[PATCH v2] send-pack: never fetch when checking exclusions

[Jonathan Tan <jonathantanmy@google.com>]

When building the packfile to be sent, send_pack() is given a list of
remote refs to be used as exclusions. For each ref, it first checks if
the ref exists locally, and if it does, passes it with a "^" prefix to
pack-objects. However, in a partial clone, the check may trigger a lazy
fetch.

The additional commit ancestry information obtained during such fetches
may show that certain objects that would have been sent are already
known to the server, resulting in a smaller pack being sent. But this is
at the cost of fetching from many possibly unrelated refs, and the lazy
fetches do not help at all in the typical case where the client is
up-to-date with the upstream of the branch being pushed.

Ensure that these lazy fetches do not occur.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
> For example, would this change mean that the resulting pack may
> include stuff that are reachable from the (missing) negative objects
> that would not otherwise have to be sent if these objects were
> available (or made available by the lazy fetching), and we are
> making a trade-off to send possibly more in order for not fetching?
> Have we laid enough on the table to help readers if such a trade-off
> (if we are making one, that is) strikes the right balance?

Thanks for your comments. I've expanded the commit message.
---
 send-pack.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/send-pack.c b/send-pack.c
index 6dc16c3211..34c77cbb1a 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -40,7 +40,8 @@ int option_parse_push_signed(const struct option *opt,
 
 static void feed_object(const struct object_id *oid, FILE *fh, int negative)
 {
-	if (negative && !has_object_file(oid))
+	if (negative &&
+	    !has_object_file_with_flags(oid, OBJECT_INFO_SKIP_FETCH_OBJECT))
 		return;
 
 	if (negative)
-- 
2.23.0.581.g78d2f28ef7-goog