Make this pattern work

    event_init();
    signal_set(&ev, SIGCHLD, sigchldhandler, NULL);
    signal_add(&ev, NULL);
    if (fork() == 0) {
        ...
    }
    ...
    event_dispatch();

even if the child exits before event_dispatch() is reached.

With this change, evsignal_process() runs with signals unblocked, so
timing is critical. The original code is then racy:

    TAILQ_FOREACH(ev, &signalqueue, ev_signal_next) {
            ncalls = evsigcaught[EVENT_SIGNAL(ev)];         <-- point A
            if (ncalls) {
                    if (!(ev->ev_events & EV_PERSIST))
                            event_del(ev);
                    event_active(ev, EV_SIGNAL, ncalls);
            }
    }

    memset(evsigcaught, 0, sizeof(evsigcaught));            <-- point B
    evsignal_caught = 0;                                    <-- point C

For any signal that is not already pending (ncalls == 0 when reaching
its point A), if it arrives between then and its ncalls being cleared in
point B, it will be lost. If it arrives after then but before
evsignal_caught in point C, it will be arbitrarily delayed (until
another signal comes along to set evsignal_caught).

Solve by

* moving the evsignal_caught to before checking the individual signals
  for delivery (but after evsignal_caught is checked itself, in a different
  function).
* setting "evsigcaught[x] = 0" only when we are setting that signal active.

Index: devpoll.c
===================================================================
RCS file: /cvsroot/levent/libevent/devpoll.c,v
retrieving revision 1.9
diff -u -r1.9 devpoll.c
--- devpoll.c	15 Jul 2006 02:39:31 -0000	1.9
+++ devpoll.c	19 Oct 2006 04:42:22 -0000
@@ -66,7 +66,6 @@
 	struct pollfd *events;
 	int nevents;
 	int dpfd;
-	sigset_t evsigmask;
 	struct pollfd *changes;
 	int nchanges;
 };
@@ -180,7 +179,7 @@
 		return (NULL);
 	}
 
-	evsignal_init(&devpollop->evsigmask);
+	evsignal_init();
 
 	return (devpollop);
 }
@@ -209,7 +208,7 @@
 		devpollop->nfds = nfds;
 	}
 
-	return (evsignal_recalc(&devpollop->evsigmask));
+	return (0);
 }
 
 int
@@ -221,9 +220,6 @@
 	struct evdevpoll *evdp;
 	int i, res, timeout;
 
-	if (evsignal_deliver(&devpollop->evsigmask) == -1)
-		return (-1);
-
 	if (devpollop->nchanges)
 		devpoll_commit(devpollop);
 
@@ -235,9 +231,6 @@
 
 	res = ioctl(devpollop->dpfd, DP_POLL, &dvp);
 
-	if (evsignal_recalc(&devpollop->evsigmask) == -1)
-		return (-1);
-
 	if (res == -1) {
 		if (errno != EINTR) {
 			event_warn("ioctl: DP_POLL");
@@ -301,7 +294,7 @@
 	int fd, events;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_add(&devpollop->evsigmask, ev));
+		return (evsignal_add(ev));
 
 	fd = ev->ev_fd;
 	if (fd >= devpollop->nfds) {
@@ -356,7 +349,7 @@
 	int needwritedelete = 1, needreaddelete = 1;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_del(&devpollop->evsigmask, ev));
+		return (evsignal_del(ev));
 
 	fd = ev->ev_fd;
 	if (fd >= devpollop->nfds)
Index: epoll.c
===================================================================
RCS file: /cvsroot/levent/libevent/epoll.c,v
retrieving revision 1.18
diff -u -r1.18 epoll.c
--- epoll.c	28 Mar 2006 04:40:54 -0000	1.18
+++ epoll.c	19 Oct 2006 04:42:22 -0000
@@ -68,7 +68,6 @@
 	struct epoll_event *events;
 	int nevents;
 	int epfd;
-	sigset_t evsigmask;
 };
 
 void *epoll_init	(void);
@@ -150,7 +149,7 @@
 	}
 	epollop->nfds = nfiles;
 
-	evsignal_init(&epollop->evsigmask);
+	evsignal_init();
 
 	return (epollop);
 }
@@ -179,7 +178,7 @@
 		epollop->nfds = nfds;
 	}
 
-	return (evsignal_recalc(&epollop->evsigmask));
+	return (0);
 }
 
 int
@@ -190,15 +189,9 @@
 	struct evepoll *evep;
 	int i, res, timeout;
 
-	if (evsignal_deliver(&epollop->evsigmask) == -1)
-		return (-1);
-
 	timeout = tv->tv_sec * 1000 + (tv->tv_usec + 999) / 1000;
 	res = epoll_wait(epollop->epfd, events, epollop->nevents, timeout);
 
