1 files changed, 55 insertions, 26 deletions

diff --git a/src/backend/port/unix_latch.c b/src/backend/port/unix_latch.c
index e7be7ec1179..104401d0feb 100644
--- a/src/backend/port/unix_latch.c
+++ b/src/backend/port/unix_latch.c
@@ -283,27 +283,31 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 	do
 	{
 		/*
-		 * Clear the pipe, then check if the latch is set already. If someone
-		 * sets the latch between this and the poll()/select() below, the
-		 * setter will write a byte to the pipe (or signal us and the signal
-		 * handler will do that), and the poll()/select() will return
-		 * immediately.
+		 * Check if the latch is set already. If so, leave loop immediately,
+		 * avoid blocking again. We don't attempt to report any other events
+		 * that might also be satisfied.
+		 *
+		 * If someone sets the latch between this and the poll()/select()
+		 * below, the setter will write a byte to the pipe (or signal us and
+		 * the signal handler will do that), and the poll()/select() will
+		 * return immediately.
+		 *
+		 * If there's a pending byte in the self pipe, we'll notice whenever
+		 * blocking. Only clearing the pipe in that case avoids having to
+		 * drain it every time WaitLatchOrSocket() is used. Should the
+		 * pipe-buffer fill up we're still ok, because the pipe is in
+		 * nonblocking mode. It's unlikely for that to happen, because the
+		 * self pipe isn't filled unless we're blocking (waiting = true), or
+		 * from inside a signal handler in latch_sigusr1_handler().
 		 *
 		 * Note: we assume that the kernel calls involved in drainSelfPipe()
 		 * and SetLatch() will provide adequate synchronization on machines
 		 * with weak memory ordering, so that we cannot miss seeing is_set if
 		 * the signal byte is already in the pipe when we drain it.
 		 */
-		drainSelfPipe();
-
 		if ((wakeEvents & WL_LATCH_SET) && latch->is_set)
 		{
 			result |= WL_LATCH_SET;
-
-			/*
-			 * Leave loop immediately, avoid blocking again. We don't attempt
-			 * to report any other events that might also be satisfied.
-			 */
 			break;
 		}
 
@@ -313,24 +317,26 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		 */
 #if defined(LATCH_USE_POLL)
 		nfds = 0;
+
+		/* selfpipe is always in pfds[0] */
+		pfds[0].fd = selfpipe_readfd;
+		pfds[0].events = POLLIN;
+		pfds[0].revents = 0;
+		nfds++;
+
 		if (wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
 		{
-			/* socket, if used, is always in pfds[0] */
-			pfds[0].fd = sock;
-			pfds[0].events = 0;
+			/* socket, if used, is always in pfds[1] */
+			pfds[1].fd = sock;
+			pfds[1].events = 0;
 			if (wakeEvents & WL_SOCKET_READABLE)
-				pfds[0].events |= POLLIN;
+				pfds[1].events |= POLLIN;
 			if (wakeEvents & WL_SOCKET_WRITEABLE)
-				pfds[0].events |= POLLOUT;
-			pfds[0].revents = 0;
+				pfds[1].events |= POLLOUT;
+			pfds[1].revents = 0;
 			nfds++;
 		}
 
-		pfds[nfds].fd = selfpipe_readfd;
-		pfds[nfds].events = POLLIN;
-		pfds[nfds].revents = 0;
-		nfds++;
-
 		if (wakeEvents & WL_POSTMASTER_DEATH)
 		{
 			/* postmaster fd, if used, is always in pfds[nfds - 1] */
@@ -364,19 +370,27 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		else
 		{
 			/* at least one event occurred, so check revents values */
+
+			if (pfds[0].revents & POLLIN)
+			{
+				/* There's data in the self-pipe, clear it. */
+				drainSelfPipe();
+			}
+
 			if ((wakeEvents & WL_SOCKET_READABLE) &&
-				(pfds[0].revents & POLLIN))
+				(pfds[1].revents & POLLIN))
 			{
 				/* data available in socket, or EOF/error condition */
 				result |= WL_SOCKET_READABLE;
 			}
 			if ((wakeEvents & WL_SOCKET_WRITEABLE) &&
-				(pfds[0].revents & POLLOUT))
+				(pfds[1].revents & POLLOUT))
 			{
 				/* socket is writable */
 				result |= WL_SOCKET_WRITEABLE;
 			}
-			if (pfds[0].revents & (POLLHUP | POLLERR | POLLNVAL))
+			if ((wakeEvents & WL_SOCKET_WRITEABLE) &&
+				(pfds[1].revents & (POLLHUP | POLLERR | POLLNVAL)))
 			{
 				/* EOF/error condition */
 				if (wakeEvents & WL_SOCKET_READABLE)
@@ -468,6 +482,11 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		else
 		{
 			/* at least one event occurred, so check masks */
+			if (FD_ISSET(selfpipe_readfd, &input_mask))
+			{
+				/* There's data in the self-pipe, clear it. */
+				drainSelfPipe();
+			}
 			if ((wakeEvents & WL_SOCKET_READABLE) && FD_ISSET(sock, &input_mask))
 			{
 				/* data available in socket, or EOF */
@@ -498,6 +517,16 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		}
 #endif   /* LATCH_USE_SELECT */
 
+		/*
+		 * Check again whether latch is set, the arrival of a signal/self-byte
+		 * might be what stopped our sleep. It's not required for correctness
+		 * to signal the latch as being set (we'd just loop if there's no
+		 * other event), but it seems good to report an arrived latch asap.
+		 * This way we also don't have to compute the current timestamp again.
+		 */
+		if ((wakeEvents & WL_LATCH_SET) && latch->is_set)
+			result |= WL_LATCH_SET;
+
 		/* If we're not done, update cur_timeout for next iteration */
 		if (result == 0 && (wakeEvents & WL_TIMEOUT))
 		{