14 files changed, 455 insertions, 100 deletions

diff --git a/Documentation/config/push.txt b/Documentation/config/push.txt
index 21b256e..f2667b2 100644
--- a/Documentation/config/push.txt
+++ b/Documentation/config/push.txt
@@ -120,3 +120,10 @@ push.useForceIfIncludes::
 	`--force-if-includes` as an option to linkgit:git-push[1]
 	in the command line. Adding `--no-force-if-includes` at the
 	time of push overrides this configuration setting.
+
+push.negotiate::
+	If set to "true", attempt to reduce the size of the packfile
+	sent by rounds of negotiation in which the client and the
+	server attempt to find commits in common. If "false", Git will
+	rely solely on the server's ref advertisement to find commits
+	in common.
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index f4ed141..a1e3136 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -346,6 +346,14 @@ explained below.
 	client should download from all given URIs. Currently, the
 	protocols supported are "http" and "https".
 
+If the 'wait-for-done' feature is advertised, the following argument
+can be included in the client's request.
+
+    wait-for-done
+	Indicates to the server that it should never send "ready", but
+	should wait for the client to say "done" before sending the
+	packfile.
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
 header. Most sections are sent only when the packfile is sent.
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 97c4fe6..dfde96a 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -83,6 +83,7 @@ static struct string_list server_options = STRING_LIST_INIT_DUP;
 static struct string_list negotiation_tip = STRING_LIST_INIT_NODUP;
 static int fetch_write_commit_graph = -1;
 static int stdin_refspecs = 0;
+static int negotiate_only;
 
 static int git_fetch_config(const char *k, const char *v, void *cb)
 {
@@ -205,6 +206,8 @@ static struct option builtin_fetch_options[] = {
 			TRANSPORT_FAMILY_IPV6),
 	OPT_STRING_LIST(0, "negotiation-tip", &negotiation_tip, N_("revision"),
 			N_("report that we have only objects reachable from this object")),
+	OPT_BOOL(0, "negotiate-only", &negotiate_only,
+		 N_("do not fetch a packfile; instead, print ancestors of negotiation tips")),
 	OPT_PARSE_LIST_OBJECTS_FILTER(&filter_options),
 	OPT_BOOL(0, "auto-maintenance", &enable_auto_gc,
 		 N_("run 'maintenance --auto' after fetching")),
@@ -2043,7 +2046,29 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (remote) {
+	if (negotiate_only) {
+		struct oidset acked_commits = OIDSET_INIT;
+		struct oidset_iter iter;
+		const struct object_id *oid;
+
+		if (!remote)
+			die(_("must supply remote when using --negotiate-only"));
+		gtransport = prepare_transport(remote, 1);
+		if (gtransport->smart_options) {
+			gtransport->smart_options->acked_commits = &acked_commits;
+		} else {
+			warning(_("Protocol does not support --negotiate-only, exiting."));
+			return 1;
+		}
+		if (server_options.nr)
+			gtransport->server_options = &server_options;
+		result = transport_fetch_refs(gtransport, NULL);
+
+		oidset_iter_init(&acked_commits, &iter);
+		while ((oid = oidset_iter_next(&iter)))
+			printf("%s\n", oid_to_hex(oid));
+		oidset_clear(&acked_commits);
+	} else if (remote) {
 		if (filter_options.choice || has_promisor_remote())
 			fetch_one_setup_partial(remote);
 		result = fetch_one(remote, argc, argv, prune_tags_ok, stdin_refspecs);
diff --git a/fetch-pack.c b/fetch-pack.c
index 2318ebe..c135635 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -23,6 +23,8 @@
 #include "fetch-negotiator.h"
 #include "fsck.h"
 #include "shallow.h"
+#include "commit-reach.h"
+#include "commit-graph.h"
 
 static int transfer_unpack_limit = -1;
 static int fetch_unpack_limit = -1;
@@ -45,6 +47,8 @@ static struct string_list uri_protocols = STRING_LIST_INIT_DUP;
 /* Remember to update object flag allocation in object.h */
 #define COMPLETE	(1U << 0)
 #define ALTERNATE	(1U << 1)
+#define COMMON		(1U << 6)
+#define REACH_SCRATCH	(1U << 7)
 
