1;
                *sha1++ = val;
                hex += 2;
        }
        return 0;
}

static inline int offset_1st_component(const char *path)
{
        if (has_dos_drive_prefix(path))
                return 2 + (path[2] == '/');
        return *path == '/';
}

int safe_create_leading_directories(char *path)
{
        char *pos = path + offset_1st_component(path);
        struct stat st;

        while (pos) {
                pos = strchr(pos, '/');
                if (!pos)
                        break;
                *pos = 0;
                if (!stat(path, &st)) {
                        /* path exists */
                        if (!S_ISDIR(st.st_mode)) {
                                *pos = '/';
                                return -3;
                        }
                }
                else if (mkdir(path, 0777)) {
                        *pos = '/';
                        return -1;
                }
                else if (adjust_shared_perm(path)) {
                        *pos = '/';
                        return -2;
                }
                *pos++ = '/';
        }
        return 0;
}

int safe_create_leading_directories_const(const char *path)
{
        /* path points to cache entries, so xstrdup before messing with it */
        char *buf = xstrdup(path);
        int result = safe_create_leading_directories(buf);
        free(buf);
        return result;
}

char *sha1_to_hex(const unsigned char *sha1)
{
        static int bufno;
        static char hexbuffer[4][50];
        static const char hex[] = "0123456789abcdef";
        char *buffer = hexbuffer[3 & ++bufno], *buf = buffer;
        int i;

        for (i = 0; i < 20; i++) {
                unsigned int val = *sha1++;
                *buf++ = hex[val >> 4];
                *buf++ = hex[val & 0xf];
        }
        *buf = '\0';

        return buffer;
}

static void fill_sha1_path(char *pathbuf, const unsigned char *sha1)
{
        int i;
        for (i = 0; i < 20; i++) {
                static char hex[] = "0123456789abcdef";
                unsigned int val = sha1[i];
                char *pos = pathbuf + i*2 + (i > 0);
                *pos++ = hex[val >> 4];
                *pos = hex[val & 0xf];
        }
}

/*
 * NOTE! This returns a statically allocated buffer, so you have to be
 * careful about using it. Do an "xstrdup()" if you need to save the
 * filename.
 *
 * Also note that this returns the location for creating.  Reading
 * SHA1 file can happen from any alternate directory listed in the
 * DB_ENVIRONMENT environment variable if it is not found in
 * the primary object database.
 */
char *sha1_file_name(const unsigned char *sha1)
{
        static char *name, *base;

        if (!base) {
                const char *sha1_file_directory = get_object_directory();
                int len = strlen(sha1_file_directory);
                base = xmalloc(len + 60);
                memcpy(base, sha1_file_directory, len);
                memset(base+len, 0, 60);
                base[len] = '/';
                base[len+3] = '/';
                name = base + len + 1;
        }
        fill_sha1_path(name, sha1);
        return base;
}

static char *sha1_get_pack_name(const unsigned char *sha1,
                                char **name, char **base, const char *which)
{
        static const char hex[] = "0123456789abcdef";
        char *buf;
        int i;

        if (!*base) {
                const char *sha1_file_directory = get_object_directory();
                int len = strlen(sha1_file_directory);
                *base = xmalloc(len + 60);
                sprintf(*base, "%s/pack/pack-1234567890123456789012345678901234567890.%s",
                        sha1_file_directory, which);
                *name = *base + len + 11;
        }

        buf = *name;

        for (i = 0; i < 20; i++) {
                unsigned int val = *sha1++;
                *buf++ = hex[val >> 4];
                *buf++ = hex[val & 0xf];
        }

        return *base;
}

char *sha1_pack_name(const unsigned char *sha1)
{
        static char *name, *base;

        return sha1_get_pack_name(sha1, &name, &base, "pack");
}

char *sha1_pack_index_name(const unsigned char *sha1)
{
        static char *name, *base;

        return sha1_get_pack_name(sha1, &name, &base, "idx");
}

struct alternate_object_database *alt_odb_list;
static struct alternate_object_database **alt_odb_tail;

static void read_info_alternates(const char * alternates, int depth);

/*
 * Prepare alternate object database registry.
 *
 * The variable alt_odb_list points at the list of struct
 * alternate_object_database.  The elements on this list come from
 * non-empty elements from colon separated ALTERNATE_DB_ENVIRONMENT
 * environment variable, and $GIT_OBJECT_DIRECTORY/info/alternates,
 * whose contents is similar to that environment variable but can be
 * LF separated.  Its base points at a statically allocated buffer that
 * contains "/the/directory/corresponding/to/.git/objects/...", while
 * its name points just after the slash at the end of ".git/objects/"
 * in the example above, and has enough space to hold 40-byte hex
 * SHA1, an extra slash for the first level indirection, and the
 * terminating NUL.
 */
static int link_alt_odb_entry(const char * entry, int len, const char * relative_base, int depth)
{
        struct stat st;
        const char *objdir = get_object_directory();
        struct alternate_object_database *ent;
        struct alternate_object_database *alt;
        /* 43 = 40-byte + 2 '/' + terminating NUL */
        int pfxlen = len;
        int entlen = pfxlen + 43;
        int base_len = -1;

        if (!is_absolute_path(entry) && relative_base) {
                /* Relative alt-odb */
                if (base_len < 0)
                        base_len = strlen(relative_base) + 1;
                entlen += base_len;
                pfxlen += base_len;
        }
        ent = xmalloc(sizeof(*ent) + entlen);

        if (!is_absolute_path(entry) && relative_base) {
                memcpy(ent->base, relative_base, base_len - 1);
                ent->base[base_len - 1] = '/';
                memcpy(ent->base + base_len, entry, len);
        }
        else
                memcpy(ent->base, entry, pfxlen);

        ent->name = ent->base + pfxlen + 1;
        ent->base[pfxlen + 3] = '/';
        ent->base[pfxlen] = ent->base[entlen-1] = 0;

        /* Detect cases where alternate disappeared */
        if (stat(ent->base, &st) || !S_ISDIR(st.st_mode)) {
                error("object directory %s does not exist; "
                      "check .git/objects/info/alternates.",
                      ent->base);
                free(ent);
                return -1;
        }

