From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4DB0AC433DF for ; Thu, 4 Jun 2020 18:50:10 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1C624206C3 for ; Thu, 4 Jun 2020 18:50:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="NEYGoiWi" 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certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 55E56107ACF5; Thu, 4 Jun 2020 18:48:58 +0000 (UTC) Received: from [10.10.117.188] (ovpn-117-188.rdu2.redhat.com [10.10.117.188]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1ECC360C80; Thu, 4 Jun 2020 18:48:56 +0000 (UTC) Subject: Re: [PATCH v2 10/16] python/machine.py: Handle None events in event_wait To: Kevin Wolf References: <20200602214528.12107-1-jsnow@redhat.com> <20200602214528.12107-11-jsnow@redhat.com> <20200604142043.GH4512@linux.fritz.box> From: John Snow Autocrypt: addr=jsnow@redhat.com; prefer-encrypt=mutual; keydata= mQINBFTKefwBEAChvwqYC6saTzawbih87LqBYq0d5A8jXYXaiFMV/EvMSDqqY4EY6whXliNO IYzhgrPEe7ZmPxbCSe4iMykjhwMh5byIHDoPGDU+FsQty2KXuoxto+ZdrP9gymAgmyqdk3aV vzzmCa3cOppcqKvA0Kqr10UeX/z4OMVV390V+DVWUvzXpda45/Sxup57pk+hyY52wxxjIqef rj8u5BN93s5uCVTus0oiVA6W+iXYzTvVDStMFVqnTxSxlpZoH5RGKvmoWV3uutByQyBPHW2U 1Y6n6iEZ9MlP3hcDqlo0S8jeP03HaD4gOqCuqLceWF5+2WyHzNfylpNMFVi+Hp0H/nSDtCvQ 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DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , qemu-block@nongnu.org, qemu-devel@nongnu.org, Max Reitz , Cleber Rosa , philmd@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/4/20 10:20 AM, Kevin Wolf wrote: > Am 02.06.2020 um 23:45 hat John Snow geschrieben: >> If the timeout is 0, we can get None back. Handle this explicitly. >> >> Signed-off-by: John Snow > > Subject line: This is events_wait(), not event_wait(). Both functions > exist. > >> @@ -562,6 +564,8 @@ def _match(event): >> # Poll for new events >> while True: >> event = self._qmp.pull_event(wait=timeout) >> + if event is None: >> + break >> if _match(event): >> return event >> self._events.append(event) > > Hm... How could this ever work? I guess we just never really tested > whether timeouts actually time out? > It's weirder than you think. pull_event, when a timeout is supplied, will pass that timeout to QEMUMonitorProtocol.__get_events, which populates the received event queue but does not return data on the stack. If there are no events in the queue and we are given a timeout, we will raise QMPTimeoutError if no event appears in that timeframe. pull_event() will only return None in the case that you don't give it a timeout or tell it to wait. The typing of events_wait allows us to specify a timeout of 0 here, which is treated as no-wait down the stack, which may return None if there was no such event ready. Thus, break and also return None. (I should update the docstring here.) > (It's still somewhat unintuitive that receiving an unrelated event > resets the timeout, but not the problem of this series...) > > Kevin > Yes, it's weird. It's not a timeout on this one event, it's a timeout on event-receiving activity. In practice, it works quite well for the iotest suite where there are often not other event drivers present. It was a quick hack (that I introduced!) to ensure that iotests would halt in some finite amount of time. --js