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, 3 insertions, 2 deletions

diff --git a/builtin-fetch.c b/builtin-fetch.c
index de6f3074b1..8c86974cbe 100644
--- a/builtin-fetch.c
+++ b/builtin-fetch.c
@@ -10,6 +10,7 @@
 #include "transport.h"
 #include "run-command.h"
 #include "parse-options.h"
+#include "sigchain.h"
 
 static const char * const builtin_fetch_usage[] = {
 	"git fetch [options] [<repository> <refspec>...]",
@@ -58,7 +59,7 @@ static void unlock_pack(void)
 static void unlock_pack_on_signal(int signo)
 {
 	unlock_pack();
-	signal(SIGINT, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -672,7 +673,7 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		ref_nr = j;
 	}
 
-	signal(SIGINT, unlock_pack_on_signal);
+	sigchain_push(SIGINT, unlock_pack_on_signal);
 	atexit(unlock_pack);
 	exit_code = do_fetch(transport,
 			parse_fetch_refspec(ref_nr, refs), ref_nr);