        /* Prevent the common mistake of listing the same
         * thing twice, or object directory itself.
         */
        for (alt = alt_odb_list; alt; alt = alt->next) {
                if (!memcmp(ent->base, alt->base, pfxlen)) {
                        free(ent);
                        return -1;
                }
        }
        if (!memcmp(ent->base, objdir, pfxlen)) {
                free(ent);
                return -1;
        }

        /* add the alternate entry */
        *alt_odb_tail = ent;
        alt_odb_tail = &(ent->next);
        ent->next = NULL;

        /* recursively add alternates */
        read_info_alternates(ent->base, depth + 1);

        ent->base[pfxlen] = '/';

        return 0;
}

static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
                                 const char *relative_base, int depth)
{
        const char *cp, *last;

        if (depth > 5) {
                error("%s: ignoring alternate object stores, nesting too deep.",
                                relative_base);
                return;
        }

        last = alt;
        while (last < ep) {
                cp = last;
                if (cp < ep && *cp == '#') {
                        while (cp < ep && *cp != sep)
                                cp++;
                        last = cp + 1;
                        continue;
                }
                while (cp < ep && *cp != sep)
                        cp++;
                if (last != cp) {
                        if (!is_absolute_path(last) && depth) {
                                error("%s: ignoring relative alternate object store %s",
                                                relative_base, last);
                        } else {
                                link_alt_odb_entry(last, cp - last,
                                                relative_base, depth);
                        }
                }
                while (cp < ep && *cp == sep)
                        cp++;
                last = cp;
        }
}

static void read_info_alternates(const char * relative_base, int depth)
{
        char *map;
        size_t mapsz;
        struct stat st;
        const char alt_file_name[] = "info/alternates";
        /* Given that relative_base is no longer than PATH_MAX,
           ensure that "path" has enough space to append "/", the
           file name, "info/alternates", and a trailing NUL.  */
        char path[PATH_MAX + 1 + sizeof alt_file_name];
        int fd;

        sprintf(path, "%s/%s", relative_base, alt_file_name);
        fd = open(path, O_RDONLY);
        if (fd < 0)
                return;
        if (fstat(fd, &st) || (st.st_size == 0)) {
                close(fd);
                return;
        }
        mapsz = xsize_t(st.st_size);
        map = xmmap(NULL, mapsz, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        link_alt_odb_entries(map, map + mapsz, '\n', relative_base, depth);

        munmap(map, mapsz);
}

void add_to_alternates_file(const char *reference)
{
        struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
        int fd = hold_lock_file_for_append(lock, git_path("objects/info/alternates"), 1);
        char *alt = mkpath("%s/objects\n", reference);
        write_or_die(fd, alt, strlen(alt));
        if (commit_lock_file(lock))
                die("could not close alternates file");
        if (alt_odb_tail)
                link_alt_odb_entries(alt, alt + strlen(alt), '\n', NULL, 0);
}

void prepare_alt_odb(void)
{
        const char *alt;

        if (alt_odb_tail)
                return;

        alt = getenv(ALTERNATE_DB_ENVIRONMENT);
        if (!alt) alt = "";

        alt_odb_tail = &alt_odb_list;
        link_alt_odb_entries(alt, alt + strlen(alt), PATH_SEP, NULL, 0);

        read_info_alternates(get_object_directory(), 0);
}

static int has_loose_object(const unsigned char *sha1)
{
        char *name = sha1_file_name(sha1);
        struct alternate_object_database *alt;

        if (!access(name, F_OK))
                return 1;
        prepare_alt_odb();
        for (alt = alt_odb_list; alt; alt = alt->next) {
                name = alt->name;
                fill_sha1_path(name, sha1);
                if (!access(alt->base, F_OK))
                        return 1;
        }
        return 0;
}

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 size_t peak_pack_mapped;
static size_t pack_mapped;
struct packed_git *packed_git;

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));
}

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 = open(path, O_RDONLY);
        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;
        int ret;

        if (p->index_data)
                return 0;

        idx_name = xstrdup(p->pack_name);
        strcpy(idx_name + strlen(idx_name) - strlen(".pack"), ".idx");
        ret = check_packed_git_idx(idx_name, p);
        free(idx_name);
        return ret;
}

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, int keep_fd)
{
        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;
                        if (!lru_p->windows && lru_p->pack_fd != keep_fd) {
                                close(lru_p->pack_fd);
                                lru_p->pack_fd = -1;
                        }
                }
                free(lru_w);
                pack_open_windows--;
                return 1;
        }
        return 0;
}

void release_pack_memory(size_t need, int fd)
{
        size_t cur = pack_mapped;
        while (need >= (cur - pack_mapped) && unuse_one_window(NULL, fd))
                ; /* 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);
        }
}

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

/*
 * 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;

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

        p->pack_fd = open(p->pack_name, O_RDONLY);
        if (p->pack_fd < 0 || fstat(p->pack_fd, &st))
                return -1;

        /* 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. */
        if (read_in_full(p->pack_fd, &hdr, sizeof(hdr)) != 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);
        if (read_in_full(p->pack_fd, sha1, sizeof(sha1)) != 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;
        if (p->pack_fd != -1) {
                close(p->pack_fd);
                p->pack_fd = -1;
        }
        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 int *left)
{
        struct pack_window *win = *w_cursor;

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

        /* Since packfiles end in a hash of their content and its
         * 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 (offset > (p->pack_size - 20))
                die("offset beyond end of packfile (truncated pack?)");

        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;
                        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, p->pack_fd))
                                ; /* nothing */
                        win->base = xmmap(NULL, win->len,
                                PROT_READ, MAP_PRIVATE,
                                p->pack_fd, win->offset);
                        if (win->base == MAP_FAILED)
                                die("packfile %s cannot be mapped: %s",
                                        p->pack_name,
                                        strerror(errno));
                        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;
}

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

struct packed_git *add_packed_git(const char *path, int path_len, int local)
{
        struct stat st;
        struct packed_git *p = alloc_packed_git(path_len + 2);