 /*
  * After sending this many "have"s if we do not get any new ACK , we
@@ -1195,11 +1199,9 @@ static void add_common(struct strbuf *req_buf, struct oidset *common)
 }
 
 static int add_haves(struct fetch_negotiator *negotiator,
-		     int seen_ack,
 		     struct strbuf *req_buf,
-		     int *haves_to_send, int *in_vain)
+		     int *haves_to_send)
 {
-	int ret = 0;
 	int haves_added = 0;
 	const struct object_id *oid;
 
@@ -1209,41 +1211,29 @@ static int add_haves(struct fetch_negotiator *negotiator,
 			break;
 	}
 
-	*in_vain += haves_added;
-	if (!haves_added || (seen_ack && *in_vain >= MAX_IN_VAIN)) {
-		/* Send Done */
-		packet_buf_write(req_buf, "done\n");
-		ret = 1;
-	}
-
 	/* Increase haves to send on next round */
 	*haves_to_send = next_flush(1, *haves_to_send);
 
-	return ret;
+	return haves_added;
 }
 
-static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
-			      struct fetch_pack_args *args,
-			      const struct ref *wants, struct oidset *common,
-			      int *haves_to_send, int *in_vain,
-			      int sideband_all, int seen_ack)
+static void write_fetch_command_and_capabilities(struct strbuf *req_buf,
+						 const struct string_list *server_options)
 {
-	int ret = 0;
 	const char *hash_name;
-	struct strbuf req_buf = STRBUF_INIT;
 
 	if (server_supports_v2("fetch", 1))
-		packet_buf_write(&req_buf, "command=fetch");
+		packet_buf_write(req_buf, "command=fetch");
 	if (server_supports_v2("agent", 0))
-		packet_buf_write(&req_buf, "agent=%s", git_user_agent_sanitized());
+		packet_buf_write(req_buf, "agent=%s", git_user_agent_sanitized());
 	if (advertise_sid && server_supports_v2("session-id", 0))
-		packet_buf_write(&req_buf, "session-id=%s", trace2_session_id());
-	if (args->server_options && args->server_options->nr &&
+		packet_buf_write(req_buf, "session-id=%s", trace2_session_id());
+	if (server_options && server_options->nr &&
 	    server_supports_v2("server-option", 1)) {
 		int i;
-		for (i = 0; i < args->server_options->nr; i++)
-			packet_buf_write(&req_buf, "server-option=%s",
-					 args->server_options->items[i].string);
+		for (i = 0; i < server_options->nr; i++)
+			packet_buf_write(req_buf, "server-option=%s",
+					 server_options->items[i].string);
 	}
 
 	if (server_feature_v2("object-format", &hash_name)) {
@@ -1251,13 +1241,26 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		if (hash_algo_by_ptr(the_hash_algo) != hash_algo)
 			die(_("mismatched algorithms: client %s; server %s"),
 			    the_hash_algo->name, hash_name);
-		packet_buf_write(&req_buf, "object-format=%s", the_hash_algo->name);
+		packet_buf_write(req_buf, "object-format=%s", the_hash_algo->name);
 	} else if (hash_algo_by_ptr(the_hash_algo) != GIT_HASH_SHA1) {
 		die(_("the server does not support algorithm '%s'"),
 		    the_hash_algo->name);
 	}
+	packet_buf_delim(req_buf);
+}
+
+static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
+			      struct fetch_pack_args *args,
+			      const struct ref *wants, struct oidset *common,
+			      int *haves_to_send, int *in_vain,
+			      int sideband_all, int seen_ack)
+{
+	int haves_added;
+	int done_sent = 0;
+	struct strbuf req_buf = STRBUF_INIT;
+
+	write_fetch_command_and_capabilities(&req_buf, args->server_options);
 
-	packet_buf_delim(&req_buf);
 	if (args->use_thin_pack)
 		packet_buf_write(&req_buf, "thin-pack");
 	if (args->no_progress)
@@ -1312,9 +1315,13 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 	/* Add all of the common commits we've found in previous rounds */
 	add_common(&req_buf, common);
 
-	/* Add initial haves */
-	ret = add_haves(negotiator, seen_ack, &req_buf,
-			haves_to_send, in_vain);
+	haves_added = add_haves(negotiator, &req_buf, haves_to_send);
+	*in_vain += haves_added;
+	if (!haves_added || (seen_ack && *in_vain >= MAX_IN_VAIN)) {
+		/* Send Done */
+		packet_buf_write(&req_buf, "done\n");
+		done_sent = 1;
+	}
 
