[PATCH v2] fetch-pack: do not reset in_vain on non-novel acks

[Jonathan Tan <jonathantanmy@google.com>]

The MAX_IN_VAIN mechanism was introduced in commit f061e5f ("fetch-pack:
give up after getting too many "ack continue"", 2006-05-24) to stop ref
negotiation if a number of consecutive "have"s have been sent with no
corresponding new acks. This is to stop the client from digging too deep
in an irrelevant side branch in vain without ever finding a common
ancestor. A use case (as described in that commit) is the scenario in
which the local repository has more roots than the remote repository.

However, during a negotiation in which stateless RPCs are used,
MAX_IN_VAIN will (almost) never trigger (in the more-roots scenario
above and others) because in each new request, the client has to inform
the server of objects it already has and knows the server has (to remind
the server of the state), which the server then acks.

Make fetch-pack only consider, as new acks for the purpose of
MAX_IN_VAIN, acks for objects for which the client has never received an
ack before in this session.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---

Thanks for your comments - I really appreciate them.

Update from original:
o removed redundant text from commit message and comment in patch
o mentioned stopping the client from digging too deep in the commit
  message

I tried looking at creating a helper function to reduce both the size
and the nesting level of the loop, but it seems to me that a helper
function can't be extracted so easily because the logic is quite
intertwined with the rest of the function. For example, the "if
(args->stateless_rpc..." block uses 6 variables from the outer scope:
args, ack, commit, result_sha1, req_buf, and state_len (and in_vain, but
this can be the return value of the function). Expanding it wider would
allow us to make some of those 6 local, but also introduce new ones from
the outer scope.

 fetch-pack.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fetch-pack.c b/fetch-pack.c
index 85e77af..413937e 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -428,10 +428,17 @@ static int find_common(struct fetch_pack_args *args,
 						const char *hex = sha1_to_hex(result_sha1);
 						packet_buf_write(&req_buf, "have %s\n", hex);
 						state_len = req_buf.len;
-					}
+						/*
+						 * Reset in_vain because an ack
+						 * for this commit has not been
+						 * seen.
+						 */
+						in_vain = 0;
+					} else if (!args->stateless_rpc
+						   || ack != ACK_common)
+						in_vain = 0;
 					mark_common(commit, 0, 1);
 					retval = 0;
-					in_vain = 0;
 					got_continue = 1;
 					if (ack == ACK_ready) {
 						clear_prio_queue(&rev_list);
-- 
2.8.0.rc3.226.g39d4020