-	if (evsignal_recalc(&epollop->evsigmask) == -1)
-		return (-1);
-
 	if (res == -1) {
 		if (errno != EINTR) {
 			event_warn("epoll_wait");
@@ -262,7 +255,7 @@
 	int fd, op, events;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_add(&epollop->evsigmask, ev));
+		return (evsignal_add(ev));
 
 	fd = ev->ev_fd;
 	if (fd >= epollop->nfds) {
@@ -311,7 +304,7 @@
 	int needwritedelete = 1, needreaddelete = 1;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_del(&epollop->evsigmask, ev));
+		return (evsignal_del(ev));
 
 	fd = ev->ev_fd;
 	if (fd >= epollop->nfds)
Index: evport.c
===================================================================
RCS file: /cvsroot/levent/libevent/evport.c,v
retrieving revision 1.2
diff -u -r1.2 evport.c
--- evport.c	9 Aug 2006 01:04:12 -0000	1.2
+++ evport.c	19 Oct 2006 04:42:22 -0000
@@ -117,7 +117,6 @@
 
 struct evport_data {
 	int 		ed_port;	/* event port for system events  */
-	sigset_t 	ed_sigmask;	/* for evsignal 		 */
 	int		ed_nevents;	/* number of allocated fdi's 	 */
 	struct fd_info *ed_fds;		/* allocated fdi table 		 */
 	/* fdi's that we need to reassoc */
@@ -173,7 +172,7 @@
 	evpd->ed_nevents = DEFAULT_NFDS;
 	memset(&evpd->ed_pending, 0, EVENTS_PER_GETN * sizeof(struct fd_info*));
 
-	evsignal_init(&evpd->ed_sigmask);
+	evsignal_init();
 
 	return (evpd);
 }
@@ -330,11 +329,6 @@
 		}
 	}
 
-	
-
-	if (evsignal_deliver(&epdp->ed_sigmask) == -1)
-		return (-1);
-
 	if ((res = port_getn(epdp->ed_port, pevtlist, EVENTS_PER_GETN, 
 		    &nevents, &ts)) == -1) {
 		if (errno == EINTR) {
@@ -410,9 +404,6 @@
 
 	check_evportop(epdp);
 
-	if (evsignal_recalc(&epdp->ed_sigmask) == -1)
-		return (-1);
-
 	return (0);
 }
 
@@ -426,7 +417,7 @@
 {
 	struct evport_data *evpd = arg;
 	check_evportop(evpd);
-	return (evsignal_recalc(&evpd->ed_sigmask));
+	return (0);
 }
 
 
@@ -448,7 +439,7 @@
 	 * Delegate, if it's not ours to handle.
 	 */
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_add(&evpd->ed_sigmask, ev));
+		return (evsignal_add(ev));
 
 	/*
 	 * If necessary, grow the file descriptor info table
@@ -489,7 +480,7 @@
 	 * Delegate, if it's not ours to handle
 	 */
 	if (ev->ev_events & EV_SIGNAL) {
-		return (evsignal_del(&evpd->ed_sigmask, ev));
+		return (evsignal_del(ev));
 	}
 
 	if (evpd->ed_nevents < ev->ev_fd) {
Index: evsignal.h
===================================================================
RCS file: /cvsroot/levent/libevent/evsignal.h,v
retrieving revision 1.2
diff -u -r1.2 evsignal.h
--- evsignal.h	4 Oct 2003 23:27:26 -0000	1.2
+++ evsignal.h	19 Oct 2006 04:42:22 -0000
@@ -27,11 +27,9 @@
 #ifndef _EVSIGNAL_H_
 #define _EVSIGNAL_H_
 
-void evsignal_init(sigset_t *);
+void evsignal_init(void);
 void evsignal_process(void);
-int evsignal_recalc(sigset_t *);
-int evsignal_deliver(sigset_t *);
-int evsignal_add(sigset_t *, struct event *);
-int evsignal_del(sigset_t *, struct event *);
+int evsignal_add(struct event *);
+int evsignal_del(struct event *);
 
 #endif /* _EVSIGNAL_H_ */
Index: poll.c
===================================================================
RCS file: /cvsroot/levent/libevent/poll.c,v
retrieving revision 1.21
diff -u -r1.21 poll.c
--- poll.c	28 Mar 2006 04:40:54 -0000	1.21
+++ poll.c	19 Oct 2006 04:42:23 -0000
@@ -66,7 +66,6 @@
 	int *idxplus1_by_fd; /* Index into event_set by fd; we add 1 so
 			      * that 0 (which is easy to memset) can mean
 			      * "no entry." */
-	sigset_t evsigmask;
 };
 
 void *poll_init	(void);
@@ -98,7 +97,7 @@
 	if (!(pollop = calloc(1, sizeof(struct pollop))))
 		return (NULL);
 