 	/* Send request */
 	packet_buf_flush(&req_buf);
@@ -1322,7 +1329,7 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		die_errno(_("unable to write request to remote"));
 
 	strbuf_release(&req_buf);
-	return ret;
+	return done_sent;
 }
 
 /*
@@ -1351,35 +1358,11 @@ static int process_section_header(struct packet_reader *reader,
 	return ret;
 }
 
-enum common_found {
-	/*
-	 * No commit was found to be possessed by both the client and the
-	 * server, and "ready" was not received.
-	 */
-	NO_COMMON_FOUND,
-
-	/*
-	 * At least one commit was found to be possessed by both the client and
-	 * the server, and "ready" was not received.
-	 */
-	COMMON_FOUND,
-
-	/*
-	 * "ready" was received, indicating that the server is ready to send
-	 * the packfile without any further negotiation.
-	 */
-	READY
-};
-
-static enum common_found process_acks(struct fetch_negotiator *negotiator,
-				      struct packet_reader *reader,
-				      struct oidset *common)
+static int process_ack(struct fetch_negotiator *negotiator,
+		       struct packet_reader *reader,
+		       struct object_id *common_oid,
+		       int *received_ready)
 {
-	/* received */
-	int received_ready = 0;
-	int received_ack = 0;
-
-	process_section_header(reader, "acknowledgments", 0);
 	while (packet_reader_read(reader) == PACKET_READ_NORMAL) {
 		const char *arg;
 
@@ -1387,20 +1370,17 @@ static enum common_found process_acks(struct fetch_negotiator *negotiator,
 			continue;
 
 		if (skip_prefix(reader->line, "ACK ", &arg)) {
-			struct object_id oid;
-			received_ack = 1;
-			if (!get_oid_hex(arg, &oid)) {
+			if (!get_oid_hex(arg, common_oid)) {
 				struct commit *commit;
-				oidset_insert(common, &oid);
-				commit = lookup_commit(the_repository, &oid);
+				commit = lookup_commit(the_repository, common_oid);
 				if (negotiator)
 					negotiator->ack(negotiator, commit);
 			}
-			continue;
+			return 1;
 		}
 
 		if (!strcmp(reader->line, "ready")) {
-			received_ready = 1;
+			*received_ready = 1;
 			continue;
 		}
 
@@ -1418,13 +1398,12 @@ static enum common_found process_acks(struct fetch_negotiator *negotiator,
 	 * sent. Therefore, a DELIM is expected if "ready" is sent, and a FLUSH
 	 * otherwise.
 	 */
-	if (received_ready && reader->status != PACKET_READ_DELIM)
+	if (*received_ready && reader->status != PACKET_READ_DELIM)
 		die(_("expected packfile to be sent after 'ready'"));
-	if (!received_ready && reader->status != PACKET_READ_FLUSH)
+	if (!*received_ready && reader->status != PACKET_READ_FLUSH)
 		die(_("expected no other sections to be sent after no 'ready'"));
 
-	return received_ready ? READY :
-		(received_ack ? COMMON_FOUND : NO_COMMON_FOUND);
+	return 0;
 }
 
 static void receive_shallow_info(struct fetch_pack_args *args,
@@ -1548,10 +1527,10 @@ enum fetch_state {
 	FETCH_DONE,
 };
 
-static void do_check_stateless_delimiter(const struct fetch_pack_args *args,
+static void do_check_stateless_delimiter(int stateless_rpc,
 					 struct packet_reader *reader)
 {
-	check_stateless_delimiter(args->stateless_rpc, reader,
+	check_stateless_delimiter(stateless_rpc, reader,
 				  _("git fetch-pack: expected response end packet"));
 }
 
