Re: [PATCH v9 3/3] iotests: test nbd reconnect

[Eric Blake <eblake@redhat.com>]

On 9/24/19 3:31 AM, Vladimir Sementsov-Ogievskiy wrote:

>>> +def qemu_nbd_popen(*args):
>>> +    '''Run qemu-nbd in daemon mode and return the parent's exit code'''
>>> +    return subprocess.Popen(qemu_nbd_args + ['--persistent'] + list(args))
>>> +
>>
>> Should you also use a pid file here, and wait for the existence of the
>> pid file before returning (rather than hard-coding sleep(1))?
> 
> What do you mean / how to do it?
> 
> We want to wait until listening socket is prepared..

In shell:

qemu-nbd --pid-file=/path/to/file ...
while [ ! -e /path/to/file ]; do
   sleep ... # fractional second, or exponential, or whatever...
done
# Now the listening socket is indeed prepared

You'd have to translate that idiom to python.

Or:

pre-open Unix socket at /path/to/socket
LISTEN_PID=... LISTEN_FDS=1 qemu-nbd ... 3<>/path/to/socket

Now the socket is pre-created and passed into qemu-nbd via systemd 
socket activation, so you know the listening socket is ready without 
having to do any loop at all.  Here's a patch in libnbd where we just 
switched from waiting for the port to appear (because the test predated 
qemu-nbd pidfile support) to instead using socket activation, for reference:
https://github.com/libguestfs/libnbd/commit/352331d177

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org