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docs: show how to spawn qemu-storage-daemon with fd passing

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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 <rjones@redhat.com>:
89113f484e

Thanks to Daniel P. Berrangé <berrange@redhat.com> for suggestions on
how to get this working. Now let's document it!

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210301172728.135331-2-stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2021-03-01 17:27:27 +00:00 committed by Kevin Wolf
parent 03d2b412aa
commit 3f14b909eb
1 changed files with 40 additions and 2 deletions
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42
docs/tools/qemu-storage-daemon.rst
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@ -101,10 +101,12 @@ Standard options:
.. option:: --nbd-server addr.type=inet,addr.host=<host>,addr.port=<port>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
--nbd-server addr.type=unix,addr.path=<path>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
--nbd-server addr.type=fd,addr.str=<fd>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
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=<fd>`` and ``--export
type=vhost-user-blk,addr.type=fd,addr.str=<fd>`` options.
Export raw image file ``disk.img`` over NBD UNIX domain socket ``nbd.sock``::
$ qemu-storage-daemon \