@@ -1573,6 +1552,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	struct fetch_negotiator negotiator_alloc;
 	struct fetch_negotiator *negotiator;
 	int seen_ack = 0;
+	struct object_id common_oid;
+	int received_ready = 0;
 	struct string_list packfile_uris = STRING_LIST_INIT_DUP;
 	int i;
 	struct strvec index_pack_args = STRVEC_INIT;
@@ -1631,22 +1612,22 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 			break;
 		case FETCH_PROCESS_ACKS:
 			/* Process ACKs/NAKs */
-			switch (process_acks(negotiator, &reader, &common)) {
-			case READY:
+			process_section_header(&reader, "acknowledgments", 0);
+			while (process_ack(negotiator, &reader, &common_oid,
+					   &received_ready)) {
+				in_vain = 0;
+				seen_ack = 1;
+				oidset_insert(&common, &common_oid);
+			}
+			if (received_ready) {
 				/*
 				 * Don't check for response delimiter; get_pack() will
 				 * read the rest of this response.
 				 */
 				state = FETCH_GET_PACK;
-				break;
-			case COMMON_FOUND:
-				in_vain = 0;
-				seen_ack = 1;
-				/* fallthrough */
-			case NO_COMMON_FOUND:
-				do_check_stateless_delimiter(args, &reader);
+			} else {
+				do_check_stateless_delimiter(args->stateless_rpc, &reader);
 				state = FETCH_SEND_REQUEST;
-				break;
 			}
 			break;
 		case FETCH_GET_PACK:
@@ -1668,7 +1649,7 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 				     packfile_uris.nr ? &index_pack_args : NULL,
 				     sought, nr_sought, &fsck_options.gitmodules_found))
 				die(_("git fetch-pack: fetch failed."));
-			do_check_stateless_delimiter(args, &reader);
+			do_check_stateless_delimiter(args->stateless_rpc, &reader);
 
 			state = FETCH_DONE;
 			break;
@@ -1985,6 +1966,105 @@ cleanup:
 	return ref_cpy;
 }
 
+static int add_to_object_array(const struct object_id *oid, void *data)
+{
+	struct object_array *a = data;
+
+	add_object_array(lookup_object(the_repository, oid), "", a);
+	return 0;
+}
+
+static void clear_common_flag(struct oidset *s)
+{
+	struct oidset_iter iter;
+	const struct object_id *oid;
+	oidset_iter_init(s, &iter);
+
+	while ((oid = oidset_iter_next(&iter))) {
+		struct object *obj = lookup_object(the_repository, oid);
+		obj->flags &= ~COMMON;
+	}
+}
+
+void negotiate_using_fetch(const struct oid_array *negotiation_tips,
+			   const struct string_list *server_options,
+			   int stateless_rpc,
+			   int fd[],
+			   struct oidset *acked_commits)
+{
+	struct fetch_negotiator negotiator;
+	struct packet_reader reader;
+	struct object_array nt_object_array = OBJECT_ARRAY_INIT;
+	struct strbuf req_buf = STRBUF_INIT;
+	int haves_to_send = INITIAL_FLUSH;
+	int in_vain = 0;
+	int seen_ack = 0;
+	int last_iteration = 0;
+	timestamp_t min_generation = GENERATION_NUMBER_INFINITY;
+
+	fetch_negotiator_init(the_repository, &negotiator);
+	mark_tips(&negotiator, negotiation_tips);
+
+	packet_reader_init(&reader, fd[0], NULL, 0,
+			   PACKET_READ_CHOMP_NEWLINE |
+			   PACKET_READ_DIE_ON_ERR_PACKET);
+
+	oid_array_for_each((struct oid_array *) negotiation_tips,
+			   add_to_object_array,
+			   &nt_object_array);
+
+	while (!last_iteration) {
+		int haves_added;
+		struct object_id common_oid;
+		int received_ready = 0;
+
+		strbuf_reset(&req_buf);
+		write_fetch_command_and_capabilities(&req_buf, server_options);
+
+		packet_buf_write(&req_buf, "wait-for-done");
+
+		haves_added = add_haves(&negotiator, &req_buf, &haves_to_send);
+		in_vain += haves_added;
+		if (!haves_added || (seen_ack && in_vain >= MAX_IN_VAIN))
+			last_iteration = 1;
+
+		/* Send request */
+		packet_buf_flush(&req_buf);
+		if (write_in_full(fd[1], req_buf.buf, req_buf.len) < 0)
+			die_errno(_("unable to write request to remote"));
+
+		/* Process ACKs/NAKs */
+		process_section_header(&reader, "acknowledgments", 0);
+		while (process_ack(&negotiator, &reader, &common_oid,
+				   &received_ready)) {
+			struct commit *commit = lookup_commit(the_repository,
+							      &common_oid);
+			if (commit) {
+				timestamp_t generation;
+
+				parse_commit_or_die(commit);
+				commit->object.flags |= COMMON;
+				generation = commit_graph_generation(commit);
+				if (generation < min_generation)
+					min_generation = generation;
+			}
+			in_vain = 0;
+			seen_ack = 1;
+			oidset_insert(acked_commits, &common_oid);
+		}
+		if (received_ready)
+			die(_("unexpected 'ready' from remote"));
+		else
+			do_check_stateless_delimiter(stateless_rpc, &reader);
+		if (can_all_from_reach_with_flag(&nt_object_array, COMMON,
+						 REACH_SCRATCH, 0,
+						 min_generation))
+			last_iteration = 1;
+	}
+	clear_common_flag(acked_commits);
+	strbuf_release(&req_buf);
+}
+
 int report_unmatched_refs(struct ref **sought, int nr_sought)
 {
 	int i, ret = 0;
diff --git a/fetch-pack.h b/fetch-pack.h
index f114d72..7f94a2a 100644
--- a/fetch-pack.h
+++ b/fetch-pack.h
@@ -5,6 +5,7 @@
 #include "run-command.h"
 #include "protocol.h"
 #include "list-objects-filter-options.h"
+#include "oidset.h"
 
