The connectivity check is executed via git-receive-pack(1) to verify
that a client has provided all references that are required to satisfy a
set of reference updates. What the connectivity check does is to walk
the object graph with all reference tips as starting points while all
preexisting reference tips are marked as uninteresting.

Preexisting references are currently marked uninteresting by passing
`--not --all` to git-rev-list(1). Some users of the connectivity check
may have a better picture of which objects should be regarded as
uninteresting though, e.g. by reusing information from the reference
advertisement when serving a push.

Add a new field to `struct check_connected_options` that allows callers
to replace the `--not --all` logic with their own set of object IDs they
regard as uninteresting.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 connected.c | 9 ++++++++-
 connected.h | 7 +++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/connected.c b/connected.c
index 74a20cb32e..2a4c4e0025 100644
--- a/connected.c
+++ b/connected.c
@@ -98,7 +98,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 	strvec_push(&rev_list.args, "--stdin");
 	if (has_promisor_remote())
 		strvec_push(&rev_list.args, "--exclude-promisor-objects");
-	if (!opt->is_deepening_fetch) {
+	if (!opt->is_deepening_fetch && !opt->reachable_oids_fn) {
 		strvec_push(&rev_list.args, "--not");
 		strvec_push(&rev_list.args, "--all");
 	}
@@ -125,6 +125,13 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 
 	rev_list_in = xfdopen(rev_list.in, "w");
 
+	if (opt->reachable_oids_fn) {
+		const struct object_id *reachable_oid;
+		while ((reachable_oid = opt->reachable_oids_fn(opt->reachable_oids_data)) != NULL)
+			if (fprintf(rev_list_in, "^%s\n", oid_to_hex(reachable_oid)) < 0)
+				break;
+	}
+
 	do {
 		/*
 		 * If index-pack already checked that:
diff --git a/connected.h b/connected.h
index 6e59c92aa3..f09c7d7884 100644
--- a/connected.h
+++ b/connected.h
@@ -46,6 +46,13 @@ struct check_connected_options {
 	 * during a fetch.
 	 */
 	unsigned is_deepening_fetch : 1;
+
+	/*
+	 * If non-NULL, use this iterator to determine the set of reachable
+	 * objects instead of marking all references as unreachable.
+	 */
+	oid_iterate_fn reachable_oids_fn;
+	void *reachable_oids_data;
 };
 
 #define CHECK_CONNECTED_INIT { 0 }
-- 
2.38.1