        /*
         * Make sure a corresponding .pack file exists and that
         * the index looks sane.
         */
        path_len -= strlen(".idx");
        if (path_len < 1) {
                free(p);
                return NULL;
        }
        memcpy(p->pack_name, path, path_len);
        strcpy(p->pack_name + 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;
}

struct packed_git *parse_pack_index(unsigned char *sha1)
{
        const char *idx_path = sha1_pack_index_name(sha1);
        const char *path = sha1_pack_name(sha1);
        struct packed_git *p = alloc_packed_git(strlen(path) + 1);

        strcpy(p->pack_name, path);
        hashcpy(p->sha1, sha1);
        if (check_packed_git_idx(idx_path, p)) {
                free(p);
                return NULL;
        }

        return p;
}

void install_packed_git(struct packed_git *pack)
{
        pack->next = packed_git;
        packed_git = pack;
}

static void prepare_packed_git_one(char *objdir, int local)
{
        /* Ensure that this buffer is large enough so that we can
           append "/pack/" without clobbering the stack even if
           strlen(objdir) were PATH_MAX.  */
        char path[PATH_MAX + 1 + 4 + 1 + 1];
        int len;
        DIR *dir;
        struct dirent *de;

        sprintf(path, "%s/pack", objdir);
        len = strlen(path);
        dir = opendir(path);
        if (!dir) {
                if (errno != ENOENT)
                        error("unable to open object pack directory: %s: %s",
                              path, strerror(errno));
                return;
        }
        path[len++] = '/';
        while ((de = readdir(dir)) != NULL) {
                int namelen = strlen(de->d_name);
                struct packed_git *p;

                if (!has_extension(de->d_name, ".idx"))
                        continue;

                if (len + namelen + 1 > sizeof(path))
                        continue;

                /* Don't reopen a pack we already have. */
                strcpy(path + len, de->d_name);
                for (p = packed_git; p; p = p->next) {
                        if (!memcmp(path, p->pack_name, len + namelen - 4))
                                break;
                }
                if (p)
                        continue;
                /* See if it really is a valid .idx file with corresponding
                 * .pack file that we can map.
                 */
                p = add_packed_git(path, len + namelen, local);
                if (!p)
                        continue;
                install_packed_git(p);
        }
        closedir(dir);
}

static int sort_pack(const void *a_, const void *b_)
{
        struct packed_git *a = *((struct packed_git **)a_);
        struct packed_git *b = *((struct packed_git **)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)
{
        struct packed_git **ary, *p;
        int i, n;

        for (n = 0, p = packed_git; p; p = p->next)
                n++;
        if (n < 2)
                return;

        /* prepare an array of packed_git for easier sorting */
        ary = xcalloc(n, sizeof(struct packed_git *));
        for (n = 0, p = packed_git; p; p = p->next)
                ary[n++] = p;

        qsort(ary, n, sizeof(struct packed_git *), sort_pack);

        /* link them back again */
        for (i = 0; i < n - 1; i++)
                ary[i]->next = ary[i + 1];
        ary[n - 1]->next = NULL;
        packed_git = ary[0];

        free(ary);
}

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) {
                alt->name[-1] = 0;
                prepare_packed_git_one(alt->base, 0);
                alt->name[-1] = '/';
        }
        rearrange_packed_git();
        prepare_packed_git_run_once = 1;
}

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

static 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 + 20 * i))
                        return;
        p->bad_object_sha1 = xrealloc(p->bad_object_sha1, 20 * (p->num_bad_objects + 1));
        hashcpy(p->bad_object_sha1 + 20 * p->num_bad_objects, sha1);
        p->num_bad_objects++;
}

static int 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 1;
        return 0;
}

int check_sha1_signature(const unsigned char *sha1, void *map, unsigned long size, const char *type)
{
        unsigned char real_sha1[20];
        hash_sha1_file(map, size, type, real_sha1);
        return hashcmp(sha1, real_sha1) ? -1 : 0;
}

static int git_open_noatime(const char *name)
{
        static int sha1_file_open_flag = O_NOATIME;
        int fd = open(name, O_RDONLY | sha1_file_open_flag);

        /* Might the failure be due to O_NOATIME? */
        if (fd < 0 && errno != ENOENT && sha1_file_open_flag) {
                fd = open(name, O_RDONLY);
                if (fd >= 0)
                        sha1_file_open_flag = 0;
        }
        return fd;
}

static int open_sha1_file(const unsigned char *sha1)
{
        int fd;
        char *name = sha1_file_name(sha1);
        struct alternate_object_database *alt;

        fd = git_open_noatime(name);
        if (fd >= 0)
                return fd;

        prepare_alt_odb();
        errno = ENOENT;
        for (alt = alt_odb_list; alt; alt = alt->next) {
                name = alt->name;
                fill_sha1_path(name, sha1);
                fd = git_open_noatime(alt->base);
                if (fd >= 0)
                        return fd;
        }
        return -1;
}

static void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
{
        void *map;
        int fd;

        fd = open_sha1_file(sha1);
        map = NULL;
        if (fd >= 0) {
                struct stat st;

                if (!fstat(fd, &st)) {
                        *size = xsize_t(st.st_size);
                        map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
                }
                close(fd);
        }
        return map;
}

static int legacy_loose_object(unsigned char *map)
{
        unsigned int word;

        /*
         * Is it a zlib-compressed buffer? If so, the first byte
         * must be 0x78 (15-bit window size, deflated), and the
         * first 16-bit word is evenly divisible by 31
         */
        word = (map[0] << 8) + map[1];
        if (map[0] == 0x78 && !(word % 31))
                return 1;
        else
                return 0;
}

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

        c = buf[used++];
        *type = (c >> 4) & 7;
        size = c & 15;
        shift = 4;
        while (c & 0x80) {
                if (len <= used)
                        return 0;
                if (sizeof(long) * 8 <= shift)
                        return 0;
                c = buf[used++];
                size += (c & 0x7f) << shift;
                shift += 7;
        }
        *sizep = size;
        return used;
}

static int unpack_sha1_header(z_stream *stream, unsigned char *map, unsigned long mapsize, void *buffer, unsigned long bufsiz)
{
        unsigned long size, used;
        static const char valid_loose_object_type[8] = {
                0, /* OBJ_EXT */
                1, 1, 1, 1, /* "commit", "tree", "blob", "tag" */
                0, /* "delta" and others are invalid in a loose object */
        };
        enum object_type type;

        /* Get the data stream */
        memset(stream, 0, sizeof(*stream));
        stream->next_in = map;
        stream->avail_in = mapsize;
        stream->next_out = buffer;
        stream->avail_out = bufsiz;

        if (legacy_loose_object(map)) {
                inflateInit(stream);
                return inflate(stream, 0);
        }