 struct oid_array;
 
@@ -82,6 +83,19 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       enum protocol_version version);
 
 /*
+ * Execute the --negotiate-only mode of "git fetch", adding all known common
+ * commits to acked_commits.
+ *
+ * In the capability advertisement that has happened prior to invoking this
+ * function, the "wait-for-done" capability must be present.
+ */
+void negotiate_using_fetch(const struct oid_array *negotiation_tips,
+			   const struct string_list *server_options,
+			   int stateless_rpc,
+			   int fd[],
+			   struct oidset *acked_commits);
+
+/*
  * Print an appropriate error message for each sought ref that wasn't
  * matched.  Return 0 if all sought refs were matched, otherwise 1.
  */
diff --git a/object.h b/object.h
index 87a6da4..8bca310 100644
--- a/object.h
+++ b/object.h
@@ -60,7 +60,7 @@ struct object_array {
 /*
  * object flag allocation:
  * revision.h:               0---------10         15             23------26
- * fetch-pack.c:             01
+ * fetch-pack.c:             01    67
  * negotiator/default.c:       2--5
  * walker.c:                 0-2
  * upload-pack.c:                4       11-----14  16-----19
diff --git a/send-pack.c b/send-pack.c
index 5f215b1..9cb9f71 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -56,7 +56,9 @@ static void feed_object(const struct object_id *oid, FILE *fh, int negative)
 /*
  * Make a pack stream and spit it out into file descriptor fd
  */
-static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struct send_pack_args *args)
+static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
+			struct oid_array *negotiated,
+			struct send_pack_args *args)
 {
 	/*
 	 * The child becomes pack-objects --revs; we feed
@@ -94,8 +96,10 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struc
 	 * parameters by writing to the pipe.
 	 */
 	po_in = xfdopen(po.in, "w");
-	for (i = 0; i < extra->nr; i++)
-		feed_object(&extra->oid[i], po_in, 1);
+	for (i = 0; i < advertised->nr; i++)
+		feed_object(&advertised->oid[i], po_in, 1);
+	for (i = 0; i < negotiated->nr; i++)
+		feed_object(&negotiated->oid[i], po_in, 1);
 
 	while (refs) {
 		if (!is_null_oid(&refs->old_oid))
@@ -409,11 +413,55 @@ static void reject_invalid_nonce(const char *nonce, int len)
 	}
 }
 
