receive-pack: allow hooks to ignore its standard input stream

The pre-receive and post-receive hooks were designed to be an
improvement over old style update and post-update hooks, which take
the update information on their command line and are limited by the
command line length limit.  The same information is fed from the
standard input to pre/post-receive hooks instead to lift this
limitation.  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.

If a hook does not want to look at all the information, it is easy
to send 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),
and this has already been done by existing hooks that are written
carefully.

However, because there is no good way to consistently fail hooks
that do not consume the input fully (a small push may result in a
short update record that may fit within the pipe buffer, to which
the receive-pack process may manage to write before the hook has a
chance to exit without reading anything, which will not result in a
death-by-SIGPIPE of receive-pack), it can lead to a hard to diagnose
"once in a blue moon" phantom failure.

Lift this "hooks must consume their input fully" mandate.  A mandate
that is not enforced strictly is not helping us to catch mistakes in
hooks.  If a hook has a good reason to decide the outcome of its
operation without reading the information we feed it, let it do so
as it pleases.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index f93ac454b4133f5c1e7cb1675a618b4c0b3174cd..516386fdd3b757ecde1402c1ce02e9be2f540b9f 100644 (file)
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -15,6 +15,7 @@
 #include "connected.h"
 #include "argv-array.h"
 #include "version.h"
+#include "sigchain.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
 
@@ -288,6 +289,8 @@ 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;
@@ -299,6 +302,9 @@ 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);
 }
 

diff --git a/t/t5401-update-hooks.sh b/t/t5401-update-hooks.sh
index 17bcb0b04096eabb29c13a025fd2afe8d1e4623b..7f278d8ce932f34420c337571ec043ffb5e318e9 100755 (executable)
--- a/t/t5401-update-hooks.sh
+++ b/t/t5401-update-hooks.sh
@@ -135,4 +135,17 @@ test_expect_success 'send-pack stderr contains hook messages' '
        test_cmp expect actual
 '
 
+test_expect_success 'pre-receive hook that forgets to read its input' '
+       write_script victim.git/hooks/pre-receive <<-\EOF &&
+       exit 0
+       EOF
+       rm -f victim.git/hooks/update victim.git/hooks/post-update &&
+
+       for v in $(test_seq 100 999)
+       do
+               git branch branch_$v master || return
+       done &&
+       git push ./victim.git "+refs/heads/*:refs/heads/*"
+'
+
 test_done