        /*
         * There used to be a second loose object header format which
         * was meant to mimic the in-pack format, allowing for direct
         * copy of the object data.  This format turned up not to be
         * really worth it and we don't write it any longer.  But we
         * can still read it.
         */
        used = unpack_object_header_gently(map, mapsize, &type, &size);
        if (!used || !valid_loose_object_type[type])
                return -1;
        map += used;
        mapsize -= used;

        /* Set up the stream for the rest.. */
        stream->next_in = map;
        stream->avail_in = mapsize;
        inflateInit(stream);

        /* And generate the fake traditional header */
        stream->total_out = 1 + snprintf(buffer, bufsiz, "%s %lu",
                                         typename(type), size);
        return 0;
}

static void *unpack_sha1_rest(z_stream *stream, void *buffer, unsigned long size, const unsigned char *sha1)
{
        int bytes = strlen(buffer) + 1;
        unsigned char *buf = xmalloc(1+size);
        unsigned long n;
        int status = Z_OK;

        n = stream->total_out - bytes;
        if (n > size)
                n = size;
        memcpy(buf, (char *) buffer + bytes, n);
        bytes = n;
        if (bytes <= size) {
                /*
                 * The above condition must be (bytes <= size), not
                 * (bytes < size).  In other words, even though we
                 * expect no more output and set avail_out to zer0,
                 * the input zlib stream may have bytes that express
                 * "this concludes the stream", and we *do* want to
                 * eat that input.
                 *
                 * Otherwise we would not be able to test that we
                 * consumed all the input to reach the expected size;
                 * we also want to check that zlib tells us that all
                 * went well with status == Z_STREAM_END at the end.
                 */
                stream->next_out = buf + bytes;
                stream->avail_out = size - bytes;
                while (status == Z_OK)
                        status = inflate(stream, Z_FINISH);
        }
        buf[size] = 0;
        if (status == Z_STREAM_END && !stream->avail_in) {
                inflateEnd(stream);
                return buf;
        }

        if (status < 0)
                error("corrupt loose object '%s'", sha1_to_hex(sha1));
        else if (stream->avail_in)
                error("garbage at end of loose object '%s'",
                      sha1_to_hex(sha1));
        free(buf);
        return NULL;
}

/*
 * We used to just use "sscanf()", but that's actually way
 * too permissive for what we want to check. So do an anal
 * object header parse by hand.
 */
static int parse_sha1_header(const char *hdr, unsigned long *sizep)
{
        char type[10];
        int i;
        unsigned long size;

        /*
         * The type can be at most ten bytes (including the
         * terminating '\0' that we add), and is followed by
         * a space.
         */
        i = 0;
        for (;;) {
                char c = *hdr++;
                if (c == ' ')
                        break;
                type[i++] = c;
                if (i >= sizeof(type))
                        return -1;
        }
        type[i] = 0;

        /*
         * The length must follow immediately, and be in canonical
         * decimal format (ie "010" is not valid).
         */
        size = *hdr++ - '0';
        if (size > 9)
                return -1;
        if (size) {
                for (;;) {
                        unsigned long c = *hdr - '0';
                        if (c > 9)
                                break;
                        hdr++;
                        size = size * 10 + c;
                }
        }
        *sizep = size;

        /*
         * The length must be followed by a zero byte
         */
        return *hdr ? -1 : type_from_string(type);
}

static void *unpack_sha1_file(void *map, unsigned long mapsize, enum object_type *type, unsigned long *size, const unsigned char *sha1)
{
        int ret;
        z_stream stream;
        char hdr[8192];

        ret = unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr));
        if (ret < Z_OK || (*type = parse_sha1_header(hdr, size)) < 0)
                return NULL;

        return unpack_sha1_rest(&stream, hdr, *size, sha1);
}

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;
        z_stream stream;
        int st;

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

        inflateInit(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = inflate(&stream, Z_FINISH);
                curpos += stream.next_in - in;
        } while ((st == Z_OK || st == Z_BUF_ERROR) &&
                 stream.total_out < sizeof(delta_head));
        inflateEnd(&stream);
        if ((st != Z_STREAM_END) && stream.total_out != sizeof(delta_head))
                die("delta data unpack-initial failed");

        /* 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));
}

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 >= 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;
}

/* forward declaration for a mutually recursive function */
static int packed_object_info(struct packed_git *p, off_t offset,
                              unsigned long *sizep);

static int packed_delta_info(struct packed_git *p,
                             struct pack_window **w_curs,
                             off_t curpos,
                             enum object_type type,
                             off_t obj_offset,
                             unsigned long *sizep)
{
        off_t base_offset;

        base_offset = get_delta_base(p, w_curs, &curpos, type, obj_offset);
        type = packed_object_info(p, base_offset, NULL);

        /* We choose to only get the type of the base object and
         * ignore potentially corrupt pack file that expects the delta
         * based on a base with a wrong size.  This saves tons of
         * inflate() calls.
         */
        if (sizep)
                *sizep = get_size_from_delta(p, w_curs, curpos);

        return type;
}

static int unpack_object_header(struct packed_git *p,
                                struct pack_window **w_curs,
                                off_t *curpos,
                                unsigned long *sizep)
{
        unsigned char *base;
        unsigned int 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 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_gently(base, left, &type, sizep);
        if (!used)
                die("object offset outside of pack file");
        *curpos += used;

        return type;
}

const char *packed_object_info_detail(struct packed_git *p,
                                      off_t obj_offset,
                                      unsigned long *size,
                                      unsigned long *store_size,
                                      unsigned int *delta_chain_length,
                                      unsigned char *base_sha1)
{
        struct pack_window *w_curs = NULL;
        off_t curpos;
        unsigned long dummy;
        unsigned char *next_sha1;
        enum object_type type;
        struct revindex_entry *revidx;

