[PATCH 4/7] builtin/gc: stop processing log file on signal

From: Patrick Steinhardt <ps@pks.im>
Date: Aug 13, 2024, 07:17 UTC
Message-ID: <812c61c9b66d7608e41c6c1d00a6e22e995cef06.1723533091.git.ps@pks.im>
In-Reply-To: <cover.1723533091.git.ps@pks.im>

When detaching, git-gc(1) will redirect its stderr to a "gc.log" log file, which is then used to surface errors of a backgrounded process to the user. To ensure that the file is properly managed on abnormal exit paths, we install both signal and exit handlers that try to either commit the underlying lock file or roll it back in case there wasn't any error.

This logic is severly broken when handling signals though, as we end up calling all kinds of functions that are not signal safe. This includes malloc(3P) via `git_path()`, fprintf(3P), fflush(3P) and many more functions. The consequence can be anything, from deadlocks to crashes. Unfortunately, we cannot really do much about this without a larger refactoring.

The least-worst thing we can do is to not set up the signal handler in the first place. This will still cause us to remove the lockfile, as the underlying tempfile subsystem already knows to unlink locks when receiving a signal. But it may cause us to remove the lock even in the case where it would have contained actual errors, which is a change in behaviour.

The consequence is that "gc.log" will not be committed, and thus subsequent calls to `git gc --auto` won't bail out because of this. Arguably though, it is better to retry garbage collection rather than having the process run into a potentially-corrupted state.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/gc.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index a93cfa147e..f815557081 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -109,13 +109,6 @@ static void process_log_file_at_exit(void)
 	process_log_file();
 }
 
-static void process_log_file_on_signal(int signo)
-{
-	process_log_file();
-	sigchain_pop(signo);
-	raise(signo);
-}
-
 static int gc_config_is_timestamp_never(const char *var)
 {
 	const char *value;
@@ -807,7 +800,6 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 					  git_path("gc.log"),
 					  LOCK_DIE_ON_ERROR);
 		dup2(get_lock_file_fd(&log_lock), 2);
-		sigchain_push_common(process_log_file_on_signal);
 		atexit(process_log_file_at_exit);
 	}

-- 
2.46.0.46.g406f326d27.dirty