chain kill signals for cleanup functions

If a piece of code wanted to do some cleanup before exiting
(e.g., cleaning up a lockfile or a tempfile), our usual
strategy was to install a signal handler that did something
like this:

  do_cleanup(); /* actual work */
  signal(signo, SIG_DFL); /* restore previous behavior */
  raise(signo); /* deliver signal, killing ourselves */

For a single handler, this works fine. However, if we want
to clean up two _different_ things, we run into a problem.
The most recently installed handler will run, but when it
removes itself as a handler, it doesn't put back the first
handler.

This patch introduces sigchain, a tiny library for handling
a stack of signal handlers. You sigchain_push each handler,
and use sigchain_pop to restore whoever was before you in
the stack.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/lockfile.c b/lockfile.c
index 858915553..3cd57dc38 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -2,6 +2,7 @@
  * Copyright (c) 2005, Junio C Hamano
  */
 #include "cache.h"
+#include "sigchain.h"
 
 static struct lock_file *lock_file_list;
 static const char *alternate_index_output;
@@ -24,7 +25,7 @@ static void remove_lock_file(void)
 static void remove_lock_file_on_signal(int signo)
 {
 	remove_lock_file();
-	signal(signo, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -136,11 +137,11 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 	lk->fd = open(lk->filename, O_RDWR | O_CREAT | O_EXCL, 0666);
 	if (0 <= lk->fd) {
 		if (!lock_file_list) {
-			signal(SIGINT, remove_lock_file_on_signal);
-			signal(SIGHUP, remove_lock_file_on_signal);
-			signal(SIGTERM, remove_lock_file_on_signal);
-			signal(SIGQUIT, remove_lock_file_on_signal);
-			signal(SIGPIPE, remove_lock_file_on_signal);
+			sigchain_push(SIGINT, remove_lock_file_on_signal);
+			sigchain_push(SIGHUP, remove_lock_file_on_signal);
+			sigchain_push(SIGTERM, remove_lock_file_on_signal);
+			sigchain_push(SIGQUIT, remove_lock_file_on_signal);
+			sigchain_push(SIGPIPE, remove_lock_file_on_signal);
 			atexit(remove_lock_file);
 		}
 		lk->owner = getpid();