        *delta_chain_length = 0;
        curpos = obj_offset;
        type = unpack_object_header(p, &w_curs, &curpos, size);

        revidx = find_pack_revindex(p, obj_offset);
        *store_size = revidx[1].offset - obj_offset;

        for (;;) {
                switch (type) {
                default:
                        die("pack %s contains unknown object type %d",
                            p->pack_name, type);
                case OBJ_COMMIT:
                case OBJ_TREE:
                case OBJ_BLOB:
                case OBJ_TAG:
                        unuse_pack(&w_curs);
                        return typename(type);
                case OBJ_OFS_DELTA:
                        obj_offset = get_delta_base(p, &w_curs, &curpos, type, obj_offset);
                        if (!obj_offset)
                                die("pack %s contains bad delta base reference of type %s",
                                    p->pack_name, typename(type));
                        if (*delta_chain_length == 0) {
                                revidx = find_pack_revindex(p, obj_offset);
                                hashcpy(base_sha1, nth_packed_object_sha1(p, revidx->nr));
                        }
                        break;
                case OBJ_REF_DELTA:
                        next_sha1 = use_pack(p, &w_curs, curpos, NULL);
                        if (*delta_chain_length == 0)
                                hashcpy(base_sha1, next_sha1);
                        obj_offset = find_pack_entry_one(next_sha1, p);
                        break;
                }
                (*delta_chain_length)++;
                curpos = obj_offset;
                type = unpack_object_header(p, &w_curs, &curpos, &dummy);
        }
}

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

        type = unpack_object_header(p, &w_curs, &curpos, &size);

        switch (type) {
        case OBJ_OFS_DELTA:
        case OBJ_REF_DELTA:
                type = packed_delta_info(p, &w_curs, curpos,
                                         type, obj_offset, sizep);
                break;
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                if (sizep)
                        *sizep = size;
                break;
        default:
                die("pack %s contains unknown object type %d",
                    p->pack_name, type);
        }
        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;
        z_stream stream;
        unsigned char *buffer, *in;

        buffer = xmalloc(size + 1);
        buffer[size] = 0;
        memset(&stream, 0, sizeof(stream));
        stream.next_out = buffer;
        stream.avail_out = size;

        inflateInit(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = inflate(&stream, Z_FINISH);
                curpos += stream.next_in - in;
        } while (st == Z_OK || st == Z_BUF_ERROR);
        inflateEnd(&stream);
        if ((st != Z_STREAM_END) || stream.total_out != size) {
                free(buffer);
                return NULL;
        }

        return buffer;
}

#define MAX_DELTA_CACHE (256)

static size_t delta_base_cached;

static struct delta_base_cache_lru_list {
        struct delta_base_cache_lru_list *prev;
        struct delta_base_cache_lru_list *next;
} delta_base_cache_lru = { &delta_base_cache_lru, &delta_base_cache_lru };

static struct delta_base_cache_entry {
        struct delta_base_cache_lru_list lru;
        void *data;
        struct packed_git *p;
        off_t base_offset;
        unsigned long size;
        enum object_type type;
} delta_base_cache[MAX_DELTA_CACHE];

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

        hash = (unsigned long)p + (unsigned long)base_offset;
        hash += (hash >> 8) + (hash >> 16);
        return hash % MAX_DELTA_CACHE;
}

static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
        unsigned long *base_size, enum object_type *type, int keep_cache)
{
        void *ret;
        unsigned long hash = pack_entry_hash(p, base_offset);
        struct delta_base_cache_entry *ent = delta_base_cache + hash;

        ret = ent->data;
        if (ret && ent->p == p && ent->base_offset == base_offset)
                goto found_cache_entry;
        return unpack_entry(p, base_offset, type, base_size);

found_cache_entry:
        if (!keep_cache) {
                ent->data = NULL;
                ent->lru.next->prev = ent->lru.prev;
                ent->lru.prev->next = ent->lru.next;
                delta_base_cached -= ent->size;
        } else {
                ret = xmemdupz(ent->data, ent->size);
        }
        *type = ent->type;
        *base_size = ent->size;
        return ret;
}

static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
{
        if (ent->data) {
                free(ent->data);
                ent->data = NULL;
                ent->lru.next->prev = ent->lru.prev;
                ent->lru.prev->next = ent->lru.next;
                delta_base_cached -= ent->size;
        }
}

static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
        void *base, unsigned long base_size, enum object_type type)
{
        unsigned long hash = pack_entry_hash(p, base_offset);
        struct delta_base_cache_entry *ent = delta_base_cache + hash;
        struct delta_base_cache_lru_list *lru;

        release_delta_base_cache(ent);
        delta_base_cached += base_size;

        for (lru = delta_base_cache_lru.next;
             delta_base_cached > delta_base_cache_limit
             && lru != &delta_base_cache_lru;
             lru = lru->next) {
                struct delta_base_cache_entry *f = (void *)lru;
                if (f->type == OBJ_BLOB)
                        release_delta_base_cache(f);
        }
        for (lru = delta_base_cache_lru.next;
             delta_base_cached > delta_base_cache_limit
             && lru != &delta_base_cache_lru;
             lru = lru->next) {
                struct delta_base_cache_entry *f = (void *)lru;
                release_delta_base_cache(f);
        }

        ent->p = p;
        ent->base_offset = base_offset;
        ent->type = type;
        ent->data = base;
        ent->size = base_size;
        ent->lru.next = &delta_base_cache_lru;
        ent->lru.prev = delta_base_cache_lru.prev;
        delta_base_cache_lru.prev->next = &ent->lru;
        delta_base_cache_lru.prev = &ent->lru;
}

static void *unpack_delta_entry(struct packed_git *p,
                                struct pack_window **w_curs,
                                off_t curpos,
                                unsigned long delta_size,
                                off_t obj_offset,
                                enum object_type *type,
                                unsigned long *sizep)
{
        void *delta_data, *result, *base;
        unsigned long base_size;
        off_t base_offset;

        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);
                return NULL;
        }
        unuse_pack(w_curs);
        base = cache_or_unpack_entry(p, base_offset, &base_size, type, 0);
        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 = find_pack_revindex(p, base_offset);
                const unsigned char *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)base_offset,
                      p->pack_name);
                mark_bad_packed_object(p, base_sha1);
                base = read_object(base_sha1, type, &base_size);
                if (!base)
                        return NULL;
        }

        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);
                free(base);
                return NULL;
        }
        result = patch_delta(base, base_size,
                             delta_data, delta_size,
                             sizep);
        if (!result)
                die("failed to apply delta");
        free(delta_data);
        add_delta_base_cache(p, base_offset, base, base_size, *type);
        return result;
}

void *unpack_entry(struct packed_git *p, off_t obj_offset,
                   enum object_type *type, unsigned long *sizep)
{
        struct pack_window *w_curs = NULL;
        off_t curpos = obj_offset;
        void *data;

