+struct alternate_object_database *alt_odb_list;
+static struct alternate_object_database **alt_odb_tail;
+
+/*
+ * 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 void link_alt_odb_entries(const char *alt, const char *ep, int sep,
+ const char *relative_base)
+{
+ const char *cp, *last;
+ struct alternate_object_database *ent;
+ const char *objdir = get_object_directory();
+ int base_len = -1;
+
+ last = alt;
+ while (last < ep) {
+ cp = last;
+ if (cp < ep && *cp == '#') {
+ while (cp < ep && *cp != sep)
+ cp++;
+ last = cp + 1;
+ continue;
+ }
+ for ( ; cp < ep && *cp != sep; cp++)
+ ;
+ if (last != cp) {
+ struct stat st;
+ struct alternate_object_database *alt;
+ /* 43 = 40-byte + 2 '/' + terminating NUL */
+ int pfxlen = cp - last;
+ int entlen = pfxlen + 43;
+
+ if (*last != '/' && 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 (*last != '/' && relative_base) {
+ memcpy(ent->base, relative_base, base_len - 1);
+ ent->base[base_len - 1] = '/';
+ memcpy(ent->base + base_len,
+ last, cp - last);
+ }
+ else
+ memcpy(ent->base, last, 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);
+ goto bad;
+ }
+ ent->base[pfxlen] = '/';
+
+ /* 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))
+ goto bad;
+ if (!memcmp(ent->base, objdir, pfxlen)) {
+ bad:
+ free(ent);
+ }
+ else {
+ *alt_odb_tail = ent;
+ alt_odb_tail = &(ent->next);
+ ent->next = NULL;
+ }
+ }
+ while (cp < ep && *cp == sep)
+ cp++;
+ last = cp;
+ }
+}
+
+void prepare_alt_odb(void)
+{
+ char path[PATH_MAX];
+ char *map;
+ int fd;
+ struct stat st;
+ char *alt;
+
+ alt = getenv(ALTERNATE_DB_ENVIRONMENT);
+ if (!alt) alt = "";
+
+ if (alt_odb_tail)
+ return;
+ alt_odb_tail = &alt_odb_list;
+ link_alt_odb_entries(alt, alt + strlen(alt), ':', NULL);
+
+ sprintf(path, "%s/info/alternates", get_object_directory());
+ fd = open(path, O_RDONLY);
+ if (fd < 0)
+ return;
+ if (fstat(fd, &st) || (st.st_size == 0)) {
+ close(fd);
+ return;
+ }
+ map = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+ close(fd);
+ if (map == MAP_FAILED)
+ return;
+
+ link_alt_odb_entries(map, map + st.st_size, '\n',
+ get_object_directory());
+ munmap(map, st.st_size);
+}
+
+static char *find_sha1_file(const unsigned char *sha1, struct stat *st)
+{
+ char *name = sha1_file_name(sha1);
+ struct alternate_object_database *alt;
+
+ if (!stat(name, st))
+ return name;
+ prepare_alt_odb();
+ for (alt = alt_odb_list; alt; alt = alt->next) {
+ name = alt->name;
+ fill_sha1_path(name, sha1);
+ if (!stat(alt->base, st))
+ return alt->base;
+ }
+ return NULL;
+}
+
+#define PACK_MAX_SZ (1<<26)
+static int pack_used_ctr;
+static unsigned long pack_mapped;
+struct packed_git *packed_git;
+
+static int check_packed_git_idx(const char *path, unsigned long *idx_size_,
+ void **idx_map_)
+{
+ void *idx_map;
+ unsigned int *index;
+ unsigned long idx_size;
+ int nr, i;
+ int fd = open(path, O_RDONLY);
+ struct stat st;
+ if (fd < 0)
+ return -1;
+ if (fstat(fd, &st)) {
+ close(fd);
+ return -1;
+ }
+ idx_size = st.st_size;
+ idx_map = mmap(NULL, idx_size, PROT_READ, MAP_PRIVATE, fd, 0);
+ close(fd);
+ if (idx_map == MAP_FAILED)
+ return -1;
+
+ index = idx_map;
+ *idx_map_ = idx_map;
+ *idx_size_ = idx_size;
+
+ /* check index map */
+ if (idx_size < 4*256 + 20 + 20)
+ return error("index file too small");
+ nr = 0;
+ for (i = 0; i < 256; i++) {
+ unsigned int n = ntohl(index[i]);
+ if (n < nr)
+ return error("non-monotonic index");
+ nr = n;
+ }
+
+ /*
+ * 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)
+ return error("wrong index file size");
+
+ return 0;
+}
+
+static int unuse_one_packed_git(void)
+{
+ struct packed_git *p, *lru = NULL;
+
+ for (p = packed_git; p; p = p->next) {
+ if (p->pack_use_cnt || !p->pack_base)
+ continue;
+ if (!lru || p->pack_last_used < lru->pack_last_used)
