write(2) can hit the same EAGAIN/EWOULDBLOCK errors as read(2),
so busy-looping on a non-blocking FD is a waste of resources.
Currently, I do not know of a way for this happen:
* the NonBlocking directive in systemd does not apply to stdin,
stdout, or stderr.
* xinetd provides no way to set the non-blocking flag at all
But theoretically, it's possible a careless C10K HTTP server
could use pipe2(..., O_NONBLOCK) to setup a pipe for
git-http-backend with only the intent to use non-blocking reads;
but accidentally leave non-blocking set on the write end passed
as stdout to git-upload-pack.
Followup-to: 1079c4be0b720 ("xread: poll on non blocking fds")
Signed-off-by: Eric Wong <[email protected]>
---
wrapper.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/wrapper.c b/wrapper.c
index f1155d0..d973f86 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -274,8 +274,26 @@ ssize_t xwrite(int fd, const void *buf, size_t len)
len = MAX_IO_SIZE;
while (1) {
nr = write(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 = POLLOUT;
+ pfd.fd = fd;
+ /*
+ * it is OK if this poll() failed; we
+ * want to leave this infinite loop
+ * only when write() returns with
+ * success, or an expected failure,
+ * which would be checked by the next
+ * call to write(2).
+ */
+ poll(&pfd, 1, -1);
+ continue;
+ }
+ }
+
return nr;
}
}
--
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