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>

http-push.c

@@ -10,6 +10,7 @@
 #include "exec_cmd.h"
 #include "remote.h"
 #include "list-objects.h"
+#include "sigchain.h"
 
 #include <expat.h>
 
@@ -1363,7 +1364,7 @@ static void remove_locks(void)
 static void remove_locks_on_signal(int signo)
 {
 	remove_locks();
-	signal(signo, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -2261,10 +2262,10 @@ int main(int argc, char **argv)
 		goto cleanup;
 	}
 
-	signal(SIGINT, remove_locks_on_signal);
-	signal(SIGHUP, remove_locks_on_signal);
-	signal(SIGQUIT, remove_locks_on_signal);
-	signal(SIGTERM, remove_locks_on_signal);
+	sigchain_push(SIGINT, remove_locks_on_signal);
+	sigchain_push(SIGHUP, remove_locks_on_signal);
+	sigchain_push(SIGQUIT, remove_locks_on_signal);
+	sigchain_push(SIGTERM, remove_locks_on_signal);
 
 	/* Check whether the remote has server info files */
 	remote->can_update_info_refs = 0;