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Re: [PATCH] tests/qemu_iotests: Minimize usage of used ports

From: Eric Blake
Subject: Re: [PATCH] tests/qemu_iotests: Minimize usage of used ports
Date: Thu, 6 Feb 2020 10:48:26 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/6/20 10:37 AM, Max Reitz wrote:

thank you and I am sorry for not digging deep enough. This week my CI failed 

01:24:06 DEBUG| [stdout] +ERROR: test_inet (__main__.QemuNBD)
01:24:06 DEBUG| [stdout] 
01:24:06 DEBUG| [stdout] +Traceback (most recent call last):
01:24:06 DEBUG| [stdout] +  File "147", line 85, in setUp
01:24:06 DEBUG| [stdout] +    self.vm.launch()
01:24:06 DEBUG| [stdout] +  File 
 line 302, in launch
01:24:06 DEBUG| [stdout] +    self._launch()
01:24:06 DEBUG| [stdout] +  File 
 line 319, in _launch
01:24:06 DEBUG| [stdout] +    self._pre_launch()
01:24:06 DEBUG| [stdout] +  File 
 line 106, in _pre_launch
01:24:06 DEBUG| [stdout] +    super(QEMUQtestMachine, self)._pre_launch()
01:24:06 DEBUG| [stdout] +  File 
 line 270, in _pre_launch
01:24:06 DEBUG| [stdout] +    self._qmp = 
qmp.QEMUMonitorProtocol(self._vm_monitor, server=True)
01:24:06 DEBUG| [stdout] +  File 
 line 60, in __init__
01:24:06 DEBUG| [stdout] +    self.__sock.bind(self.__address)
01:24:06 DEBUG| [stdout] +OSError: [Errno 98] Address already in use

Was this test 147?  If so, see:

because that failure matches what I was seeing.

I made the mistake of reproducing this on my home system using the qemu 
revision that I had and assuming it's caused by a used port. So I limited the 
port range and used nc to occupy the port. It sort-of reproduced but instead of 
Address already in use it hanged until I kill the nc process. Then it failed 

+Traceback (most recent call last):
+  File "147", line 124, in test_inet
+    flatten_sock_addr(address))
+  File "147", line 59, in client_test
+    self.assert_qmp(result, 'return', {})
+  File "/home/medic/Work/Projekty/qemu/tests/qemu-iotests/iotests.py", line 
821, in assert_qmp
+    result = self.dictpath(d, path)
+  File "/home/medic/Work/Projekty/qemu/tests/qemu-iotests/iotests.py", line 
797, in dictpath
+    self.fail('failed path traversal for "%s" in "%s"' % (path, str(d)))
+AssertionError: failed path traversal for "return" in "{'error': {'class': 
'GenericError', 'desc': 'Failed to read initial magic: Unexpected end-of-file before all bytes were 

That's a secondary failure, I assume if the initial bug is fixed we are less likely to hit the secondary one; but the secondary one may still be worth fixing.

After a brief study I thought qemu is not doing the job well enough and wanted to add a 
protection. Anyway after a more thorough overview I came to a different conclusion and 
that is that we are facing the same issue as with incoming migration about a year ago. 
What happened is that I started "nc -l localhost 32789" which results in:

nc      26758 medic    3u  IPv6 9579487      0t0  TCP localhost:32789 (LISTEN)

Then we start the server by "_try_server_up" where qemu-nbd detects the port is 
occupied on IPv6 but available on IPv4, so it claims it:
nc        26758 medic    3u  IPv6 9579487      0t0  TCP localhost:32789 (LISTEN)
qemu-nbd  26927 medic    4u  IPv4 9591857      0t0  TCP localhost:32789 (LISTEN)

and reports success. Then we try to connect but the hotplugged VM first 
attempts to connect on the IPv6 address and hangs for infinity.

Now is this an expected behavior? If so then we need the find_free_address (but 
preferably directly in _try_server_up just before starting the qemu-nbd) to 
leave as little time-frame for collision as possible. Otherwise the test is 
alright and qemu-nbd needs a fix to bail out in case some address is already 
used (IIRC this is what incoming migration does).

Ah, OK.

Well, expected behavior...  It’s a shame, that’s what it is.

In libnbd, we recently improved the testsuite by switching over to systemd-style fd passing: instead of asking qemu-nbd to open a random port (and hoping it is available), we instead pre-open the port (where failure is under our control) and then pass in that fd with environment variables to qemu-nbd, which in turn guarantees that qemu-nbd won't hit failures in trying to use the port. Maybe we should utilize that more in qemu's own testsuite.

Also, I need to revisit my proposed patches for letting qemu-nbd support TLS over Unix sockets, as that's another way to avoid TCP contention (right now, qemu has an anachronistic prohibition against the combination of TLS and Unix sockets).

My second mistake was testing this on the old code-base and rebasing it only 
before sending the patch (without testing after the rebase). If I were to test 
it first, I would have found out that the real reproducer is simply running the 
test as the commit 8dff69b9415b4287e900358744b732195e1ab2e2 broke it.

So basically there are 2 actions:

1. fix the test as on my system it fails in 100% of cases, bisect says the 
first bad commit is 8dff69b9415b4287e900358744b732195e1ab2e2. Would anyone have 
time in digging into this? I already spent way too much time on this and don't 
really know what that commit is trying to do.

Yep, I’ve sent a patch:


Ah, so we did notice the same problem.

2. decide on the behavior when IPv4/6 is already in use (bail-out or start).
2a. In case it should bail-out than the test is correct and there is no need 
for my patch. On the other hand qemu-nbd has to be fixed

I don’t think it makes much sense to let qemu’s NBD server ensure that
it claims both IPv4 and IPv6 in case the user specifies a
non-descriptive hostname.

2b. Otherwise I can send a v2 that will check the port in the _try_server_up 
just before starting qemu-nbd to minimize the risk of using a utilized port (or 
should you decide it's not worth checking, I can simply forget about this)

Hm.  It wouldn’t be fully reliable, but, well...  The risk would be minimal.

OTOH, would it work if we just did a %s/localhost/ in the
test?  We have specific cases for IPv6, so I think it makes sense to
force IPv4 in all other cases.

Except then it will fail on machines configured for IPv6-only.

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

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