        *type = unpack_object_header(p, &w_curs, &curpos, sizep);
        switch (*type) {
        case OBJ_OFS_DELTA:
        case OBJ_REF_DELTA:
                data = unpack_delta_entry(p, &w_curs, curpos, *sizep,
                                          obj_offset, type, sizep);
                break;
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                data = unpack_compressed_entry(p, &w_curs, curpos, *sizep);
                break;
        default:
                data = NULL;
                error("unknown object type %i at offset %"PRIuMAX" in %s",
                      *type, (uintmax_t)obj_offset, p->pack_name);
        }
        unuse_pack(&w_curs);
        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;
        }
}

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;
                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 use_lookup = -1;
        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);

        if (use_lookup < 0)
                use_lookup = !!getenv("GIT_USE_LOOKUP");
        if (use_lookup) {
                int pos = sha1_entry_pos(index, stride, 0,
                                         lo, hi, p->num_objects, sha1);
                if (pos < 0)
                        return 0;
                return nth_packed_object_offset(p, pos);
        }

        do {
                unsigned mi = (lo + hi) / 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;
        } while (lo < hi);
        return 0;
}

int matches_pack_name(struct packed_git *p, const char *name)
{
        const char *last_c, *c;

        if (!strcmp(p->pack_name, name))
                return 1;

        for (c = p->pack_name, last_c = c; *c;)
                if (*c == '/')
                        last_c = ++c;
                else
                        ++c;
        if (!strcmp(last_c, name))
                return 1;

        return 0;
}

static int find_pack_entry(const unsigned char *sha1, struct pack_entry *e, const char **ignore_packed)
{
        static struct packed_git *last_found = (void *)1;
        struct packed_git *p;
        off_t offset;

        prepare_packed_git();
        if (!packed_git)
                return 0;
        p = (last_found == (void *)1) ? packed_git : last_found;

        do {
                if (ignore_packed) {
                        const char **ig;
                        for (ig = ignore_packed; *ig; ig++)
                                if (matches_pack_name(p, *ig))
                                        break;
                        if (*ig)
                                goto next;
                }

                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))
                                        goto next;
                }

                offset = find_pack_entry_one(sha1, p);
                if (offset) {
                        /*
                         * 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 (p->pack_fd == -1 && open_packed_git(p)) {
                                error("packfile %s cannot be accessed", p->pack_name);
                                goto next;
                        }
                        e->offset = offset;
                        e->p = p;
                        hashcpy(e->sha1, sha1);
                        last_found = p;
                        return 1;
                }

                next:
                if (p == last_found)
                        p = packed_git;
                else
                        p = p->next;
                if (p == last_found)
                        p = p->next;
        } while (p);
        return 0;
}

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 sha1_loose_object_info(const unsigned char *sha1, unsigned long *sizep)
{
        int status;
        unsigned long mapsize, size;
        void *map;
        z_stream stream;
        char hdr[32];

        map = map_sha1_file(sha1, &mapsize);
        if (!map)
                return error("unable to find %s", sha1_to_hex(sha1));
        if (unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr)) < 0)
                status = error("unable to unpack %s header",
                               sha1_to_hex(sha1));
        else if ((status = parse_sha1_header(hdr, &size)) < 0)
                status = error("unable to parse %s header", sha1_to_hex(sha1));
        else if (sizep)
                *sizep = size;
        inflateEnd(&stream);
        munmap(map, mapsize);
        return status;
}

int sha1_object_info(const unsigned char *sha1, unsigned long *sizep)
{
        struct pack_entry e;

        if (!find_pack_entry(sha1, &e, NULL)) {
                reprepare_packed_git();
                if (!find_pack_entry(sha1, &e, NULL))
                        return sha1_loose_object_info(sha1, sizep);
        }
        return packed_object_info(e.p, e.offset, sizep);
}

static void *read_packed_sha1(const unsigned char *sha1,
                              enum object_type *type, unsigned long *size)
{
        struct pack_entry e;
        void *data;

        if (!find_pack_entry(sha1, &e, NULL))
                return NULL;
        data = cache_or_unpack_entry(e.p, e.offset, size, type, 1);
        if (!data) {
                /*
                 * We're probably in deep shit, but let's try to fetch
                 * the required object anyway from another pack or loose.
                 * This should happen only in the presence of a corrupted
                 * pack, and is better than failing outright.
                 */
                error("failed to read object %s at offset %"PRIuMAX" from %s",
                      sha1_to_hex(sha1), (uintmax_t)e.offset, e.p->pack_name);
                mark_bad_packed_object(e.p, sha1);
                data = read_object(sha1, type, size);
        }
        return data;
}

/*
 * This is meant to hold a *small* number of objects that you would
 * want read_sha1_file() to be able to return, but yet you do not want
 * to write them into the object store (e.g. a browse-only
 * application).
 */
static struct cached_object {
        unsigned char sha1[20];
        enum object_type type;
        void *buf;
        unsigned long size;
} *cached_objects;
static int cached_object_nr, cached_object_alloc;

static struct cached_object empty_tree = {
        /* empty tree sha1: 4b825dc642cb6eb9a060e54bf8d69288fbee4904 */
        "\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60"
        "\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04",
        OBJ_TREE,
        "",
        0
};

static struct cached_object *find_cached_object(const unsigned char *sha1)
{
        int i;
        struct cached_object *co = cached_objects;

        for (i = 0; i < cached_object_nr; i++, co++) {
                if (!hashcmp(co->sha1, sha1))
                        return co;
        }
        if (!hashcmp(sha1, empty_tree.sha1))
                return &empty_tree;
        return NULL;
}

int pretend_sha1_file(void *buf, unsigned long len, enum object_type type,
                      unsigned char *sha1)
{
        struct cached_object *co;

        hash_sha1_file(buf, len, typename(type), sha1);
        if (has_sha1_file(sha1) || find_cached_object(sha1))
                return 0;
        if (cached_object_alloc <= cached_object_nr) {
                cached_object_alloc = alloc_nr(cached_object_alloc);
                cached_objects = xrealloc(cached_objects,
                                          sizeof(*cached_objects) *
                                          cached_object_alloc);
        }
        co = &cached_objects[cached_object_nr++];
        co->size = len;
        co->type = type;
        co->buf = xmalloc(len);
        memcpy(co->buf, buf, len);
        hashcpy(co->sha1, sha1);
        return 0;
}

void *read_object(const unsigned char *sha1, enum object_type *type,
                  unsigned long *size)
{
        unsigned long mapsize;
        void *map, *buf;
        struct cached_object *co;

