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

From: Max Reitz
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 3/3] iotests: Allow 147 to be run concurrently
Date: Wed, 23 Jan 2019 14:12:55 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

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.

>> For the IPv6 test, we need a different range, though (just above that
>> one).  This is because "localhost" resolves to both and ::1.
>> This means that if you bind to it, it will bind to both, if possible, or
>> just one if the other is already in use.  Therefore, if the IPv6 test
>> has already taken [::1]:some_port and we then try to take
>> localhost:some_port, that will work -- only the second server will be
>> bound to alone and not [::1]:some_port in addition.
>> So we have two different servers on the same port, one for IPv4 and one
>> for IPv6.
>> But when we then try to connect to the server through
>> localhost:some_port, we will always end up at the IPv6 one (as long as
>> it is up), and this may not be the one we want.
>> Thus, we must make sure not to create an IPv6-only NBD server on the
>> same port as a normal "dual-stack" NBD server -- which is done by using
>> distinct port ranges, as explained above.
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  tests/qemu-iotests/147 | 98 +++++++++++++++++++++++++++++-------------
>>  1 file changed, 68 insertions(+), 30 deletions(-)
>> @@ -88,17 +92,29 @@ class QemuNBD(NBDBlockdevAddBase):
>>          except OSError:
>>              pass
>> +    def _try_server_up(self, *args):
>> +        status, msg = qemu_nbd_pipe('-f', imgfmt, test_img, *args)
>> +        if status == 0:
>> +            return True
>> +        if 'Address already in use' in msg:
>> +            return False
>> +        self.fail(msg)
> 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.


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