Disable signalfd by default

signalfd may behave differently to sigaction/signal, so to avoid
breaking libevent users (like [1], [2]) disable it by default.

  [1]: https://github.com/tmux/tmux/pull/3621
  [2]: https://github.com/tmux/tmux/pull/3626

Also signalfd is not that perfect:
- you need to SIG_BLOCK the signal before
  - blocked signals are not reset on exec
  - blocked signals are allowed to coalesce - so in case of multiple
    signals sent you may get the signal only once (ok for most of the
    signals, but may be a problem for SIGCHLD, though you may call
    waitpid() in a loop or use pidfd)
- and also one implementation problem -
  sigprocmask is unspecified in a multithreaded process

Refs:
- https://lwn.net/Articles/415684/
- https://ldpreload.com/blog/signalfd-is-useless

Refs: https://github.com/libevent/libevent/issues/1460
Refs: #1342 (cc @dmantipov)

CMakeLists.txt

@@ -1509,6 +1509,7 @@ if (NOT EVENT__DISABLE_TESTS)
         else()
             add_backend_test(${BACKEND} "${BACKEND_ENV_VARS}")
         endif()
+        add_backend_test(signalfd_${BACKEND} "${BACKEND_ENV_VARS};EVENT_USE_SIGNALFD=1")
     endforeach()
 
     #

include/event2/event.h

@@ -599,9 +599,11 @@ enum event_base_config_flag {
 	 */
 	EVENT_BASE_FLAG_EPOLL_DISALLOW_TIMERFD = 0x40,
 
-	/** Do not use signalfd(2) to handle signals even if supported.
+	/** Use signalfd(2) to handle signals over sigaction/signal.
+	 *
+	 * But note, that in some edge cases signalfd() may works differently.
 	 */
-	EVENT_BASE_FLAG_DISALLOW_SIGNALFD = 0x80,
+	EVENT_BASE_FLAG_USE_SIGNALFD = 0x80,
 };
 
 /**

signalfd.c

@@ -205,8 +205,8 @@ sigfd_del(struct event_base *base, int signo, short old, short events, void *p)
 int sigfd_init_(struct event_base *base)
 {
 	EVUTIL_ASSERT(base != NULL);
-	if ((base->flags & EVENT_BASE_FLAG_DISALLOW_SIGNALFD) ||
-	    getenv("EVENT_DISALLOW_SIGNALFD"))
+	if (!(base->flags & EVENT_BASE_FLAG_USE_SIGNALFD) &&
+	    !getenv("EVENT_USE_SIGNALFD"))
 		return -1;
 	base->evsigsel = &sigfdops;
 	return 0;

test/include.am

@@ -80,6 +80,8 @@ test_runner_changelist: $(top_srcdir)/test/test.sh
 	$(top_srcdir)/test/test.sh -b "" -c
 test_runner_timerfd_changelist: $(top_srcdir)/test/test.sh
 	$(top_srcdir)/test/test.sh -b "" -T
+test_runner_timerfd_changelist: $(top_srcdir)/test/test.sh
+	$(top_srcdir)/test/test.sh -b "" -S
 
 DISTCLEANFILES += test/regress.gen.c test/regress.gen.h
 

test/test.sh

@@ -50,6 +50,7 @@ setup () {
 	done
 	unset EVENT_EPOLL_USE_CHANGELIST
 	unset EVENT_PRECISE_TIMER
+	unset EVENT_USE_SIGNALFD
 }
 
 announce () {
@@ -138,10 +139,12 @@ do_test() {
 	    EVENT_EPOLL_USE_CHANGELIST=yes; export EVENT_EPOLL_USE_CHANGELIST
 	elif test "$2" = "(timerfd)" ; then
 	    EVENT_PRECISE_TIMER=1; export EVENT_PRECISE_TIMER
+	elif test "$2" = "(signalfd)" ; then
+	    EVENT_USE_SIGNALFD=1; export EVENT_USE_SIGNALFD
 	elif test "$2" = "(timerfd+changelist)" ; then
 	    EVENT_EPOLL_USE_CHANGELIST=yes; export EVENT_EPOLL_USE_CHANGELIST
 	    EVENT_PRECISE_TIMER=1; export EVENT_PRECISE_TIMER
-        fi
+	fi
 
 	run_tests
 }
@@ -153,6 +156,7 @@ usage()
   -t   - run timerfd test
   -c   - run changelist test
   -T   - run timerfd+changelist test
+  -S   - run signalfd test
 EOL
 }
 main()
@@ -161,13 +165,15 @@ main()
 	timerfd=0
 	changelist=0
 	timerfd_changelist=0
+	signalfd=0
 
-	while getopts "b:tcT" c; do
+	while getopts "b:tcTS" c; do
 		case "$c" in
 			b) backends="$OPTARG";;
 			t) timerfd=1;;
 			c) changelist=1;;
 			T) timerfd_changelist=1;;
+			S) signalfd=1;;
 			?*) usage && exit 1;;
 		esac
 	done
@@ -179,6 +185,7 @@ main()
 	[ $timerfd_changelist -eq 0 ] || do_test EPOLL "(timerfd+changelist)"
 	for i in $backends; do
 		do_test $i
+		[ $signalfd -eq 0 ] || do_test $i "(signalfd)"
 	done
 
 	if test "$FAILED" = "yes"; then