[RFC] allowing hooks to ignore input?

[Junio C Hamano <gitster@pobox.com>]

The pre-receive and post-receive hooks were designed to be an
improvement over old style update and post-update hooks that used to
take the update information on the command line and were limited by
the command line length limit.  They take the same information from
their standard input.  It has been mandatory for these new style
hooks to consume the update information fully from the standard
input stream.  Otherwise, they would risk killing the receive-pack
process via SIGPIPE.

This is easy, and it has already been done by existing hooks that
are written correctly, to work around, if a hook does not want to
look at all the information, by sending its standard input to
/dev/null (perhaps a niche use of hook might need to know only the
fact that a push was made, without having to know what objects have
been pushed to update which refs).

However, because there is no good way to consistently fail hooks
that do not consume the input fully, it can lead to a hard to
diagnose "once in a blue moon" phantom failure.

I wonder if this "you must consume the input fully", which is a
mandate that is not enforced strictly, is not helping us to catch
mistakes in hooks more than it is hurting us.  Perhaps we can do
something like the attached patch to make it optional for them to
read the input we feed?

I dunno.

 builtin/receive-pack.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 30560a7..9d9d16d 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -17,6 +17,7 @@
 #include "version.h"
 #include "tag.h"
 #include "gpg-interface.h"
+#include "sigchain.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
 
@@ -500,6 +501,8 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 		return code;
 	}
 
+	sigchain_push(SIGPIPE, SIG_IGN);
+
 	while (1) {
 		const char *buf;
 		size_t n;
@@ -511,6 +514,9 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 	close(proc.in);
 	if (use_sideband)
 		finish_async(&muxer);
+
+	sigchain_pop(SIGPIPE);
+
 	return finish_command(&proc);
 }