Re: [RFC] have tail always detect broken pipes

[Pádraig Brady]

On 07/06/17 01:00, Pádraig Brady wrote:
> The following will hang indefinitely, due to no further data
> being written through the pipe, and thus no SIGPIPE generated:
> 
>   $ tail -f /etc/hosts | sleep 1
> 
> A more practical use case might be:
> 
>   tail -f file.log | grep -q trigger &&
>   process_immediately
> 
> Another case where one might notice this is that sometimes
> (depending on how slow the reader is) the following will hang,
> and you may not notice the issue until data arrives
> 
>   tail -f file.log | grep_typo filter
> 
> Below is a quick hack to support this,
> and tail now terminates promptly for the above cases.
> 
> The implementation is a proof on concept done in a few minutes
> (and may change from poll() to select() if possible).
> A disadvantage is the extra syscalls to do the polling.
> Actually I may be able to combine the existing select()
> with this new poll(), and thus wait for changes in the
> output as well as the inotify descriptor.
> 
> cheers,
> Pádraig.
> 
> 
> diff --git a/src/tail.c b/src/tail.c

> +/* If the output has gone away, then terminate
> +   as we would if we had written to this output.  */
> +static void
> +check_output (void)
> +{
> +  struct pollfd pfd = {.fd = STDOUT_FILENO, .events = POLLERR};
> +  if (poll (&pfd, 1, 0) >= 0 && (pfd.revents & POLLERR))
> +    raise (SIGPIPE);
> +}
> +

The attached changes from using poll() to using select(),
which was already used in tail(1), and avoids the repeated
poll() calls in the normal case.

cheers,
Pádraig

tail-pipe.diff
Description: Text Data