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-clone.c b/builtin-clone.c
index f1a1a0c36..18b939233 100644
--- a/builtin-clone.c
+++ b/builtin-clone.c
@@ -19,6 +19,7 @@
 #include "strbuf.h"
 #include "dir.h"
 #include "pack-refs.h"
+#include "sigchain.h"
 
 /*
  * Overall FIXMEs:
@@ -288,7 +289,7 @@ static void remove_junk(void)
 static void remove_junk_on_signal(int signo)
 {
 	remove_junk();
-	signal(SIGINT, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -438,7 +439,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	}
 	junk_git_dir = git_dir;
 	atexit(remove_junk);
-	signal(SIGINT, remove_junk_on_signal);
+	sigchain_push(SIGINT, remove_junk_on_signal);
 
 	setenv(CONFIG_ENVIRONMENT, xstrdup(mkpath("%s/config", git_dir)), 1);