midx: avoid opening multiple MIDXs when writing

Opening multiple instance of the same MIDX can lead to problems like two
separate packed_git structures which represent the same pack being added
to the repository's object store.

The above scenario can happen because prepare_midx_pack() checks if
`m->packs[pack_int_id]` is NULL in order to determine if a pack has been
opened and installed in the repository before. But a caller can
construct two copies of the same MIDX by calling get_multi_pack_index()
and load_multi_pack_index() since the former manipulates the
object store directly but the latter is a lower-level routine which
allocates a new MIDX for each call.

So if prepare_midx_pack() is called on multiple MIDXs with the same
pack_int_id, then that pack will be installed twice in the object
store's packed_git pointer.

This can lead to problems in, for e.g., the pack-bitmap code, which does
something like the following (in pack-bitmap.c:open_pack_bitmap()):

    struct bitmap_index *bitmap_git = ...;
    for (p = get_all_packs(r); p; p = p->next) {
      if (open_pack_bitmap_1(bitmap_git, p) == 0)
        ret = 0;
    }

which is a problem if two copies of the same pack exist in the
packed_git list because pack-bitmap.c:open_pack_bitmap_1() contains a
conditional like the following:

    if (bitmap_git->pack || bitmap_git->midx) {
      /* ignore extra bitmap file; we can only handle one */
      warning("ignoring extra bitmap file: %s", packfile->pack_name);
      close(fd);
      return -1;
    }

Avoid this scenario by not letting write_midx_internal() open a MIDX
that isn't also pointed at by the object store. So long as this is the
case, other routines should prefer to open MIDXs with
get_multi_pack_index() or reprepare_packed_git() instead of creating
instances on their own. Because get_multi_pack_index() returns
`r->object_store->multi_pack_index` if it is non-NULL, we'll only have
one instance of a MIDX open at one time, avoiding these problems.

To encourage this, drop the `struct multi_pack_index *` parameter from
`write_midx_internal()`, and rely instead on the `object_dir` to find
(or initialize) the correct MIDX instance.

Likewise, replace the call to `close_midx()` with
`close_object_store()`, since we're about to replace the MIDX with a new
one and should invalidate the object store's memory of any MIDX that
might have existed beforehand.

Note that this now forbids passing object directories that don't belong
to alternate repositories over `--object-dir`, since before we would
have happily opened a MIDX in any directory, but now restrict ourselves
to only those reachable by `r->objects->multi_pack_index` (and alternate
MIDXs that we can see by walking the `next` pointer).

As far as I can tell, supporting arbitrary directories with
`--object-dir` was a historical accident, since even the documentation
says `<alt>` when referring to the value passed to this option.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-multi-pack-index.txt

@@ -23,6 +23,8 @@ OPTIONS
 	Use given directory for the location of Git objects. We check
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
++
+`<dir>` must be an alternate of the current repository.
 
 --[no-]progress::
 	Turn progress on/off explicitly. If neither is specified, progress is

builtin/commit-graph.c

@@ -43,28 +43,6 @@ static struct opts_commit_graph {
 	int enable_changed_paths;
 } opts;
 
-static struct object_directory *find_odb(struct repository *r,
-					 const char *obj_dir)
-{
-	struct object_directory *odb;
-	char *obj_dir_real = real_pathdup(obj_dir, 1);
-	struct strbuf odb_path_real = STRBUF_INIT;
-
-	prepare_alt_odb(r);
-	for (odb = r->objects->odb; odb; odb = odb->next) {
-		strbuf_realpath(&odb_path_real, odb->path, 1);
-		if (!strcmp(obj_dir_real, odb_path_real.buf))
-			break;
-	}
-
-	free(obj_dir_real);
-	strbuf_release(&odb_path_real);
-
-	if (!odb)
-		die(_("could not find object directory matching %s"), obj_dir);
-	return odb;
-}
-
 static int graph_verify(int argc, const char **argv)
 {
 	struct commit_graph *graph = NULL;

midx.c

@@ -893,7 +893,7 @@ static int midx_checksum_valid(struct multi_pack_index *m)
 	return hashfile_checksum_valid(m->data, m->data_len);
 }
 