+ lru = p;
+ }
+ if (!lru)
+ return 0;
+ munmap(lru->pack_base, lru->pack_size);
+ lru->pack_base = NULL;
+ return 1;
+}
+
+void unuse_packed_git(struct packed_git *p)
+{
+ p->pack_use_cnt--;
+}
+
+int use_packed_git(struct packed_git *p)
+{
+ if (!p->pack_size) {
+ struct stat st;
+ // We created the struct before we had the pack
+ stat(p->pack_name, &st);
+ if (!S_ISREG(st.st_mode))
+ die("packfile %s not a regular file", p->pack_name);
+ p->pack_size = st.st_size;
+ }
+ if (!p->pack_base) {
+ int fd;
+ struct stat st;
+ void *map;
+
+ pack_mapped += p->pack_size;
+ while (PACK_MAX_SZ < pack_mapped && unuse_one_packed_git())
+ ; /* nothing */
+ fd = open(p->pack_name, O_RDONLY);
+ if (fd < 0)
+ die("packfile %s cannot be opened", p->pack_name);
+ if (fstat(fd, &st)) {
+ close(fd);
+ die("packfile %s cannot be opened", p->pack_name);
+ }
+ if (st.st_size != p->pack_size)
+ die("packfile %s size mismatch.", p->pack_name);
+ map = mmap(NULL, p->pack_size, PROT_READ, MAP_PRIVATE, fd, 0);
+ close(fd);
+ if (map == MAP_FAILED)
+ die("packfile %s cannot be mapped.", p->pack_name);
+ p->pack_base = map;
+
+ /* Check if the pack file matches with the index file.
+ * this is cheap.
+ */
+ if (memcmp((char*)(p->index_base) + p->index_size - 40,
+ p->pack_base + p->pack_size - 20, 20)) {
+
+ die("packfile %s does not match index.", p->pack_name);
+ }
+ }
+ p->pack_last_used = pack_used_ctr++;
+ p->pack_use_cnt++;
+ return 0;
+}
+
+struct packed_git *add_packed_git(char *path, int path_len, int local)
+{
+ struct stat st;
+ struct packed_git *p;
+ unsigned long idx_size;
+ void *idx_map;
+ unsigned char sha1[20];
+
+ if (check_packed_git_idx(path, &idx_size, &idx_map))
+ return NULL;
+
+ /* do we have a corresponding .pack file? */
+ strcpy(path + path_len - 4, ".pack");
+ if (stat(path, &st) || !S_ISREG(st.st_mode)) {
+ munmap(idx_map, idx_size);
+ return NULL;
+ }
+ /* ok, it looks sane as far as we can check without
+ * actually mapping the pack file.
+ */
+ p = xmalloc(sizeof(*p) + path_len + 2);
+ strcpy(p->pack_name, path);
+ p->index_size = idx_size;
+ p->pack_size = st.st_size;
+ p->index_base = idx_map;
+ p->next = NULL;
+ p->pack_base = NULL;
+ p->pack_last_used = 0;
+ p->pack_use_cnt = 0;
+ p->pack_local = local;
+ if ((path_len > 44) && !get_sha1_hex(path + path_len - 44, sha1))
+ memcpy(p->sha1, sha1, 20);
+ return p;
+}
+
+struct packed_git *parse_pack_index(unsigned char *sha1)
+{
+ char *path = sha1_pack_index_name(sha1);
+ return parse_pack_index_file(sha1, path);
+}
+
+struct packed_git *parse_pack_index_file(const unsigned char *sha1, char *idx_path)
+{
+ struct packed_git *p;
+ unsigned long idx_size;
+ void *idx_map;
+ char *path;
+
+ if (check_packed_git_idx(idx_path, &idx_size, &idx_map))
+ return NULL;
+
+ path = sha1_pack_name(sha1);
+
+ p = xmalloc(sizeof(*p) + strlen(path) + 2);
+ strcpy(p->pack_name, path);
+ p->index_size = idx_size;
+ p->pack_size = 0;
+ p->index_base = idx_map;
+ p->next = NULL;
+ p->pack_base = NULL;
+ p->pack_last_used = 0;
+ p->pack_use_cnt = 0;
+ memcpy(p->sha1, sha1, 20);
+ 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)
+{
+ char path[PATH_MAX];
+ 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 (strcmp(de->d_name + namelen - 4, ".idx"))
+ continue;
+
+ /* we have .idx. Is it a file we can map? */
+ strcpy(path + len, de->d_name);
+ p = add_packed_git(path, len + namelen, local);
+ if (!p)
+ continue;
+ p->next = packed_git;
+ packed_git = p;
+ }
+ closedir(dir);
+}
+
+void prepare_packed_git(void)
+{
+ static int run_once = 0;
+ struct alternate_object_database *alt;
+
+ if (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] = '/';
+ }
+ run_once = 1;
+}
+
+int check_sha1_signature(const unsigned char *sha1, void *map, unsigned long size, const char *type)