sha1_file.c: make sure packs in an alternate odb is named properly.

We somehow ended up registering packs in alternate object
directories as "dir/object//pack/pack-*", which confusd the
update-server-info code very badly.  Also we did not attempt to
detect a mistake of listing the object directory itself as one
of the alternates. This does not lead to incorrect behaviour,
but is simply wasteful, so try to do so when we are trivially
able to.

Signed-off-by: Junio C Hamano <junkio@cox.net>

sha1_file.c

@@ -184,8 +184,8 @@ static struct alternate_object_database **alt_odb_tail;
  * 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 exactly in the same format as that environment
- * variable.  Its base points at a statically allocated buffer that
+ * 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
@@ -197,6 +197,7 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 {
 	const char *cp, *last;
 	struct alternate_object_database *ent;
+	const char *objdir = get_object_directory();
 	int base_len = -1;
 
 	last = alt;
@@ -211,6 +212,7 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 		for ( ; cp < ep && *cp != sep; cp++)
 			;
 		if (last != cp) {
+			struct alternate_object_database *alt;
 			/* 43 = 40-byte + 2 '/' + terminating NUL */
 			int pfxlen = cp - last;
 			int entlen = pfxlen + 43;
@@ -223,9 +225,7 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 				pfxlen += base_len;
 			}
 			ent = xmalloc(sizeof(*ent) + entlen);
-			*alt_odb_tail = ent;
-			alt_odb_tail = &(ent->next);
-			ent->next = NULL;
+
 			if (*last != '/' && relative_base) {
 				memcpy(ent->base, relative_base, base_len - 1);
 				ent->base[base_len - 1] = '/';
@@ -237,6 +237,22 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 			ent->name = ent->base + pfxlen + 1;
 			ent->base[pfxlen] = ent->base[pfxlen + 3] = '/';
 			ent->base[entlen-1] = 0;
+
+			/* 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++;
@@ -531,8 +547,9 @@ void prepare_packed_git(void)
 	prepare_packed_git_one(get_object_directory(), 1);
 	prepare_alt_odb();
 	for (alt = alt_odb_list; alt; alt = alt->next) {
-		alt->name[0] = 0;
+		alt->name[-1] = 0;
 		prepare_packed_git_one(alt->base, 0);
+		alt->name[-1] = '/';
 	}
 	run_once = 1;
 }