        co = find_cached_object(sha1);
        if (co) {
                *type = co->type;
                *size = co->size;
                return xmemdupz(co->buf, co->size);
        }

        buf = read_packed_sha1(sha1, type, size);
        if (buf)
                return buf;
        map = map_sha1_file(sha1, &mapsize);
        if (map) {
                buf = unpack_sha1_file(map, mapsize, type, size, sha1);
                munmap(map, mapsize);
                return buf;
        }
        reprepare_packed_git();
        return read_packed_sha1(sha1, type, size);
}

void *read_sha1_file(const unsigned char *sha1, enum object_type *type,
                     unsigned long *size)
{
        void *data = read_object(sha1, type, size);
        /* legacy behavior is to die on corrupted objects */
        if (!data && (has_loose_object(sha1) || has_packed_and_bad(sha1)))
                die("object %s is corrupted", sha1_to_hex(sha1));
        return data;
}

void *read_object_with_reference(const unsigned char *sha1,
                                 const char *required_type_name,
                                 unsigned long *size,
                                 unsigned char *actual_sha1_return)
{
        enum object_type type, required_type;
        void *buffer;
        unsigned long isize;
        unsigned char actual_sha1[20];

        required_type = type_from_string(required_type_name);
        hashcpy(actual_sha1, sha1);
        while (1) {
                int ref_length = -1;
                const char *ref_type = NULL;

                buffer = read_sha1_file(actual_sha1, &type, &isize);
                if (!buffer)
                        return NULL;
                if (type == required_type) {
                        *size = isize;
                        if (actual_sha1_return)
                                hashcpy(actual_sha1_return, actual_sha1);
                        return buffer;
                }
                /* Handle references */
                else if (type == OBJ_COMMIT)
                        ref_type = "tree ";
                else if (type == OBJ_TAG)
                        ref_type = "object ";
                else {
                        free(buffer);
                        return NULL;
                }
                ref_length = strlen(ref_type);

                if (ref_length + 40 > isize ||
                    memcmp(buffer, ref_type, ref_length) ||
                    get_sha1_hex((char *) buffer + ref_length, actual_sha1)) {
                        free(buffer);
                        return NULL;
                }
                free(buffer);
                /* Now we have the ID of the referred-to object in
                 * actual_sha1.  Check again. */
        }
}

static void write_sha1_file_prepare(const void *buf, unsigned long len,
                                    const char *type, unsigned char *sha1,
                                    char *hdr, int *hdrlen)
{
        SHA_CTX c;

        /* Generate the header */
        *hdrlen = sprintf(hdr, "%s %lu", type, len)+1;

        /* Sha1.. */
        SHA1_Init(&c);
        SHA1_Update(&c, hdr, *hdrlen);
        SHA1_Update(&c, buf, len);
        SHA1_Final(sha1, &c);
}

/*
 * Move the just written object into its final resting place
 */
int move_temp_to_file(const char *tmpfile, const char *filename)
{
        int ret = link(tmpfile, filename);

        /*
         * Coda hack - coda doesn't like cross-directory links,
         * so we fall back to a rename, which will mean that it
         * won't be able to check collisions, but that's not a
         * big deal.
         *
         * The same holds for FAT formatted media.
         *
         * When this succeeds, we just return 0. We have nothing
         * left to unlink.
         */
        if (ret && ret != EEXIST) {
                if (!rename(tmpfile, filename))
                        return 0;
                ret = errno;
        }
        unlink(tmpfile);
        if (ret) {
                if (ret != EEXIST) {
                        return error("unable to write sha1 filename %s: %s\n", filename, strerror(ret));
                }
                /* FIXME!!! Collision check here ? */
        }

        return 0;
}

static int write_buffer(int fd, const void *buf, size_t len)
{
        if (write_in_full(fd, buf, len) < 0)
                return error("file write error (%s)", strerror(errno));
        return 0;
}

int hash_sha1_file(const void *buf, unsigned long len, const char *type,
                   unsigned char *sha1)
{
        char hdr[32];
        int hdrlen;
        write_sha1_file_prepare(buf, len, type, sha1, hdr, &hdrlen);
        return 0;
}

/* Finalize a file on disk, and close it. */
static void close_sha1_file(int fd)
{
        if (fsync_object_files)
                fsync_or_die(fd, "sha1 file");
        fchmod(fd, 0444);
        if (close(fd) != 0)
                die("unable to write sha1 file");
}

/* Size of directory component, including the ending '/' */
static inline int directory_size(const char *filename)
{
        const char *s = strrchr(filename, '/');
        if (!s)
                return 0;
        return s - filename + 1;
}

/*
 * This creates a temporary file in the same directory as the final
 * 'filename'
 *
 * We want to avoid cross-directory filename renames, because those
 * can have problems on various filesystems (FAT, NFS, Coda).
 */
static int create_tmpfile(char *buffer, size_t bufsiz, const char *filename)
{
        int fd, dirlen = directory_size(filename);

        if (dirlen + 20 > bufsiz) {
                errno = ENAMETOOLONG;
                return -1;
        }
        memcpy(buffer, filename, dirlen);
        strcpy(buffer + dirlen, "tmp_obj_XXXXXX");
        fd = mkstemp(buffer);
        if (fd < 0 && dirlen) {
                /* Make sure the directory exists */
                memcpy(buffer, filename, dirlen);
                buffer[dirlen-1] = 0;
                if (mkdir(buffer, 0777) || adjust_shared_perm(buffer))
                        return -1;

                /* Try again */
                strcpy(buffer + dirlen - 1, "/tmp_obj_XXXXXX");
                fd = mkstemp(buffer);
        }
        return fd;
}

static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
                              void *buf, unsigned long len, time_t mtime)
{
        int fd, size, ret;
        unsigned char *compressed;
        z_stream stream;
        char *filename;
        static char tmpfile[PATH_MAX];

        filename = sha1_file_name(sha1);
        fd = create_tmpfile(tmpfile, sizeof(tmpfile), filename);
        if (fd < 0) {
                if (errno == EPERM)
                        return error("insufficient permission for adding an object to repository database %s\n", get_object_directory());
                else
                        return error("unable to create temporary sha1 filename %s: %s\n", tmpfile, strerror(errno));
        }

