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=-20.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,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 07D76C433E0 for ; Mon, 1 Mar 2021 16:58:38 +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 A213064F5D for ; Mon, 1 Mar 2021 16:58:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A213064F5D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:54256 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lGlsS-0001gd-Ny for qemu-devel@archiver.kernel.org; Mon, 01 Mar 2021 11:58:36 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:56900) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lGlqc-0000dh-Vy for qemu-devel@nongnu.org; Mon, 01 Mar 2021 11:56:43 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:41934) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lGlqb-0000zE-10 for qemu-devel@nongnu.org; Mon, 01 Mar 2021 11:56:42 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614617800; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=P8OiGXdRHnlsq4P1WPnZO7Sn/4Ms0moWr/6fImuyR2k=; b=DyxOXNPBbts0b/wf8k0cSfEaentTfIp0Kv5ekyqKIeqqVug/FzMp4nEk7wvxFDiLpV6FHp nyO3x5BwfzRUX09FGA7bA7XXk3V6ncTl+1b2pekcVDippBhUz6uGbrhJmEQTLyi0HAaUJL HngTE3Pm63LbOwroUskh1gfp8iqyH6c= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-214-Xw0IIif1MdeRTGplxTt0IQ-1; Mon, 01 Mar 2021 11:56:26 -0500 X-MC-Unique: Xw0IIif1MdeRTGplxTt0IQ-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DA474107ACE4; Mon, 1 Mar 2021 16:56:24 +0000 (UTC) Received: from redhat.com (ovpn-113-132.ams2.redhat.com [10.36.113.132]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BB14F6060F; Mon, 1 Mar 2021 16:56:20 +0000 (UTC) Date: Mon, 1 Mar 2021 16:56:17 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Stefan Hajnoczi Subject: Re: [PATCH] docs: show how to spawn qemu-storage-daemon with fd passing Message-ID: References: <20210301153159.35660-1-stefanha@redhat.com> <20210301153906.GZ30079@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.5 (2021-01-21) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, 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_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , "Richard W.M. Jones" , qemu-block@nongnu.org, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Mar 01, 2021 at 04:50:14PM +0000, Stefan Hajnoczi wrote: > On Mon, Mar 01, 2021 at 03:44:42PM +0000, Daniel P. Berrangé wrote: > > On Mon, Mar 01, 2021 at 03:39:06PM +0000, Richard W.M. Jones wrote: > > > On Mon, Mar 01, 2021 at 03:31:59PM +0000, Stefan Hajnoczi wrote: > > > > The QMP monitor, NBD server, and vhost-user-blk export all support file > > > > descriptor passing. This is a useful technique because it allows the > > > > parent process to spawn and wait for qemu-storage-daemon without busy > > > > waiting, which may delay startup due to arbitrary sleep() calls. > > > > > > > > This Python example is inspired by the test case written for libnbd by > > > > Richard W.M. Jones : > > > > https://gitlab.com/nbdkit/libnbd/-/commit/89113f484effb0e6c322314ba75c1cbe07a04543 > > > > > > > > Thanks to Daniel P. Berrangé for suggestions on > > > > how to get this working. Now let's document it! > > > > > > > > Reported-by: Richard W.M. Jones > > > > Cc: Kevin Wolf > > > > Cc: Daniel P. Berrangé > > > > Signed-off-by: Stefan Hajnoczi > > > > --- > > > > docs/tools/qemu-storage-daemon.rst | 38 ++++++++++++++++++++++++++++-- > > > > 1 file changed, 36 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-daemon.rst > > > > index f63627eaf6..45854c131e 100644 > > > > --- a/docs/tools/qemu-storage-daemon.rst > > > > +++ b/docs/tools/qemu-storage-daemon.rst > > > > @@ -101,10 +101,12 @@ Standard options: > > > > > > > > .. option:: --nbd-server addr.type=inet,addr.host=,addr.port=[,tls-creds=][,tls-authz=][,max-connections=] > > > > --nbd-server addr.type=unix,addr.path=[,tls-creds=][,tls-authz=][,max-connections=] > > > > + --nbd-server addr.type=fd,addr.str=[,tls-creds=][,tls-authz=][,max-connections=] > > > > > > > > is a server for NBD exports. Both TCP and UNIX domain sockets are supported. > > > > - TLS encryption can be configured using ``--object`` tls-creds-* and authz-* > > > > - secrets (see below). > > > > + A listen socket can be provided via file descriptor passing (see Examples > > > > + below). TLS encryption can be configured using ``--object`` tls-creds-* and > > > > + authz-* secrets (see below). > > > > > > > > To configure an NBD server on UNIX domain socket path ``/tmp/nbd.sock``:: > > > > > > > > @@ -127,6 +129,38 @@ QMP commands:: > > > > --chardev socket,path=qmp.sock,server,nowait,id=char1 \ > > > > --monitor chardev=char1 > > > > > > > > +Launch the daemon from Python with a QMP monitor socket using file descriptor > > > > +passing so there is no need to busy wait for the QMP monitor to become > > > > +available:: > > > > + > > > > + #!/usr/bin/env python3 > > > > + import os > > > > + import subprocess > > > > + import socket > > > > + > > > > + sock_path = '/tmp/qmp-{}.sock'.format(os.getpid()) > > > > > > Not sure how much you worry about the insecure / easily guessable tmp > > > path here. I notice that there's already one in the surrounding > > > documentation (/tmp/nbd.sock) so maybe it's not a problem :-) > > > > > > > + with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as listen_sock: > > > > + listen_sock.bind(sock_path) > > > > + listen_sock.listen() > > > > + > > > > + fd = listen_sock.fileno() > > > > + > > > > + subprocess.Popen( > > > > + ['qemu-storage-daemon', > > > > + '--chardev', f'socket,fd={fd},server=on,id=char1', > > > > + '--monitor', 'chardev=char1'], > > > > + pass_fds=[fd], > > > > + ) > > > > + > > > > + qmp_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) > > > > + qmp_sock.connect(sock_path) > > > > > > A note that the order of opening the sockets is slightly different > > > from how I did it in the interop test. But I believe it makes no > > > difference, as long as you don't connect to the socket until it's in > > > the listening state, which is what you're doing here. So it should be > > > fine. > > > > Nothing here is closing listen_sock in the parent though. > > > > The trick of passing the listener FD into the child relies on the > > listener being closed in the parent, so that the parent can get > > a socket error if the child exits abnormally during startup. Keeping > > the listen socket open means the parent will wait forever for an > > accept() that never comes. > > The listen socket is closed by the context manager at the end of the > 'with' statement. This is the modern Python approach for resource > acquisition that also handles exceptions automatically. It's like RAII > in C++. Hmm, yes, I didn't remember that at first. I'm not sure that is a good idea as an example code, because people mapping this example into other languages are likely to miss that critical detail. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|