fetch-pack: avoid object flags if no_dependents

When fetch_pack() is invoked as part of another Git command (due to a
lazy fetch from a partial clone, for example), it uses object flags that
may already be used by the outer Git command.

The commit that introduced the lazy fetch feature (88e2f9ed8e
("introduce fetch-object: fetch one promisor object", 2017-12-05)) tried
to avoid this overlap, but it did not avoid it totally. It was
successful in avoiding writing COMPLETE, but did not avoid reading
COMPLETE, and did not avoid writing and reading ALTERNATE.

Ensure that no flags are written or read by fetch_pack() in the case
where it is used to perform a lazy fetch. To do this, it is sufficient
to avoid checking completeness of wanted refs (unnecessary in the case
of lazy fetches), and to avoid negotiation-related work (in the current
implementation, already, no negotiation is performed). After that was
done, the lack of overlap was verified by checking all direct and
indirect usages of COMPLETE and ALTERNATE - that they are read or
written only if no_dependents is false.

There are other possible solutions to this issue:

 (1) Split fetch-pack.{c,h} into a flag-using part and a non-flag-using
     part, and whenever no_dependents is set, only use the
     non-flag-using part.
 (2) Make fetch_pack() be able to be used with arbitrary repository
     objects. fetch_pack() should then create its own repository object
     based on the given repository object, with its own object
     hashtable, so that the flags do not conflict.

(1) is possible but invasive - some functions would need to be split;
and such invasiveness would potentially be unnecessary if we ever were
to need (2) anyway. (2) would be useful if we were to support, say,
submodules that were partial clones themselves, but I don't know when or
if the Git project plans to support those.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 59 insertions, 42 deletions

diff --git a/fetch-pack.c b/fetch-pack.c
index 88a078e9be..973d72f367 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -253,8 +253,10 @@ static int find_common(struct fetch_negotiator *negotiator,
 	if (args->stateless_rpc && multi_ack == 1)
 		die(_("--stateless-rpc requires multi_ack_detailed"));
 
-	mark_tips(negotiator, args->negotiation_tips);
-	for_each_cached_alternate(negotiator, insert_one_alternate_object);
+	if (!args->no_dependents) {
+		mark_tips(negotiator, args->negotiation_tips);
+		for_each_cached_alternate(negotiator, insert_one_alternate_object);
+	}
 
 	fetching = 0;
 	for ( ; refs ; refs = refs->next) {
@@ -271,8 +273,12 @@ static int find_common(struct fetch_negotiator *negotiator,
 		 * We use lookup_object here because we are only
 		 * interested in the case we *know* the object is
 		 * reachable and we have already scanned it.
+		 *
+		 * Do this only if args->no_dependents is false (if it is true,
+		 * we cannot trust the object flags).
 		 */
-		if (((o = lookup_object(the_repository, remote->hash)) != NULL) &&
+		if (!args->no_dependents &&
+		    ((o = lookup_object(the_repository, remote->hash)) != NULL) &&
 				(o->flags & COMPLETE)) {
 			continue;
 		}
@@ -710,31 +716,29 @@ static void mark_complete_and_common_ref(struct fetch_negotiator *negotiator,
 
 	oidset_clear(&loose_oid_set);
 
-	if (!args->no_dependents) {
-		if (!args->deepen) {
-			for_each_ref(mark_complete_oid, NULL);
-			for_each_cached_alternate(NULL, mark_alternate_complete);
-			commit_list_sort_by_date(&complete);
-			if (cutoff)
-				mark_recent_complete_commits(args, cutoff);
-		}
+	if (!args->deepen) {
+		for_each_ref(mark_complete_oid, NULL);
+		for_each_cached_alternate(NULL, mark_alternate_complete);
+		commit_list_sort_by_date(&complete);
+		if (cutoff)
+			mark_recent_complete_commits(args, cutoff);
+	}
 
-		/*
-		 * Mark all complete remote refs as common refs.
-		 * Don't mark them common yet; the server has to be told so first.
-		 */
-		for (ref = *refs; ref; ref = ref->next) {
-			struct object *o = deref_tag(the_repository,
-						     lookup_object(the_repository,
-						     ref->old_oid.hash),
-						     NULL, 0);
+	/*
+	 * Mark all complete remote refs as common refs.
+	 * Don't mark them common yet; the server has to be told so first.
+	 */
+	for (ref = *refs; ref; ref = ref->next) {
+		struct object *o = deref_tag(the_repository,
+					     lookup_object(the_repository,
+					     ref->old_oid.hash),
+					     NULL, 0);
 
