Merge branch 'jt/fix-fattening-promisor-fetch'

Fix performance regression of a recent "fatten promisor pack with
local objects" protection against an unwanted gc.

* jt/fix-fattening-promisor-fetch:
  index-pack --promisor: also check commits' trees
  index-pack --promisor: don't check blobs
  index-pack --promisor: dedup before checking links

Author: gitster

builtin/index-pack.c

@@ -155,11 +155,11 @@ static int input_fd, output_fd;
 static const char *curr_pack;
 
 /*
- * local_links is guarded by read_mutex, and record_local_links is read-only in
- * a thread.
+ * outgoing_links is guarded by read_mutex, and record_outgoing_links is
+ * read-only in a thread.
  */
-static struct oidset local_links = OIDSET_INIT;
-static int record_local_links;
+static struct oidset outgoing_links = OIDSET_INIT;
+static int record_outgoing_links;
 
 static struct thread_local_data *thread_data;
 static int nr_dispatched;
@@ -812,18 +812,41 @@ static int check_collison(struct object_entry *entry)
 	return 0;
 }
 
-static void record_if_local_object(const struct object_id *oid)
+static void record_outgoing_link(const struct object_id *oid)
 {
-	struct object_info info = OBJECT_INFO_INIT;
-	if (oid_object_info_extended(the_repository, oid, &info, 0))
-		/* Missing; assume it is a promisor object */
-		return;
-	if (info.whence == OI_PACKED && info.u.packed.pack->pack_promisor)
-		return;
-	oidset_insert(&local_links, oid);
+	oidset_insert(&outgoing_links, oid);
 }
 
-static void do_record_local_links(struct object *obj)
+static void maybe_record_name_entry(const struct name_entry *entry)
+{
+	/*
+	 * Checking only trees here results in a significantly faster packfile
+	 * indexing, but the drawback is that if the packfile to be indexed
+	 * references a local blob only directly (that is, never through a
+	 * local tree), that local blob is in danger of being garbage
+	 * collected. Such a situation may arise if we push local commits,
+	 * including one with a change to a blob in the root tree, and then the
+	 * server incorporates them into its main branch through a "rebase" or
+	 * "squash" merge strategy, and then we fetch the new main branch from
+	 * the server.
+	 *
+	 * This situation has not been observed yet - we have only noticed
+	 * missing commits, not missing trees or blobs. (In fact, if it were
+	 * believed that only missing commits are problematic, one could argue
+	 * that we should also exclude trees during the outgoing link check;
+	 * but it is safer to include them.)
+	 *
+	 * Due to the rarity of the situation (it has not been observed to
+	 * happen in real life), and because the "penalty" in such a situation
+	 * is merely to refetch the missing blob when it's needed (and this
+	 * happens only once - when refetched, the blob goes into a promisor
+	 * pack, so it won't be GC-ed, the tradeoff seems worth it.
+	*/
+	if (S_ISDIR(entry->mode))
+		record_outgoing_link(&entry->oid);
+}
+
+static void do_record_outgoing_links(struct object *obj)
 {
 	if (obj->type == OBJ_TREE) {
 		struct tree *tree = (struct tree *)obj;
@@ -837,16 +860,17 @@ static void do_record_local_links(struct object *obj)
 			 */
 			return;
 		while (tree_entry_gently(&desc, &entry))
-			record_if_local_object(&entry.oid);
+			maybe_record_name_entry(&entry);
 	} else if (obj->type == OBJ_COMMIT) {
 		struct commit *commit = (struct commit *) obj;
 		struct commit_list *parents = commit->parents;
 
+		record_outgoing_link(get_commit_tree_oid(commit));
 		for (; parents; parents = parents->next)
-			record_if_local_object(&parents->item->object.oid);
+			record_outgoing_link(&parents->item->object.oid);
 	} else if (obj->type == OBJ_TAG) {
 		struct tag *tag = (struct tag *) obj;
-		record_if_local_object(get_tagged_oid(tag));
+		record_outgoing_link(get_tagged_oid(tag));
 	}
 }
 
@@ -896,7 +920,7 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 		free(has_data);
 	}
 
-	if (strict || do_fsck_object || record_local_links) {
+	if (strict || do_fsck_object || record_outgoing_links) {
 		read_lock();
 		if (type == OBJ_BLOB) {
 			struct blob *blob = lookup_blob(the_repository, oid);
@@ -928,8 +952,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 				die(_("fsck error in packed object"));
 			if (strict && fsck_walk(obj, NULL, &fsck_options))
 				die(_("Not all child objects of %s are reachable"), oid_to_hex(&obj->oid));
-			if (record_local_links)
-				do_record_local_links(obj);
+			if (record_outgoing_links)
+				do_record_outgoing_links(obj);
 
 			if (obj->type == OBJ_TREE) {
 				struct tree *item = (struct tree *) obj;
@@ -1785,28 +1809,43 @@ static void repack_local_links(void)
 	struct strbuf line = STRBUF_INIT;
 	struct oidset_iter iter;
 	struct object_id *oid;
-	char *base_name;
+	char *base_name = NULL;
 
-	if (!oidset_size(&local_links))
+	if (!oidset_size(&outgoing_links))
 		return;
 
-	base_name = mkpathdup("%s/pack/pack", repo_get_object_directory(the_repository));
+	oidset_iter_init(&outgoing_links, &iter);
+	while ((oid = oidset_iter_next(&iter))) {
+		struct object_info info = OBJECT_INFO_INIT;
+		if (oid_object_info_extended(the_repository, oid, &info, 0))
+			/* Missing; assume it is a promisor object */
+			continue;
+		if (info.whence == OI_PACKED && info.u.packed.pack->pack_promisor)
+			continue;
 
-	strvec_push(&cmd.args, "pack-objects");
-	strvec_push(&cmd.args, "--exclude-promisor-objects-best-effort");
-	strvec_push(&cmd.args, base_name);
-	cmd.git_cmd = 1;
-	cmd.in = -1;
-	cmd.out = -1;
-	if (start_command(&cmd))
-		die(_("could not start pack-objects to repack local links"));
+		if (!cmd.args.nr) {
+			base_name = mkpathdup(
+				"%s/pack/pack",
+				repo_get_object_directory(the_repository));
+			strvec_push(&cmd.args, "pack-objects");
+			strvec_push(&cmd.args,
+				    "--exclude-promisor-objects-best-effort");
+			strvec_push(&cmd.args, base_name);
+			cmd.git_cmd = 1;
+			cmd.in = -1;
+			cmd.out = -1;
+			if (start_command(&cmd))
+				die(_("could not start pack-objects to repack local links"));
+		}
 
-	oidset_iter_init(&local_links, &iter);
-	while ((oid = oidset_iter_next(&iter))) {
 		if (write_in_full(cmd.in, oid_to_hex(oid), the_hash_algo->hexsz) < 0 ||
 		    write_in_full(cmd.in, "\n", 1) < 0)
 			die(_("failed to feed local object to pack-objects"));
 	}
+
+	if (!cmd.args.nr)
+		return;
+
 	close(cmd.in);
 
 	out = xfdopen(cmd.out, "r");
@@ -1905,7 +1944,7 @@ int cmd_index_pack(int argc,
 			} else if (skip_to_optional_arg(arg, "--keep", &keep_msg)) {
 				; /* nothing to do */
 			} else if (skip_to_optional_arg(arg, "--promisor", &promisor_msg)) {
-				record_local_links = 1;
+				record_outgoing_links = 1;
 			} else if (starts_with(arg, "--threads=")) {
 				char *end;
 				nr_threads = strtoul(arg+10, &end, 0);