        /* Set it up */
        memset(&stream, 0, sizeof(stream));
        deflateInit(&stream, zlib_compression_level);
        size = 8 + deflateBound(&stream, len+hdrlen);
        compressed = xmalloc(size);

        /* Compress it */
        stream.next_out = compressed;
        stream.avail_out = size;

        /* First header.. */
        stream.next_in = (unsigned char *)hdr;
        stream.avail_in = hdrlen;
        while (deflate(&stream, 0) == Z_OK)
                /* nothing */;

        /* Then the data itself.. */
        stream.next_in = buf;
        stream.avail_in = len;
        ret = deflate(&stream, Z_FINISH);
        if (ret != Z_STREAM_END)
                die("unable to deflate new object %s (%d)", sha1_to_hex(sha1), ret);

        ret = deflateEnd(&stream);
        if (ret != Z_OK)
                die("deflateEnd on object %s failed (%d)", sha1_to_hex(sha1), ret);

        size = stream.total_out;

        if (write_buffer(fd, compressed, size) < 0)
                die("unable to write sha1 file");
        close_sha1_file(fd);
        free(compressed);

        if (mtime) {
                struct utimbuf utb;
                utb.actime = mtime;
                utb.modtime = mtime;
                if (utime(tmpfile, &utb) < 0)
                        warning("failed utime() on %s: %s",
                                tmpfile, strerror(errno));
        }

        return move_temp_to_file(tmpfile, filename);
}

int write_sha1_file(void *buf, unsigned long len, const char *type, unsigned char *returnsha1)
{
        unsigned char sha1[20];
        char hdr[32];
        int hdrlen;

        /* Normally if we have it in the pack then we do not bother writing
         * it out into .git/objects/??/?{38} file.
         */
        write_sha1_file_prepare(buf, len, type, sha1, hdr, &hdrlen);
        if (returnsha1)
                hashcpy(returnsha1, sha1);
        if (has_sha1_file(sha1))
                return 0;
        return write_loose_object(sha1, hdr, hdrlen, buf, len, 0);
}

int force_object_loose(const unsigned char *sha1, time_t mtime)
{
        void *buf;
        unsigned long len;
        enum object_type type;
        char hdr[32];
        int hdrlen;

        if (has_loose_object(sha1))
                return 0;
        buf = read_packed_sha1(sha1, &type, &len);
        if (!buf)
                return error("cannot read sha1_file for %s", sha1_to_hex(sha1));
        hdrlen = sprintf(hdr, "%s %lu", typename(type), len) + 1;
        return write_loose_object(sha1, hdr, hdrlen, buf, len, mtime);
}

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

int has_pack_file(const unsigned char *sha1)
{
        struct stat st;
        if (stat(sha1_pack_name(sha1), &st))
                return 0;
        return 1;
}

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

int has_sha1_file(const unsigned char *sha1)
{
        struct pack_entry e;

        if (find_pack_entry(sha1, &e, NULL))
                return 1;
        return has_loose_object(sha1);
}

int index_pipe(unsigned char *sha1, int fd, const char *type, int write_object)
{
        struct strbuf buf;
        int ret;

        strbuf_init(&buf, 0);
        if (strbuf_read(&buf, fd, 4096) < 0) {
                strbuf_release(&buf);
                return -1;
        }

        if (!type)
                type = blob_type;
        if (write_object)
                ret = write_sha1_file(buf.buf, buf.len, type, sha1);
        else
                ret = hash_sha1_file(buf.buf, buf.len, type, sha1);
        strbuf_release(&buf);

        return ret;
}

int index_fd(unsigned char *sha1, int fd, struct stat *st, int write_object,
             enum object_type type, const char *path)
{
        size_t size = xsize_t(st->st_size);
        void *buf = NULL;
        int ret, re_allocated = 0;

        if (size)
                buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        if (!type)
                type = OBJ_BLOB;

        /*
         * Convert blobs to git internal format
         */
        if ((type == OBJ_BLOB) && S_ISREG(st->st_mode)) {
                struct strbuf nbuf;
                strbuf_init(&nbuf, 0);
                if (convert_to_git(path, buf, size, &nbuf,
                                   write_object ? safe_crlf : 0)) {
                        munmap(buf, size);
                        buf = strbuf_detach(&nbuf, &size);
                        re_allocated = 1;
                }
        }

        if (write_object)
                ret = write_sha1_file(buf, size, typename(type), sha1);
        else
                ret = hash_sha1_file(buf, size, typename(type), sha1);
        if (re_allocated) {
                free(buf);
                return ret;
        }
        if (size)
                munmap(buf, size);
        return ret;
}

int index_path(unsigned char *sha1, const char *path, struct stat *st, int write_object)
{
        int fd;
        char *target;
        size_t len;

        switch (st->st_mode & S_IFMT) {
        case S_IFREG:
                fd = open(path, O_RDONLY);
                if (fd < 0)
                        return error("open(\"%s\"): %s", path,
                                     strerror(errno));
                if (index_fd(sha1, fd, st, write_object, OBJ_BLOB, path) < 0)
                        return error("%s: failed to insert into database",
                                     path);
                break;
        case S_IFLNK:
                len = xsize_t(st->st_size);
                target = xmalloc(len + 1);
                if (readlink(path, target, len + 1) != st->st_size) {
                        char *errstr = strerror(errno);
                        free(target);
                        return error("readlink(\"%s\"): %s", path,
                                     errstr);
                }
                if (!write_object)
                        hash_sha1_file(target, len, blob_type, sha1);
                else if (write_sha1_file(target, len, blob_type, sha1))
                        return error("%s: failed to insert into database",
                                     path);
                free(target);
                break;
        case S_IFDIR:
                return resolve_gitlink_ref(path, "HEAD", sha1);
        default:
                return error("%s: unsupported file type", path);
        }
        return 0;
}

int read_pack_header(int fd, struct pack_header *header)
{
        if (read_in_full(fd, header, sizeof(*header)) < sizeof(*header))
                /* "eof before pack header was fully read" */
                return PH_ERROR_EOF;

        if (header->hdr_signature != htonl(PACK_SIGNATURE))
                /* "protocol error (pack signature mismatch detected)" */
                return PH_ERROR_PACK_SIGNATURE;
        if (!pack_version_ok(header->hdr_version))
                /* "protocol error (pack version unsupported)" */
                return PH_ERROR_PROTOCOL;
        return 0;
}