+static void get_commons_through_negotiation(const char *url,
+					    const struct ref *remote_refs,
+					    struct oid_array *commons)
+{
+	struct child_process child = CHILD_PROCESS_INIT;
+	const struct ref *ref;
+	int len = the_hash_algo->hexsz + 1; /* hash + NL */
+
+	child.git_cmd = 1;
+	child.no_stdin = 1;
+	child.out = -1;
+	strvec_pushl(&child.args, "fetch", "--negotiate-only", NULL);
+	for (ref = remote_refs; ref; ref = ref->next)
+		strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
+	strvec_push(&child.args, url);
+
+	if (start_command(&child))
+		die(_("send-pack: unable to fork off fetch subprocess"));
+
+	do {
+		char hex_hash[GIT_MAX_HEXSZ + 1];
+		int read_len = read_in_full(child.out, hex_hash, len);
+		struct object_id oid;
+		const char *end;
+
+		if (!read_len)
+			break;
+		if (read_len != len)
+			die("invalid length read %d", read_len);
+		if (parse_oid_hex(hex_hash, &oid, &end) || *end != '\n')
+			die("invalid hash");
+		oid_array_append(commons, &oid);
+	} while (1);
+
+	if (finish_command(&child)) {
+		/*
+		 * The information that push negotiation provides is useful but
+		 * not mandatory.
+		 */
+		warning(_("push negotiation failed; proceeding anyway with push"));
+	}
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
 	      struct oid_array *extra_have)
 {
+	struct oid_array commons = OID_ARRAY_INIT;
 	int in = fd[0];
 	int out = fd[1];
 	struct strbuf req_buf = STRBUF_INIT;
@@ -426,6 +474,7 @@ int send_pack(struct send_pack_args *args,
 	int quiet_supported = 0;
 	int agent_supported = 0;
 	int advertise_sid = 0;
+	int push_negotiate = 0;
 	int use_atomic = 0;
 	int atomic_supported = 0;
 	int use_push_options = 0;
@@ -437,6 +486,10 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config_get_bool("push.negotiate", &push_negotiate);
+	if (push_negotiate)
+		get_commons_through_negotiation(args->url, remote_refs, &commons);
+
 	git_config_get_bool("transfer.advertisesid", &advertise_sid);
 
 	/* Does the other end support the reporting? */
@@ -625,7 +678,7 @@ int send_pack(struct send_pack_args *args,
 			   PACKET_READ_DIE_ON_ERR_PACKET);
 
 	if (need_pack_data && cmds_sent) {
-		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
+		if (pack_objects(out, remote_refs, extra_have, &commons, args) < 0) {
 			if (args->stateless_rpc)
 				close(out);
 			if (git_connection_is_socket(conn))
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index f11742e..0916f76 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -191,6 +191,41 @@ test_expect_success 'fetch with pushInsteadOf (should not rewrite)' '
 	)
 '
 
+grep_wrote () {
+	object_count=$1
+	file_name=$2
+	grep 'write_pack_file/wrote.*"value":"'$1'"' $2
+}
+
+test_expect_success 'push with negotiation' '
+	# Without negotiation
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	echo now pushing without negotiation &&
+	GIT_TRACE2_EVENT="$(pwd)/event" git -c protocol.version=2 push testrepo refs/heads/main:refs/remotes/origin/main &&
+	grep_wrote 5 event && # 2 commits, 2 trees, 1 blob
+
+	# Same commands, but with negotiation
+	rm event &&
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	GIT_TRACE2_EVENT="$(pwd)/event" git -c protocol.version=2 -c push.negotiate=1 push testrepo refs/heads/main:refs/remotes/origin/main &&
+	grep_wrote 2 event # 1 commit, 1 tree
+'
+
+test_expect_success 'push with negotiation proceeds anyway even if negotiation fails' '
+	rm event &&
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	GIT_TEST_PROTOCOL_VERSION=0 GIT_TRACE2_EVENT="$(pwd)/event" \
+		git -c push.negotiate=1 push testrepo refs/heads/main:refs/remotes/origin/main 2>err &&
+	grep_wrote 5 event && # 2 commits, 2 trees, 1 blob
+	test_i18ngrep "push negotiation failed" err
+'
+
 test_expect_success 'push without wildcard' '
 	mk_empty testrepo &&
 
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index 73e74a9..930721f 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -16,7 +16,7 @@ test_expect_success 'test capability advertisement' '
 	version 2
 	agent=git/$(git version | cut -d" " -f3)
 	ls-refs=unborn
-	fetch=shallow
+	fetch=shallow wait-for-done
 	server-option
 	object-format=$(test_oid algo)
 	object-info
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 2e1243c..66af411 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -585,6 +585,49 @@ test_expect_success 'deepen-relative' '
 	test_cmp expected actual
 '
 
