[Xcb] [PATCH] Handle EAGAIN errno from poll(2) or select(2)

[Mark Kettenis]

> From: Jeremy Huddleston Sequoia <jeremyhu at apple.com>
> Date: Wed, 19 Aug 2015 16:09:33 -0700
> 
> No known fallout from this, but I spotted the possible issue when auditing
> this code to track down a related issue.  While not noted in SUS, some
> implementations (like darwin) may return EAGAIN for (possibly) transient
> kernel issues that would suggest trying again.

Well, EAGAIN suggests "try again *later*".  Presumably the kernel
would return EAGAIN immediately and therefore this change may very
well introduce a spinning loop.  That would not be good, and I'd say
returning an error would be preferable over having the application
spin.

Having the kernel return EAGAIN for a blocking poll or select would be
a serious bug IMHB.  It should just wait until resources are
available.  Does Darwin really have such a bug, or are you just trying
to strike pre-emptively?

> Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu at apple.com>
> ---
>  src/xcb_conn.c | 2 +-
>  src/xcb_in.c   | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/src/xcb_conn.c b/src/xcb_conn.c
> index 7d09637..7e49384 100644
> --- a/src/xcb_conn.c
> +++ b/src/xcb_conn.c
> @@ -487,7 +487,7 @@ int _xcb_conn_wait(xcb_connection_t *c, pthread_cond_t *cond, struct iovec **vec
>  #else
>          ret = select(c->fd + 1, &rfds, &wfds, 0, 0);
>  #endif
> -    } while (ret == -1 && errno == EINTR);
> +    } while (ret == -1 && (errno == EINTR || errno == EAGAIN));
>      if(ret < 0)
>      {
>          _xcb_conn_shutdown(c, XCB_CONN_ERROR);
> diff --git a/src/xcb_in.c b/src/xcb_in.c
> index bab4bc7..e806388 100644
> --- a/src/xcb_in.c
> +++ b/src/xcb_in.c
> @@ -386,7 +386,7 @@ static int read_block(const int fd, void *buf, const ssize_t len)
>              pfd.revents = 0;
>              do {
>                  ret = poll(&pfd, 1, -1);
> -            } while (ret == -1 && errno == EINTR);
> +            } while (ret == -1 && (errno == EINTR || errno == EAGAIN));
>  #else
>              fd_set fds;
>              FD_ZERO(&fds);
> @@ -396,7 +396,7 @@ static int read_block(const int fd, void *buf, const ssize_t len)
>              errno = 0;
>              do {
>                  ret = select(fd + 1, &fds, 0, 0, 0);
> -            } while (ret == -1 && errno == EINTR);
> +            } while (ret == -1 && (errno == EINTR || errno == EAGAIN));
>  #endif /* USE_POLL */
>          }
>          if(ret <= 0)
> -- 
> 2.5.0
> 
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>