@matheusbsbs / matheus-bagliano / commits / ef1cf0167a

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 <e@80x24.org> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Eric Wong committed Jun 26, 2016 at 23:21 UTC ef1cf0167a362cd504afdcc0eef4c3200ea6dfbb

1 file changed +20 -2

wrapper.c

+20 -2

@@ -271,8 +271,26 @@ ssize_t xwrite(int fd, const void *buf, size_t len)

271 len = MAX_IO_SIZE;

272 while (1) {

273 nr = write(fd, buf, len);

274 - if ((nr < 0) && (errno == EAGAIN || errno == EINTR))

275 - continue;

274 + if (nr < 0) {

275 + if (errno == EINTR)

276 + continue;

277 + if (errno == EAGAIN || errno == EWOULDBLOCK) {

278 + struct pollfd pfd;

279 + pfd.events = POLLOUT;

280 + pfd.fd = fd;

281 + /*

282 + * it is OK if this poll() failed; we

283 + * want to leave this infinite loop

284 + * only when write() returns with

285 + * success, or an expected failure,

286 + * which would be checked by the next

287 + * call to write(2).

288 + */

289 + poll(&pfd, 1, -1);

290 + continue;

291 + }

292 + }

293 +

294 return nr;

295 }

296 }