+setup_negotiate_only () {
+	SERVER="$1"
+	URI="$2"
+
+	rm -rf "$SERVER" client
+
+	git init "$SERVER"
+	test_commit -C "$SERVER" one
+	test_commit -C "$SERVER" two
+
+	git clone "$URI" client
+	test_commit -C client three
+}
+
+test_expect_success 'file:// --negotiate-only' '
+	SERVER="server" &&
+	URI="file://$(pwd)/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	git -c protocol.version=2 -C client fetch \
+		--no-tags \
+		--negotiate-only \
+		--negotiation-tip=$(git -C client rev-parse HEAD) \
+		origin >out &&
+	COMMON=$(git -C "$SERVER" rev-parse two) &&
+	grep "$COMMON" out
+'
+
+test_expect_success 'file:// --negotiate-only with protocol v0' '
+	SERVER="server" &&
+	URI="file://$(pwd)/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	test_must_fail git -c protocol.version=0 -C client fetch \
+		--no-tags \
+		--negotiate-only \
+		--negotiation-tip=$(git -C client rev-parse HEAD) \
+		origin 2>err &&
+	test_i18ngrep "negotiate-only requires protocol v2" err
+'
+
 # Test protocol v2 with 'http://' transport
 #
 . "$TEST_DIRECTORY"/lib-httpd.sh
@@ -1035,6 +1078,52 @@ test_expect_success 'packfile-uri with transfer.fsckobjects fails when .gitmodul
 	test_i18ngrep "disallowed submodule name" err
 '
 
+test_expect_success 'http:// --negotiate-only' '
+	SERVER="$HTTPD_DOCUMENT_ROOT_PATH/server" &&
+	URI="$HTTPD_URL/smart/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	git -c protocol.version=2 -C client fetch \
+		--no-tags \
+		--negotiate-only \
+		--negotiation-tip=$(git -C client rev-parse HEAD) \
+		origin >out &&
+	COMMON=$(git -C "$SERVER" rev-parse two) &&
+	grep "$COMMON" out
+'
+
+test_expect_success 'http:// --negotiate-only without wait-for-done support' '
+	SERVER="server" &&
+	URI="$HTTPD_URL/one_time_perl/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	echo "s/ wait-for-done/ xxxx-xxx-xxxx/" \
+		>"$HTTPD_ROOT_PATH/one-time-perl" &&
+
+	test_must_fail git -c protocol.version=2 -C client fetch \
+		--no-tags \
+		--negotiate-only \
+		--negotiation-tip=$(git -C client rev-parse HEAD) \
+		origin 2>err &&
+	test_i18ngrep "server does not support wait-for-done" err
+'
+
+test_expect_success 'http:// --negotiate-only with protocol v0' '
+	SERVER="$HTTPD_DOCUMENT_ROOT_PATH/server" &&
+	URI="$HTTPD_URL/smart/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	test_must_fail git -c protocol.version=0 -C client fetch \
+		--no-tags \
+		--negotiate-only \
+		--negotiation-tip=$(git -C client rev-parse HEAD) \
+		origin 2>err &&
+	test_i18ngrep "negotiate-only requires protocol v2" err
+'
+
 # DO NOT add non-httpd-specific tests here, because the last part of this
 # test script is only executed when httpd is available and enabled.
 
diff --git a/transport-helper.c b/transport-helper.c
index 4cd7636..4be035e 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -684,6 +684,16 @@ static int fetch(struct transport *transport,
 		return transport->vtable->fetch(transport, nr_heads, to_fetch);
 	}
 
+	/*
+	 * If we reach here, then the server, the client, and/or the transport
+	 * helper does not support protocol v2. --negotiate-only requires
+	 * protocol v2.
+	 */
+	if (data->transport_options.acked_commits) {
+		warning(_("--negotiate-only requires protocol v2"));
+		return -1;
+	}
+
 	if (!data->get_refs_list_called)
 		get_refs_list_using_list(transport, 0);
 
diff --git a/transport.c b/transport.c
index 62b6eee..6cf3da1 100644
--- a/transport.c
+++ b/transport.c
@@ -392,16 +392,29 @@ static int fetch_refs_via_pack(struct transport *transport,
 	else if (data->version <= protocol_v1)
 		die_if_server_options(transport);
 
