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, 22 insertions, 0 deletions

diff --git a/t/t0005-signals.sh b/t/t0005-signals.sh
new file mode 100755
index 0000000000..9707af7d03
--- /dev/null
+++ b/t/t0005-signals.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+
+test_description='signals work as we expect'
+. ./test-lib.sh
+
+cat >expect <<EOF
+three
+two
+one
+EOF
+
+test_expect_success 'sigchain works' '
+	test-sigchain >actual
+	case "$?" in
+	130) true ;; # POSIX w/ SIGINT=2
+	  3) true ;; # Windows
+	  *) false ;;
+	esac &&
+	test_cmp expect actual
+'
+
+test_done