[PATCH v3] receive-pack: check if client is alive before completing the push

[Robin Jarry <robin.jarry@6wind.com>]

Abort the push operation (i.e. do not migrate the objects from temporary
to permanent storage) if the client has disconnected while the
pre-receive hook was running.

This reduces the risk of inconsistencies on network errors or if the
user hits ctrl-c while the pre-receive hook is running.

Send a keepalive packet (empty) on sideband 2 (the one to report
progress). If the client has exited, receive-pack will be killed via
SIGPIPE and the push will be aborted. This only works when sideband*
capabilities are advertised by the client.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
---
v2 -> v3:
    I had missed Documentation/technical/pack-protocol.txt. Using
    sideband 2 to send the keepalive packet works.

 builtin/receive-pack.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 9f4a0b816cf9..8b0d56897c9f 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1971,6 +1971,15 @@ static void execute_commands(struct command *commands,
 		return;
 	}
 
+	/*
+	 * Send a keepalive packet on sideband 2 (progress info) to ensure that
+	 * the client has not disconnected while pre-receive was running.
+	 */
+	if (use_sideband) {
+		static const char buf[] = "0005\2";
+		write_or_die(1, buf, sizeof(buf) - 1);
+	}
+
 	/*
 	 * Now we'll start writing out refs, which means the objects need
 	 * to be in their final positions so that other processes can see them.
-- 
2.35.0.4.gfdf4c72cdf3d