+	if (data->options.acked_commits) {
+		if (data->version < protocol_v2) {
+			warning(_("--negotiate-only requires protocol v2"));
+			ret = -1;
+		} else if (!server_supports_feature("fetch", "wait-for-done", 0)) {
+			warning(_("server does not support wait-for-done"));
+			ret = -1;
+		} else {
+			negotiate_using_fetch(data->options.negotiation_tips,
+					      transport->server_options,
+					      transport->stateless_rpc,
+					      data->fd,
+					      data->options.acked_commits);
+			ret = 0;
+		}
+		goto cleanup;
+	}
+
 	refs = fetch_pack(&args, data->fd,
 			  refs_tmp ? refs_tmp : transport->remote_refs,
 			  to_fetch, nr_heads, &data->shallow,
 			  &transport->pack_lockfiles, data->version);
 
-	close(data->fd[0]);
-	close(data->fd[1]);
-	if (finish_connect(data->conn))
-		ret = -1;
-	data->conn = NULL;
 	data->got_remote_heads = 0;
 	data->options.self_contained_and_connected =
 		args.self_contained_and_connected;
@@ -412,6 +425,13 @@ static int fetch_refs_via_pack(struct transport *transport,
 	if (report_unmatched_refs(to_fetch, nr_heads))
 		ret = -1;
 
+cleanup:
+	close(data->fd[0]);
+	close(data->fd[1]);
+	if (finish_connect(data->conn))
+		ret = -1;
+	data->conn = NULL;
+
 	free_refs(refs_tmp);
 	free_refs(refs);
 	return ret;
diff --git a/transport.h b/transport.h
index 4d5db0a..1cbab11 100644
--- a/transport.h
+++ b/transport.h
@@ -47,6 +47,12 @@ struct git_transport_options {
 	 * transport_set_option().
 	 */
 	struct oid_array *negotiation_tips;
+
+	/*
+	 * If allocated, whenever transport_fetch_refs() is called, add known
+	 * common commits to this oidset instead of fetching any packfiles.
+	 */
+	struct oidset *acked_commits;
 };
 
 enum transport_family {
diff --git a/upload-pack.c b/upload-pack.c
index 5c1cd19..297b76f 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -103,6 +103,7 @@ struct upload_pack_data {
 	unsigned use_ofs_delta : 1;
 	unsigned no_progress : 1;
 	unsigned use_include_tag : 1;
+	unsigned wait_for_done : 1;
 	unsigned allow_filter : 1;
 	unsigned allow_filter_fallback : 1;
 	unsigned long tree_filter_max_depth;
@@ -1496,6 +1497,10 @@ static void process_args(struct packet_reader *request,
 			data->done = 1;
 			continue;
 		}
+		if (!strcmp(arg, "wait-for-done")) {
+			data->wait_for_done = 1;
+			continue;
+		}
 
 		/* Shallow related arguments */
 		if (process_shallow(arg, &data->shallows))
@@ -1578,7 +1583,7 @@ static int send_acks(struct upload_pack_data *data, struct oid_array *acks)
 				    oid_to_hex(&acks->oid[i]));
 	}
 
-	if (ok_to_give_up(data)) {
+	if (!data->wait_for_done && ok_to_give_up(data)) {
 		/* Send Ready */
 		packet_writer_write(&data->writer, "ready\n");
 		return 1;
@@ -1668,10 +1673,13 @@ int upload_pack_v2(struct repository *r, struct strvec *keys,
 		case FETCH_PROCESS_ARGS:
 			process_args(request, &data);
 
-			if (!data.want_obj.nr) {
+			if (!data.want_obj.nr && !data.wait_for_done) {
 				/*
-				 * Request didn't contain any 'want' lines,
-				 * guess they didn't want anything.
+				 * Request didn't contain any 'want' lines (and
+				 * the request does not contain
+				 * "wait-for-done", in which it is reasonable
+				 * to just send 'have's without 'want's); guess
+				 * they didn't want anything.
 				 */
 				state = FETCH_DONE;
 			} else if (data.haves.nr) {
@@ -1723,7 +1731,7 @@ int upload_pack_advertise(struct repository *r,
 		int allow_sideband_all_value;
 		char *str = NULL;
 
-		strbuf_addstr(value, "shallow");
+		strbuf_addstr(value, "shallow wait-for-done");
 
 		if (!repo_config_get_bool(the_repository,
 					 "uploadpack.allowfilter",