aboutsummaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorStefan Hajnoczi <stefanha@redhat.com>2021-03-01 17:27:27 +0000
committerKevin Wolf <kwolf@redhat.com>2021-03-08 14:55:19 +0100
commit3f14b909ebe7296eef6d4b1a1ed5f602ab129602 (patch)
treefce96b20cbb87121b00b8d7270b04d37d432ddcf
parent03d2b412aaf2078425f8472f31c8a9c2340969eb (diff)
docs: show how to spawn qemu-storage-daemon with fd passing
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>: https://gitlab.com/nbdkit/libnbd/-/commit/89113f484effb0e6c322314ba75c1cbe07a04543 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>
-rw-r--r--docs/tools/qemu-storage-daemon.rst42
1 files changed, 40 insertions, 2 deletions
diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-daemon.rst
index 6ce85f2f7d..5714794775 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=<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 \