@matheusbsbs / matheus-bagliano / commits / 1079c4be0b

xread: poll on non blocking fds

The man page of read(2) says: EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the read would block. EAGAIN or EWOULDBLOCK The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the read would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities. If we get an EAGAIN or EWOULDBLOCK the fd must have set O_NONBLOCK. As the intent of xread is to read as much as possible either until the fd is EOF or an actual error occurs, we can ease the feeder of the fd by not spinning the whole time, but rather wait for it politely by not busy waiting. We should not care if the call to poll failed, as we're in an infinite loop and can only get out with the correct read(). Signed-off-by: Stefan Beller <sbeller@google.com> Acked-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Stefan Beller committed Dec 15, 2015 at 16:04 UTC 1079c4be0b72003668df647f8a520fa137c7e158

1 file changed +18 -2

wrapper.c

+18 -2

@@ -236,8 +236,24 @@ ssize_t xread(int fd, void *buf, size_t len)

236 len = MAX_IO_SIZE;

237 while (1) {

238 nr = read(fd, buf, len);

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

240 - continue;

239 + if (nr < 0) {

240 + if (errno == EINTR)

241 + continue;

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

243 + struct pollfd pfd;

244 + pfd.events = POLLIN;

245 + pfd.fd = fd;

246 + /*

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

248 + * want to leave this infinite loop

249 + * only when read() returns with

250 + * success, or an expected failure,

251 + * which would be checked by the next

252 + * call to read(2).

253 + */

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

255 + }

256 + }

257 return nr;

258 }

259 }