Re: [PATCH 2/8] xread: poll on non blocking fds

[Eric Sunshine <ericsunshine@gmail.com>]

On Mon, Dec 14, 2015 at 2:37 PM, Stefan Beller <sbeller@google.com> wrote:
> 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>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
> diff --git a/wrapper.c b/wrapper.c
> index 6fcaa4d..4f720fe 100644
> --- a/wrapper.c
> +++ b/wrapper.c
> @@ -236,8 +236,17 @@ 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 */

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.

> +                               poll(&pfd, 1, -1);
> +                       }
> +               }
>                 return nr;
>         }
>  }
> --
> 2.6.4.443.ge094245.dirty
>