Re: [PATCHv2 2/7] xread: poll on non blocking fds

From: Jeff King <peff@peff.net>
Date: Dec 18, 2015, 03:13 UTC
Message-ID: <20151218031336.GA8467@sigill.intra.peff.net>
In-Reply-To: <56731E19.7050504@web.de>

On Thu, Dec 17, 2015 at 09:42:01PM +0100, Torsten Bögershausen wrote:

Show 11 quoted lines

> > Or do you mean to insert another continue in here?
> I was thinking that we run into similar loop as before:
> read() returns -1; errno = EAGAIN  /* No data to read */
> poll() returns -1; errno = EAGAIN /* poll failed. If the fd was OK, the failure may be temporaly,
>                                     as much as poll() can see.
>                                     But most probably we run out ouf memory */
> 
> So the code would look like this:
> 
>    if (!poll(&pfd, 1, -1))
>       return -1;

That changes the semantics of the function. The poll() is just a convenience to avoid spinning. If it fails, with Stefan's patch[1] the worst case is that we would spin on read() and poll(), instead of actually blocking in the poll().

But if we return on poll() failure, now the caller will see errors from poll() even though they don't know or care that we called poll() in the first place. Consider what would happen with your code if read got EAGAIN and then poll got EINTR. We would report an error, even though the whole point of xread() is to loop on these conditions.

-Peff

[1] Stefan's patch does have a bug. It forgets to "continue" after
    calling poll, which means we will block with poll and _then_ exit
    with -1, instead of restarting the loop.