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>

12 files changed, 125 insertions, 19 deletions

diff --git a/.gitignore b/.gitignore
index d9adce585a..f28a54d262 100644
--- a/.gitignore
+++ b/.gitignore
@@ -152,6 +152,7 @@ test-match-trees
 test-parse-options
 test-path-utils
 test-sha1
+test-sigchain
 common-cmds.h
 *.tar.gz
 *.dsc
diff --git a/Makefile b/Makefile
index 2b873fa99f..fd02decc01 100644
--- a/Makefile
+++ b/Makefile
@@ -388,6 +388,7 @@ LIB_H += revision.h
 LIB_H += run-command.h
 LIB_H += sha1-lookup.h
 LIB_H += sideband.h
+LIB_H += sigchain.h
 LIB_H += strbuf.h
 LIB_H += tag.h
 LIB_H += transport.h
@@ -481,6 +482,7 @@ LIB_OBJS += sha1-lookup.o
 LIB_OBJS += sha1_name.o
 LIB_OBJS += shallow.o
 LIB_OBJS += sideband.o
+LIB_OBJS += sigchain.o
 LIB_OBJS += strbuf.o
 LIB_OBJS += symlinks.o
 LIB_OBJS += tag.o
@@ -1364,6 +1366,7 @@ TEST_PROGRAMS += test-match-trees$X
 TEST_PROGRAMS += test-parse-options$X
 TEST_PROGRAMS += test-path-utils$X
 TEST_PROGRAMS += test-sha1$X
+TEST_PROGRAMS += test-sigchain$X
 
 all:: $(TEST_PROGRAMS)
 
diff --git a/builtin-clone.c b/builtin-clone.c
index f1a1a0c365..18b9392334 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);
 
diff --git a/builtin-fetch--tool.c b/builtin-fetch--tool.c
index 469b07e240..b1d7f8fb32 100644
--- a/builtin-fetch--tool.c
+++ b/builtin-fetch--tool.c
@@ -2,6 +2,7 @@
 #include "cache.h"
 #include "refs.h"
 #include "commit.h"
+#include "sigchain.h"
 
 static char *get_stdin(void)
 {
@@ -186,7 +187,7 @@ static void remove_keep(void)
 static void remove_keep_on_signal(int signo)
 {
 	remove_keep();
-	signal(SIGINT, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -245,7 +246,7 @@ static int fetch_native_store(FILE *fp,
 	char buffer[1024];
 	int err = 0;
 
-	signal(SIGINT, remove_keep_on_signal);
+	sigchain_push(SIGINT, remove_keep_on_signal);
 	atexit(remove_keep);
 
 	while (fgets(buffer, sizeof(buffer), stdin)) {
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);
diff --git a/diff.c b/diff.c
index 3cfc0b636c..9c9977d892 100644
--- a/diff.c
+++ b/diff.c
@@ -12,6 +12,7 @@
 #include "run-command.h"
 #include "utf8.h"
 #include "userdiff.h"
+#include "sigchain.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -188,7 +189,7 @@ static void remove_tempfile(void)
 static void remove_tempfile_on_signal(int signo)
 {
 	remove_tempfile();
-	signal(SIGINT, SIG_DFL);
+	sigchain_pop(signo);
 	raise(signo);
 }
 
@@ -1902,7 +1903,7 @@ static struct diff_tempfile *prepare_temp_file(const char *name,
 
 	if (!remove_tempfile_installed) {
 		atexit(remove_tempfile);
-		signal(SIGINT, remove_tempfile_on_signal);
+		sigchain_push(SIGINT, remove_tempfile_on_signal);
 		remove_tempfile_installed = 1;
 	}
 
diff --git a/http-push.c b/http-push.c
index a4b7d08663..dec395deed 100644
--- a/http-push.c
+++ b/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;
diff --git a/lockfile.c b/lockfile.c
index 8589155532..3cd57dc385 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();
diff --git a/sigchain.c b/sigchain.c
new file mode 100644
index 0000000000..a18d505e56
--- /dev/null
+++ b/sigchain.c
@@ -0,0 +1,43 @@
+#include "sigchain.h"
+#include "cache.h"
+
+#define SIGCHAIN_MAX_SIGNALS 32
+
+struct sigchain_signal {
+	sigchain_fun *old;
+	int n;
+	int alloc;
+};
+static struct sigchain_signal signals[SIGCHAIN_MAX_SIGNALS];
+
+static void check_signum(int sig)
+{
+	if (sig < 1 || sig >= SIGCHAIN_MAX_SIGNALS)
+		die("BUG: signal out of range: %d", sig);
+}
+
+int sigchain_push(int sig, sigchain_fun f)
+{
+	struct sigchain_signal *s = signals + sig;
+	check_signum(sig);
+
+	ALLOC_GROW(s->old, s->n + 1, s->alloc);
+	s->old[s->n] = signal(sig, f);
+	if (s->old[s->n] == SIG_ERR)
+		return -1;
+	s->n++;
+	return 0;
+}
+
+int sigchain_pop(int sig)
+{
+	struct sigchain_signal *s = signals + sig;
+	check_signum(sig);
+	if (s->n < 1)
+		return 0;
+
+	if (signal(sig, s->old[s->n - 1]) == SIG_ERR)
+		return -1;
+	s->n--;
+	return 0;
+}
diff --git a/sigchain.h b/sigchain.h
new file mode 100644
index 0000000000..254ebb0fa6
--- /dev/null
+++ b/sigchain.h
@@ -0,0 +1,9 @@
+#ifndef SIGCHAIN_H
+#define SIGCHAIN_H
+
+typedef void (*sigchain_fun)(int);
+
+int sigchain_push(int sig, sigchain_fun f);
+int sigchain_pop(int sig);
+
+#endif /* SIGCHAIN_H */
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
diff --git a/test-sigchain.c b/test-sigchain.c
new file mode 100644
index 0000000000..8747deac62
--- /dev/null
+++ b/test-sigchain.c
@@ -0,0 +1,22 @@
+#include "sigchain.h"
+#include "cache.h"
+
+#define X(f) \
+static void f(int sig) { \
+	puts(#f); \
+	fflush(stdout); \
+	sigchain_pop(sig); \
+	raise(sig); \
+}
+X(one)
+X(two)
+X(three)
+#undef X
+
+int main(int argc, char **argv) {
+	sigchain_push(SIGINT, one);
+	sigchain_push(SIGINT, two);
+	sigchain_push(SIGINT, three);
+	raise(SIGINT);
+	return 0;
+}