[PATCHv4 02/14] xread: poll on non blocking fds

[Stefan Beller <sbeller@google.com>]

>From the man page:
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>
---
 wrapper.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/wrapper.c b/wrapper.c
index ff49807..5517928 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -201,8 +201,17 @@ 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;
+				/* We deliberately ignore the return value */
+				poll(&pfd, 1, -1);
+			}
+		}
 		return nr;
 	}
 }
-- 
2.5.0.272.ga84127c.dirty