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 <sbel...@google.com>
Acked-by: Johannes Sixt <j...@kdbg.org>
Signed-off-by: Junio C Hamano <gits...@pobox.com>
---
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;
}
}
--
2.6.4.443.ge094245.dirty
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