[PATCHv3 02/13] xread: poll on non blocking fds

From: Stefan Beller <sbeller@google.com>
Date: Sep 21, 2015, 22:39 UTC
Message-ID: <1442875159-13027-3-git-send-email-sbeller@google.com>
In-Reply-To: <1442875159-13027-1-git-send-email-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.

So if we get an EAGAIN or EWOULDBLOCK error the fd must be nonblocking. As the intend 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.

Signed-off-by: Stefan Beller <sbeller@google.com>
---
 wrapper.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/wrapper.c b/wrapper.c
index ff49807..50267a4 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -201,8 +201,23 @@ 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;
+				int i;
+				pfd.events = POLLIN;
+				pfd.fd = fd;
+				i = poll(&pfd, 1, 100);
+				if (i < 0) {
+					if (errno == EINTR || errno == ENOMEM)
+						continue;
+					else
+						die_errno("poll");
+				}
+			}
+		}
 		return nr;
 	}
 }

-- 
2.5.0.275.ge015d2a