From ef1cf0167a362cd504afdcc0eef4c3200ea6dfbb Mon Sep 17 00:00:00 2001 From: Eric Wong Date: Sun, 26 Jun 2016 23:21:12 +0000 Subject: [PATCH] xwrite: poll on non-blocking FDs 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: 1079c4be0b720 ("xread: poll on non blocking fds") Signed-off-by: Eric Wong Reviewed-by: Jeff King Signed-off-by: Junio C Hamano --- wrapper.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-) diff --git a/wrapper.c b/wrapper.c index 9b20eb9351..f7ea6c43be 100644 --- 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; } } -- 2.11.4.GIT