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bh=pOvdoudQIuU/VcDTqIwiqfJkGiSpZYpRZlb8TAYwl0Q=; b=fz0KwLJuk1HO2gBlB5bCIKVCgZXv3UwxH6BpwkHNtAJ9qZ1KR11t91hN5JUgsENix2UbDy ZggwbfQkFvViFfubRnnVxrqTe6Myj2FWNAd5vm+0FudFOqm70fgUiVRk4gBjICfSB5c84k 55Gbl8lPoSJIcsg5ohmWTYt1FRZR7So= X-MC-Unique: ig4bBh3PMAy88G5YRk61OQ-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [PATCH v3 1/2] docs: show how to spawn qemu-storage-daemon with fd passing Date: Mon, 1 Mar 2021 17:27:27 +0000 Message-Id: <20210301172728.135331-2-stefanha@redhat.com> In-Reply-To: <20210301172728.135331-1-stefanha@redhat.com> References: <20210301172728.135331-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=216.205.24.124; envelope-from=stefanha@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_H3=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: , Cc: Kevin Wolf , =?UTF-8?q?Daniel=20P=20=2E=20Berrang=C3=A9?= , Stefan Hajnoczi , qemu-block@nongnu.org, "Richard W . M . Jones" Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) Content-Type: text/plain; charset="utf-8" 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/89113f484effb0e6c322314ba75c1cbe0= 7a04543 Thanks to Daniel P. Berrang=C3=A9 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. Berrang=C3=A9 Signed-off-by: Stefan Hajnoczi Reviewed-by: Daniel P. Berrang=C3=A9 --- v2: * Use /var/run/qmp.sock instead of /tmp/qmp-$PID.sock to prevent security issues with world-writeable directories [Rich, Daniel] --- docs/tools/qemu-storage-daemon.rst | 42 ++++++++++++++++++++++++++++-- 1 file changed, 40 insertions(+), 2 deletions(-) diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-d= aemon.rst index f63627eaf6..789a8e4a75 100644 --- a/docs/tools/qemu-storage-daemon.rst +++ b/docs/tools/qemu-storage-daemon.rst @@ -101,10 +101,12 @@ Standard options: =20 .. option:: --nbd-server addr.type=3Dinet,addr.host=3D,addr.port=3D<= port>[,tls-creds=3D][,tls-authz=3D][,max-connections=3D] --nbd-server addr.type=3Dunix,addr.path=3D[,tls-creds=3D][,tls= -authz=3D][,max-connections=3D] + --nbd-server addr.type=3Dfd,addr.str=3D[,tls-creds=3D][,tls-auth= z=3D][,max-connections=3D] =20 is a server for NBD exports. Both TCP and UNIX domain sockets are suppor= ted. - TLS encryption can be configured using ``--object`` tls-creds-* and auth= z-* - 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). =20 To configure an NBD server on UNIX domain socket path ``/tmp/nbd.sock``:: =20 @@ -127,6 +129,42 @@ QMP commands:: --chardev socket,path=3Dqmp.sock,server,nowait,id=3Dchar1 \ --monitor chardev=3Dchar1 =20 +Launch the daemon from Python with a QMP monitor socket using file descrip= tor +passing so there is no need to busy wait for the QMP monitor to become +available:: + + #!/usr/bin/env python3 + import subprocess + import socket + + sock_path =3D '/var/run/qmp.sock' + + with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as listen_sock: + listen_sock.bind(sock_path) + listen_sock.listen() + + fd =3D listen_sock.fileno() + + subprocess.Popen( + ['qemu-storage-daemon', + '--chardev', f'socket,fd=3D{fd},server=3Don,id=3Dchar1', + '--monitor', 'chardev=3Dchar1'], + pass_fds=3D[fd], + ) + + # listen_sock was automatically closed when leaving the 'with' statement + # body. If the daemon process terminated early then the following connec= t() + # will fail with "Connection refused" because no process has the listen + # socket open anymore. Launch errors can be detected this way. + + qmp_sock =3D socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) + qmp_sock.connect(sock_path) + ...QMP interaction... + +The same socket spawning approach also works with the ``--nbd-server +addr.type=3Dfd,addr.str=3D`` and ``--export +type=3Dvhost-user-blk,addr.type=3Dfd,addr.str=3D`` options. + Export raw image file ``disk.img`` over NBD UNIX domain socket ``nbd.sock`= `:: =20 $ qemu-storage-daemon \ --=20 2.29.2