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

[Max Reitz <mreitz@redhat.com>]

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On 23.01.19 15:33, Eric Blake wrote:
> 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)?

147 needs TCP sockets because that interface is tested.

Making the code reusable is not too high of a priority to me, as
normally NBD tests should just use Unix sockets.  This test is just a
special case.  But for this test, I can try to put the while loop into
an own function (that gets fed an address object without data.port), as
John has proposed.

Max

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



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