daemon: use sigaction() to install child_handler()

In a future change, the flags used for processing SIGCHLD will need to
be updated, which is only possible by using sigaction().

Factor out the call to set the signal handler and use sigaction instead
of signal for the systems that allow that, which has the added benefit
of using BSD semantics reliably and therefore not needing the rearming
call.

Signed-off-by: Carlo Marcelo Arenas Belón <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: carenas

daemon.c

@@ -912,14 +912,19 @@ static void handle(int incoming, struct sockaddr *addr, socklen_t addrlen)
 		add_child(&cld, addr, addrlen);
 }
 
-static void child_handler(int signo UNUSED)
+static void child_handler(int signo MAYBE_UNUSED)
 {
 	/*
-	 * Otherwise empty handler because systemcalls will get interrupted
-	 * upon signal receipt
-	 * SysV needs the handler to be rearmed
+	 * Otherwise empty handler because systemcalls should get interrupted
+	 * upon signal receipt.
 	 */
-	signal(SIGCHLD, child_handler);
+#ifdef USE_NON_POSIX_SIGNAL
+	/*
+	 * SysV needs the handler to be rearmed, but this is known
+	 * to trigger infinite recursion crashes at least in AIX.
+	 */
+	signal(signo, child_handler);
+#endif
 }
 
 static int set_reuse_addr(int sockfd)
@@ -1118,8 +1123,26 @@ static void socksetup(struct string_list *listen_addr, int listen_port, struct s
 	}
 }
 
+#ifndef USE_NON_POSIX_SIGNAL
+
+static void set_signal_handler(struct sigaction *psa)
+{
+	sigemptyset(&psa->sa_mask);
+	psa->sa_flags = SA_NOCLDSTOP | SA_RESTART;
+	psa->sa_handler = child_handler;
+	sigaction(SIGCHLD, psa, NULL);
+}
+
+#else
+
+static void set_signal_handler(struct sigaction *psa UNUSED)
+{
+	signal(SIGCHLD, child_handler);
+}
+
 static int service_loop(struct socketlist *socklist)
 {
+	struct sigaction sa;
 	struct pollfd *pfd;
 
 	CALLOC_ARRAY(pfd, socklist->nr);
@@ -1129,7 +1152,7 @@ static int service_loop(struct socketlist *socklist)
 		pfd[i].events = POLLIN;
 	}
 
-	signal(SIGCHLD, child_handler);
+	set_signal_handler(&sa);
 
 	for (;;) {
 		check_dead_children();