-	evsignal_init(&pollop->evsigmask);
+	evsignal_init();
 
 	return (pollop);
 }
@@ -111,9 +110,7 @@
 int
 poll_recalc(struct event_base *base, void *arg, int max)
 {
-	struct pollop *pop = arg;
-
-	return (evsignal_recalc(&pop->evsigmask));
+	return (0);
 }
 
 #ifdef CHECK_INVARIANTS
@@ -156,17 +153,11 @@
 	int res, i, sec, nfds;
 	struct pollop *pop = arg;
 
-	if (evsignal_deliver(&pop->evsigmask) == -1)
-		return (-1);
-
 	poll_check_ok(pop);
 	sec = tv->tv_sec * 1000 + (tv->tv_usec + 999) / 1000;
 	nfds = pop->nfds;
 	res = poll(pop->event_set, nfds, sec);
 
-	if (evsignal_recalc(&pop->evsigmask) == -1)
-		return (-1);
-
 	if (res == -1) {
 		if (errno != EINTR) {
                         event_warn("poll");
@@ -228,7 +219,7 @@
 	int i;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_add(&pop->evsigmask, ev));
+		return (evsignal_add(ev));
 	if (!(ev->ev_events & (EV_READ|EV_WRITE)))
 		return (0);
 
@@ -313,7 +304,7 @@
 	int i;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_del(&pop->evsigmask, ev));
+		return (evsignal_del(ev));
 
 	if (!(ev->ev_events & (EV_READ|EV_WRITE)))
 		return (0);
Index: select.c
===================================================================
RCS file: /cvsroot/levent/libevent/select.c,v
retrieving revision 1.25
diff -u -r1.25 select.c
--- select.c	28 Mar 2006 04:40:54 -0000	1.25
+++ select.c	19 Oct 2006 04:42:23 -0000
@@ -68,7 +68,6 @@
 	fd_set *event_writeset_out;
 	struct event **event_r_by_fd;
 	struct event **event_w_by_fd;
-	sigset_t evsigmask;
 };
 
 void *select_init	(void);
@@ -104,7 +103,7 @@
 
 	select_resize(sop, howmany(32 + 1, NFDBITS)*sizeof(fd_mask));
 
-	evsignal_init(&sop->evsigmask);
+	evsignal_init();
 
 	return (sop);
 }
@@ -133,7 +132,7 @@
 
 }
 #else
-#define check_selectop(sop) do {;} while (0)
+#define check_selectop(sop) do { (void) sop; } while (0)
 #endif
 
 /*
@@ -148,7 +147,7 @@
 
 	check_selectop(sop);
 
-	return (evsignal_recalc(&sop->evsigmask));
+	return (0);
 }
 
 int
@@ -164,15 +163,10 @@
 	memcpy(sop->event_writeset_out, sop->event_writeset_in,
 	       sop->event_fdsz);
 
-	if (evsignal_deliver(&sop->evsigmask) == -1)
-		return (-1);
-
 	res = select(sop->event_fds + 1, sop->event_readset_out,
 	    sop->event_writeset_out, NULL, tv);
 
 	check_selectop(sop);
-	if (evsignal_recalc(&sop->evsigmask) == -1)
-		return (-1);
 
 	if (res == -1) {
 		if (errno != EINTR) {
@@ -281,7 +275,7 @@
 	struct selectop *sop = arg;
 
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_add(&sop->evsigmask, ev));
+		return (evsignal_add(ev));
 
 	check_selectop(sop);
 	/*
@@ -332,7 +326,7 @@
 
 	check_selectop(sop);
 	if (ev->ev_events & EV_SIGNAL)
-		return (evsignal_del(&sop->evsigmask, ev));
+		return (evsignal_del(ev));
 
 	if (sop->event_fds < ev->ev_fd) {
 		check_selectop(sop);
Index: signal.c
===================================================================
RCS file: /cvsroot/levent/libevent/signal.c,v
retrieving revision 1.11
diff -u -r1.11 signal.c
--- signal.c	17 Dec 2005 20:25:22 -0000	1.11
+++ signal.c	19 Oct 2006 04:42:23 -0000
@@ -55,13 +55,14 @@
 extern struct event_list signalqueue;
 
 static sig_atomic_t evsigcaught[NSIG];
-static int needrecalc;
 volatile sig_atomic_t evsignal_caught = 0;
 
 static struct event ev_signal;
 static int ev_signal_pair[2];
 static int ev_signal_added;
 
