git daemon: avoid waking up too often

To avoid waking up unnecessarily, a pipe is set up that is only ever
written to by child_handler(), when a child disconnects, as suggested
per Junio.

This avoids waking up the main process every second to see if a child
was disconnected.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

daemon.c

@@ -16,6 +16,7 @@
 static int log_syslog;
 static int verbose;
 static int reuseaddr;
+static int child_handler_pipe[2];
 
 static const char daemon_usage[] =
 "git daemon [--verbose] [--syslog] [--export-all]\n"
@@ -788,6 +789,7 @@ static void child_handler(int signo)
 				pid = -pid;
 			dead_child[reaped % MAX_CHILDREN] = pid;
 			children_reaped = reaped + 1;
+			write(child_handler_pipe[1], &status, 1);
 			continue;
 		}
 		break;
@@ -933,29 +935,24 @@ static int service_loop(int socknum, int *socklist)
 	struct pollfd *pfd;
 	int i;
 
-	pfd = xcalloc(socknum, sizeof(struct pollfd));
+	if (pipe(child_handler_pipe) < 0)
+		die ("Could not set up pipe for child handler");
+
+	pfd = xcalloc(socknum + 1, sizeof(struct pollfd));
 
 	for (i = 0; i < socknum; i++) {
 		pfd[i].fd = socklist[i];
 		pfd[i].events = POLLIN;
 	}
+	pfd[socknum].fd = child_handler_pipe[0];
+	pfd[socknum].events = POLLIN;
 
 	signal(SIGCHLD, child_handler);
 
 	for (;;) {
 		int i;
-		int timeout;
 
-		/*
-		 * This 1-sec timeout could lead to idly looping but it is
-		 * here so that children culled in child_handler() are reported
-		 * without too much delay.  We could probably set up a pipe
-		 * to ourselves that we poll, and write to the fd from child_handler()
-		 * to wake us up (and consume it when the poll() returns...
-		 */
-		timeout = (children_spawned != children_deleted) ? 1000 : -1;
-		i = poll(pfd, socknum, timeout);
-		if (i < 0) {
+		if (poll(pfd, socknum + 1, -1) < 0) {
 			if (errno != EINTR) {
 				error("poll failed, resuming: %s",
 				      strerror(errno));
@@ -963,9 +960,9 @@ static int service_loop(int socknum, int *socklist)
 			}
 			continue;
 		}
-		if (i == 0) {
+		if (pfd[socknum].revents & POLLIN) {
+			read(child_handler_pipe[0], &i, 1);
 			check_dead_children();
-			continue;
 		}
 
 		for (i = 0; i < socknum; i++) {