Re: [PATCH 2/8] xread: poll on non blocking fds
From: Junio C Hamano <hidden>
Date: 2016-06-15 23:07:27
Eric Sunshine [off-list ref] writes:
This comment tells us what the code itself already says, but not why the value is being ignored. The reader still has to consult the commit message to learn that detail, which makes the value of the comment questionable.
Let's do this for now, then. -- >8 -- From: Stefan Beller <redacted> Date: Mon, 14 Dec 2015 11:37:12 -0800 Subject: [PATCH] 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 <redacted> Acked-by: Johannes Sixt <redacted> Signed-off-by: Junio C Hamano <redacted> --- wrapper.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-)
diff --git a/wrapper.c b/wrapper.c
index 6fcaa4d..1770efa 100644
--- a/wrapper.c
+++ b/wrapper.c@@ -236,8 +236,24 @@ ssize_t xread(int fd, void *buf, size_t len) len = MAX_IO_SIZE; while (1) { nr = read(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 = POLLIN; + pfd.fd = fd; + /* + * it is OK if this poll() failed; we + * want to leave this infinite loop + * only when read() returns with + * success, or an expected failure, + * which would be checked by the next + * call to read(2). + */ + poll(&pfd, 1, -1); + } + } return nr; } }
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2.7.0-rc0-109-gb762328