-static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+static int write_midx_internal(const char *object_dir,
 			       struct string_list *packs_to_drop,
 			       const char *preferred_pack_name,
 			       unsigned flags)
@@ -904,20 +904,26 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct write_midx_context ctx = { 0 };
+	struct multi_pack_index *cur;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
 	struct chunkfile *cf;
 
+	/* Ensure the given object_dir is local, or a known alternate. */
+	find_odb(the_repository, object_dir);
+
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name))
 		die_errno(_("unable to create leading directories of %s"),
 			  midx_name);
 
-	if (m)
-		ctx.m = m;
-	else
-		ctx.m = load_multi_pack_index(object_dir, 1);
+	for (cur = get_multi_pack_index(the_repository); cur; cur = cur->next) {
+		if (!strcmp(object_dir, cur->object_dir)) {
+			ctx.m = cur;
+			break;
+		}
+	}
 
 	if (ctx.m && !midx_checksum_valid(ctx.m)) {
 		warning(_("ignoring existing multi-pack-index; checksum mismatch"));
@@ -1119,7 +1125,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
 
 	if (ctx.m)
-		close_midx(ctx.m);
+		close_object_store(the_repository->objects);
 
 	if (ctx.nr - dropped_packs == 0) {
 		error(_("no pack files to index."));
@@ -1182,8 +1188,7 @@ int write_midx_file(const char *object_dir,
 		    const char *preferred_pack_name,
 		    unsigned flags)
 {
-	return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
-				   flags);
+	return write_midx_internal(object_dir, NULL, preferred_pack_name, flags);
 }
 
 struct clear_midx_data {
@@ -1461,8 +1466,10 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 
 	free(count);
 
-	if (packs_to_drop.nr)
-		result = write_midx_internal(object_dir, m, &packs_to_drop, NULL, flags);
+	if (packs_to_drop.nr) {
+		result = write_midx_internal(object_dir, &packs_to_drop, NULL, flags);
+		m = NULL;
+	}
 
 	string_list_clear(&packs_to_drop, 0);
 	return result;
@@ -1651,7 +1658,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 		goto cleanup;
 	}
 
-	result = write_midx_internal(object_dir, m, NULL, NULL, flags);
+	result = write_midx_internal(object_dir, NULL, NULL, flags);
 	m = NULL;
 
 cleanup:

object-file.c

@@ -820,6 +820,27 @@ out:
 	return ref_git;
 }
 
+struct object_directory *find_odb(struct repository *r, const char *obj_dir)
+{
+	struct object_directory *odb;
+	char *obj_dir_real = real_pathdup(obj_dir, 1);
+	struct strbuf odb_path_real = STRBUF_INIT;
+
+	prepare_alt_odb(r);
+	for (odb = r->objects->odb; odb; odb = odb->next) {
+		strbuf_realpath(&odb_path_real, odb->path, 1);
+		if (!strcmp(obj_dir_real, odb_path_real.buf))
+			break;
+	}
+
+	free(obj_dir_real);
+	strbuf_release(&odb_path_real);
+
+	if (!odb)
+		die(_("could not find object directory matching %s"), obj_dir);
+	return odb;
+}
+
 static void fill_alternate_refs_command(struct child_process *cmd,
 					const char *repo_path)
 {

object-store.h

@@ -38,6 +38,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 
 void prepare_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
+struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);
 int foreach_alt_odb(alt_odb_fn, void*);
 typedef void alternate_ref_fn(const struct object_id *oid, void *);

t/t5319-multi-pack-index.sh

@@ -582,7 +582,15 @@ test_expect_success 'force some 64-bit offsets with pack-objects' '
 	idx64=objects64/pack/test-64-$pack64.idx &&
 	chmod u+w $idx64 &&
 	corrupt_data $idx64 $(test_oid idxoff) "\02" &&
-	midx64=$(git multi-pack-index --object-dir=objects64 write) &&
+	# objects64 is not a real repository, but can serve as an alternate
+	# anyway so we can write a MIDX into it
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+		( cd ../objects64 && pwd ) >.git/objects/info/alternates &&
+		midx64=$(git multi-pack-index --object-dir=../objects64 write)
+	) &&
 	midx_read_expect 1 63 5 objects64 " large-offsets"
 '