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Re: [Qemu-block] [Qemu-devel] [PATCH 3/3] iotests: Allow 147 to be run c

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 3/3] iotests: Allow 147 to be run concurrently
Date: Wed, 23 Jan 2019 08:33:41 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/23/19 7:12 AM, Max Reitz wrote:
> On 21.01.19 22:02, Eric Blake wrote:
>> On 12/21/18 5:47 PM, Max Reitz wrote:
>>> To do this, we need to allow creating the NBD server on various ports
>>> instead of a single one (which may not even work if you run just one
>>> instance, because something entirely else might be using that port).
>> Can you instead reuse the ideas from nbd_server_set_tcp_port() from
>> qemu-iotests/common.nbd?
>>> So we just pick a random port in [32768, 32768 + 1024) and try to create
>>> a server there.  If that fails, we just retry until something sticks.
>> That has the advantage of checking whether a port is actually in use
>> (using 'ss' - although it does limit the test to Linux-only; perhaps
>> using socat instead of ss could make the test portable to non-Linux?)
> But doesn't that give you race conditions?  That's the point of this
> series, so you can run multiple instances of 147 concurrently.

Hmm - that does imply that common.nbd's use of ss IS racy because it
checks in linear fashion and has a TOCTTOU window (affects at least
iotest 233). Your observation that random probes within a range are less
susceptible (although not immune) to the race is correct.

>> Do you actually need to attempt a qemu-nbd process, if you take my
>> suggestion of using ss to probe for an unused port?  And if not, do we
>> still need qemu_nbd_pipe() added earlier in the series?
>>> -        address = { 'type': 'inet',
>>> -                    'data': {
>>> -                        'host': 'localhost',
>>> -                        'port': str(NBD_PORT)
>>> -                    } }
>>> -        self._server_up(address, export_name)
>>> +        while True:
>>> +            nbd_port = random.randrange(NBD_PORT_START, NBD_PORT_END)
>> common.nbd just iterates, instead of trying random ports.
> I'm not sure which is better.  Iterating gives guaranteed termination,
> trying random ports means the first one you try will usually work.

Is there any other way we can make the test more robust, perhaps by
using socket activation (that is, pre-open the port prior to starting
qemu_nbd, so that our code for finding a free socket is more easily
reusable), or by using Unix sockets for test 147 (that test seems to be
using TCP sockets only as a means to get to the real feature under test,
and not as the actual thing being tested)?

Hmm, and you made me realize that socket activation is NOT documented in
'man qemu-nbd'; I ought to fix that.

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

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