fetch: teach independent negotiation (no packfile)

Currently, the packfile negotiation step within a Git fetch cannot be
done independent of sending the packfile, even though there is at least
one application wherein this is useful. Therefore, make it possible for
this negotiation step to be done independently. A subsequent commit will
use this for one such application - push negotiation.

This feature is for protocol v2 only. (An implementation for protocol v0
would require a separate implementation in the fetch, transport, and
transport helper code.)

In the protocol, the main hindrance towards independent negotiation is
that the server can unilaterally decide to send the packfile. This is
solved by a "wait-for-done" argument: the server will then wait for the
client to say "done". In practice, the client will never say it; instead
it will cease requests once it is satisfied.

In the client, the main change lies in the transport and transport
helper code. fetch_refs_via_pack() performs everything needed - protocol
version and capability checks, and the negotiation itself.

There are 2 code paths that do not go through fetch_refs_via_pack() that
needed to be individually excluded: the bundle transport (excluded
through requiring smart_options, which the bundle transport doesn't
support) and transport helpers that do not support takeover. If or when
we support independent negotiation for protocol v0, we will need to
modify these 2 code paths to support it. But for now, report failure if
independent negotiation is requested in these cases.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/fetch-pack.h b/fetch-pack.h
index f114d72775..7f94a2a583 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.
  */