+static void evsignal_handler(int sig);
+
 /* Callback for when the signal handler write a byte to our signaling socket */
 static void
 evsignal_cb(int fd, short what, void *arg)
@@ -86,10 +87,8 @@
 #endif
 
 void
-evsignal_init(sigset_t *evsigmask)
+evsignal_init(void)
 {
-	sigemptyset(evsigmask);
-
 	/* 
 	 * Our signal handler is going to write to one end of the socket
 	 * pair to wake up our event loop.  The event loop then scans for
@@ -109,14 +108,27 @@
 }
 
 int
-evsignal_add(sigset_t *evsigmask, struct event *ev)
+evsignal_add(struct event *ev)
 {
 	int evsignal;
+	struct sigaction sa;
 
 	if (ev->ev_events & (EV_READ|EV_WRITE))
 		event_errx(1, "%s: EV_SIGNAL incompatible use", __func__);
 	evsignal = EVENT_SIGNAL(ev);
-	sigaddset(evsigmask, evsignal);
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = evsignal_handler;
+	sigfillset(&sa.sa_mask);
+	sa.sa_flags |= SA_RESTART;
+
+	if (sigaction(evsignal, &sa, NULL) == -1)
+		return (-1);
+
+	if (!ev_signal_added) {
+		ev_signal_added = 1;
+		event_add(&ev_signal, NULL);
+	}
 
 	return (0);
 }
@@ -126,13 +138,11 @@
  */
 
 int
-evsignal_del(sigset_t *evsigmask, struct event *ev)
+evsignal_del(struct event *ev)
 {
 	int evsignal;
 
 	evsignal = EVENT_SIGNAL(ev);
-	sigdelset(evsigmask, evsignal);
-	needrecalc = 1;
 
 	return (sigaction(EVENT_SIGNAL(ev),(struct sigaction *)SIG_DFL, NULL));
 }
@@ -150,63 +160,21 @@
 	errno = save_errno;
 }
 
-int
-evsignal_recalc(sigset_t *evsigmask)
-{
-	struct sigaction sa;
-	struct event *ev;
-
-	if (!ev_signal_added) {
-		ev_signal_added = 1;
-		event_add(&ev_signal, NULL);
-	}
-
-	if (TAILQ_FIRST(&signalqueue) == NULL && !needrecalc)
-		return (0);
-	needrecalc = 0;
-
-	if (sigprocmask(SIG_BLOCK, evsigmask, NULL) == -1)
-		return (-1);
-
-	/* Reinstall our signal handler. */
-	memset(&sa, 0, sizeof(sa));
-	sa.sa_handler = evsignal_handler;
-	sa.sa_mask = *evsigmask;
-	sa.sa_flags |= SA_RESTART;
-
-	TAILQ_FOREACH(ev, &signalqueue, ev_signal_next) {
-		if (sigaction(EVENT_SIGNAL(ev), &sa, NULL) == -1)
-			return (-1);
-	}
-	return (0);
-}
-
-int
-evsignal_deliver(sigset_t *evsigmask)
-{
-	if (TAILQ_FIRST(&signalqueue) == NULL)
-		return (0);
-
-	return (sigprocmask(SIG_UNBLOCK, evsigmask, NULL));
-	/* XXX - pending signals handled here */
-}
-
 void
 evsignal_process(void)
 {
 	struct event *ev;
 	sig_atomic_t ncalls;
 
+	evsignal_caught = 0;
 	TAILQ_FOREACH(ev, &signalqueue, ev_signal_next) {
 		ncalls = evsigcaught[EVENT_SIGNAL(ev)];
 		if (ncalls) {
 			if (!(ev->ev_events & EV_PERSIST))
 				event_del(ev);
 			event_active(ev, EV_SIGNAL, ncalls);
+			evsigcaught[EVENT_SIGNAL(ev)] = 0;
 		}
 	}
-
-	memset(evsigcaught, 0, sizeof(evsigcaught));
-	evsignal_caught = 0;
 }
 
Index: test/regress.c
===================================================================
RCS file: /cvsroot/levent/libevent/test/regress.c,v
retrieving revision 1.28
diff -u -r1.28 regress.c
--- test/regress.c	10 Jun 2006 22:37:21 -0000	1.28
+++ test/regress.c	19 Oct 2006 04:42:23 -0000
@@ -450,6 +450,20 @@
 
 	cleanup_test();
 }
+
+void
+test_immediatesignal(void)
+{
+	struct event ev;
+
+	printf("Immediate signal: ");
+	signal_set(&ev, SIGUSR1, signal_cb, &ev);
+	signal_add(&ev, NULL);
+	raise(SIGUSR1);
+	event_loop(EVLOOP_NONBLOCK);
+	signal_del(&ev);
+	cleanup_test();
+}
 #endif
 
 void
@@ -896,6 +910,7 @@
 	test_simpletimeout();
 #ifndef WIN32
 	test_simplesignal();
+	test_immediatesignal();
 #endif
 	test_loopexit();