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Re: [libvirt] [PATCH] util: eventpoll: Survive EBADF on macOS



On Wed, Aug 22, 2018 at 12:12:03AM +0300, Roman Bolshakov wrote:
> Fixes:
> https://www.redhat.com/archives/libvir-list/2017-January/msg00978.html
> 
> QEMU is probed through monitor fd to check capabilities during libvirtd init.
> The monitor fd is closed after probing by virQEMUCapsInitQMPCommandFree
> that calls virQEMUCapsInitQMPCommandAbort that calls qemuMonitorClose,
> the latter one notifies the event loop via an interrupt handle in
> qemuMonitorUnregister and after then closes monitor fd.
> 
> There could be a case when interrupt is sent after eventLoop is unlocked
> but before virEventPollRunOnce blocks in poll, shortly before file
> descriptor is closed by qemuMonitorClose. Then poll receives closed monitor
> fd in fdset and returns EBADF.
> 
> EBADF is not mentioned as a valid errno on macOS poll man-page but such
> behaviour can appear release-to-release, according to cpython:
> https://github.com/python/cpython/blob/master/Modules/selectmodule.c#L1161

Yeah, this really does look like broken macOS behaviour, as it should
be returning POLLNVAL instead.

> 
> The change also fixes the issue in qemucapabilitiestest. It returns
> Bad file descriptor message 25 times without the fix.
> 
> Signed-off-by: Roman Bolshakov <r bolshakov yadro com>
> ---
>  src/util/vireventpoll.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c
> index 13d278df13..6d19374c52 100644
> --- a/src/util/vireventpoll.c
> +++ b/src/util/vireventpoll.c
> @@ -643,10 +643,18 @@ int virEventPollRunOnce(void)
>          EVENT_DEBUG("Poll got error event %d", errno);
>          if (errno == EINTR || errno == EAGAIN)
>              goto retry;
> +#ifdef __APPLE__
> +        if (errno == EBADF) {
> +            ret = 0;
> +            goto dispatch;
> +        }
> +#endif
>          virReportSystemError(errno, "%s",
>                               _("Unable to poll on file handles"));
>          return -1;
>      }
> +
> + dispatch:

I don't think we should go through the dispatch code when
we get EBADF, as the contents of the 'revents' fields are
undefined after we get an error.

We should jump straight to the end where we have
"eventLoop.running = 0;"

If any FDs were in fact ready, we'll get them on the next iteration of
the loop anyway.

>      EVENT_DEBUG("Poll got %d event(s)", ret);
>  
>      virMutexLock(&eventLoop.lock);


Regards,
Daniel
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