-			if (!o || o->type != OBJ_COMMIT || !(o->flags & COMPLETE))
-				continue;
+		if (!o || o->type != OBJ_COMMIT || !(o->flags & COMPLETE))
+			continue;
 
-			negotiator->known_common(negotiator,
-						 (struct commit *)o);
-		}
+		negotiator->known_common(negotiator,
+					 (struct commit *)o);
 	}
 
 	save_commit_buffer = old_save_commit_buffer;
@@ -990,11 +994,15 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 	if (!server_supports("deepen-relative") && args->deepen_relative)
 		die(_("Server does not support --deepen"));
 
-	mark_complete_and_common_ref(&negotiator, args, &ref);
-	filter_refs(args, &ref, sought, nr_sought);
-	if (everything_local(args, &ref)) {
-		packet_flush(fd[1]);
-		goto all_done;
+	if (!args->no_dependents) {
+		mark_complete_and_common_ref(&negotiator, args, &ref);
+		filter_refs(args, &ref, sought, nr_sought);
+		if (everything_local(args, &ref)) {
+			packet_flush(fd[1]);
+			goto all_done;
+		}
+	} else {
+		filter_refs(args, &ref, sought, nr_sought);
 	}
 	if (find_common(&negotiator, args, fd, &oid, ref) < 0)
 		if (!args->keep_pack)
@@ -1040,7 +1048,7 @@ static void add_shallow_requests(struct strbuf *req_buf,
 	}
 }
 
-static void add_wants(const struct ref *wants, struct strbuf *req_buf)
+static void add_wants(int no_dependents, const struct ref *wants, struct strbuf *req_buf)
 {
 	int use_ref_in_want = server_supports_feature("fetch", "ref-in-want", 0);
 
@@ -1057,8 +1065,12 @@ static void add_wants(const struct ref *wants, struct strbuf *req_buf)
 		 * We use lookup_object here because we are only
 		 * interested in the case we *know* the object is
 		 * reachable and we have already scanned it.
+		 *
+		 * Do this only if args->no_dependents is false (if it is true,
+		 * we cannot trust the object flags).
 		 */
-		if (((o = lookup_object(the_repository, remote->hash)) != NULL) &&
+		if (!no_dependents &&
+		    ((o = lookup_object(the_repository, remote->hash)) != NULL) &&
 		    (o->flags & COMPLETE)) {
 			continue;
 		}
@@ -1155,7 +1167,7 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 	}
 
 	/* add wants */
-	add_wants(wants, &req_buf);
+	add_wants(args->no_dependents, wants, &req_buf);
 
 	if (args->no_dependents) {
 		packet_buf_write(&req_buf, "done");
@@ -1346,16 +1358,21 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 				args->deepen = 1;
 
 			/* Filter 'ref' by 'sought' and those that aren't local */
-			mark_complete_and_common_ref(&negotiator, args, &ref);
-			filter_refs(args, &ref, sought, nr_sought);
-			if (everything_local(args, &ref))
-				state = FETCH_DONE;
-			else
+			if (!args->no_dependents) {
+				mark_complete_and_common_ref(&negotiator, args, &ref);
+				filter_refs(args, &ref, sought, nr_sought);
+				if (everything_local(args, &ref))
+					state = FETCH_DONE;
+				else
+					state = FETCH_SEND_REQUEST;
+
+				mark_tips(&negotiator, args->negotiation_tips);
+				for_each_cached_alternate(&negotiator,
+							  insert_one_alternate_object);
+			} else {
+				filter_refs(args, &ref, sought, nr_sought);
 				state = FETCH_SEND_REQUEST;
-
-			mark_tips(&negotiator, args->negotiation_tips);
-			for_each_cached_alternate(&negotiator,
-						  insert_one_alternate_object);
+			}
 			break;
 		case FETCH_SEND_REQUEST:
 			if (send_fetch_request(&negotiator, fd[1], args, ref,