From 3f14b909ebe7296eef6d4b1a1ed5f602ab129602 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Mon, 1 Mar 2021 17:27:27 +0000 Subject: [PATCH] docs: show how to spawn qemu-storage-daemon with fd passing MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Message-Id: <20210301172728.135331-2-stefanha@redhat.com> Reviewed-by: Daniel P. Berrangé Reviewed-by: Richard W.M. Jones Signed-off-by: Kevin Wolf --- 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-daemon.rst index 6ce85f2f7d1a..5714794775a2 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``:: @@ -141,6 +143,42 @@ QMP commands:: --chardev socket,path=qmp.sock,server=on,wait=off,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 subprocess + import socket + + sock_path = '/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 = listen_sock.fileno() + + subprocess.Popen( + ['qemu-storage-daemon', + '--chardev', f'socket,fd={fd},server=on,id=char1', + '--monitor', 'chardev=char1'], + pass_fds=[fd], + ) + + # listen_sock was automatically closed when leaving the 'with' statement + # body. If the daemon process terminated early then the following connect() + # will fail with "Connection refused" because no process has the listen + # socket open anymore. Launch errors can be detected this way. + + qmp_sock = 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=fd,addr.str=`` and ``--export +type=vhost-user-blk,addr.type=fd,addr.str=`` options. + Export raw image file ``disk.img`` over NBD UNIX domain socket ``nbd.sock``:: $ qemu-storage-daemon \