Transitively read alternatives

When adding an alternate object store then add entries from its
info/alternates files, too.
Relative entries are only allowed in the current repository.
Loops and duplicate alternates through multiple repositories are ignored.
Just to be sure that nothing breaks it is not allow to build deep
nesting levels using info/alternates.

Signed-off-by: Martin Waitz <tali@admingilde.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

sha1_file.c

@@ -217,6 +217,8 @@ char *sha1_pack_index_name(const unsigned char *sha1)
 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.
  *
@@ -232,14 +234,85 @@ static struct alternate_object_database **alt_odb_tail;
  * 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)
+static int link_alt_odb_entry(const char * entry, int len, const char * relative_base, int depth)
 {
-	const char *cp, *last;
-	struct alternate_object_database *ent;
+	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 (*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 (*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;
@@ -249,60 +322,15 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 			last = cp + 1;
 			continue;
 		}
-		for ( ; cp < ep && *cp != sep; cp++)
-			;
+		while (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;
+			if ((*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)
@@ -311,23 +339,14 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 	}
 }
 
-void prepare_alt_odb(void)
+static void read_info_alternates(const char * relative_base, int depth)
 {
-	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);
+	char path[PATH_MAX];
+	int fd;
 
-	sprintf(path, "%s/info/alternates", get_object_directory());
+	sprintf(path, "%s/info/alternates", relative_base);
 	fd = open(path, O_RDONLY);
 	if (fd < 0)
 		return;
@@ -340,11 +359,26 @@ void prepare_alt_odb(void)
 	if (map == MAP_FAILED)
 		return;
 
-	link_alt_odb_entries(map, map + st.st_size, '\n',
-			     get_object_directory());
+	link_alt_odb_entries(map, map + st.st_size, '\n', relative_base, depth);
+
 	munmap(map, st.st_size);
 }
 
+void prepare_alt_odb(void)
+{
+	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, 0);
+
+	read_info_alternates(get_object_directory(), 0);
+}
+
 static char *find_sha1_file(const unsigned char *sha1, struct stat *st)
 {
 	char *name = sha1_file_name(sha1);