xwrite: poll on non-blocking FDs

Eric Wong · gitster · commit ef1cf0167a36 · 2016-06-27T08:34:15.000-07:00
write(2) can hit the same EAGAIN/EWOULDBLOCK errors as read(2), so busy-looping on a non-blocking FD is a waste of resources. Currently, I do not know of a way for this happen: * the NonBlocking directive in systemd does not apply to stdin, stdout, or stderr. * xinetd provides no way to set the non-blocking flag at all But theoretically, it's possible a careless C10K HTTP server could use pipe2(..., O_NONBLOCK) to setup a pipe for git-http-backend with only the intent to use non-blocking reads; but accidentally leave non-blocking set on the write end passed as stdout to git-upload-pack. Followup-to: 1079c4b ("xread: poll on non blocking fds") Signed-off-by: Eric Wong <e@80x24.org> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/wrapper.c b/wrapper.c
@@ -271,8 +271,26 @@ ssize_t xwrite(int fd, const void *buf, size_t len)
 	    len = MAX_IO_SIZE;
 	while (1) {
 		nr = write(fd, buf, len);
-		if ((nr < 0) && (errno == EAGAIN || errno == EINTR))
-			continue;
+		if (nr < 0) {
+			if (errno == EINTR)
+				continue;
+			if (errno == EAGAIN || errno == EWOULDBLOCK) {
+				struct pollfd pfd;
+				pfd.events = POLLOUT;
+				pfd.fd = fd;
+				/*
+				 * it is OK if this poll() failed; we
+				 * want to leave this infinite loop
+				 * only when write() returns with
+				 * success, or an expected failure,
+				 * which would be checked by the next
+				 * call to write(2).
+				 */
+				poll(&pfd, 1, -1);
+				continue;
+			}
+		}
+
 		return nr;
 	}
 }
