Re: [Qemu-devel] [PATCH v6 7/7] qemu.py: include debug information on launch error

[Amador Pahim <apahim@redhat.com>]

On Tue, Aug 15, 2017 at 10:57 AM, Markus Armbruster <armbru@redhat.com> wrote:
> Amador Pahim <apahim@redhat.com> writes:
>
>> When launching a VM, if an exception happens and the VM is not
>> initiated, it is useful to see the qemu command line that was executed
>> and the output of that command.
>>
>> Before this patch:
>>
>>     >>> import qemu
>>     >>> vm = qemu.QEMUMachine('qemu-system-aarch64', debug=True)
>>     >>> vm.launch()
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>       File "qemu.py", line 175, in launch
>>         self._launch()
>>       File "qemu.py", line 189, in _launch
>>         self._post_launch()
>>       File "qemu.py", line 154, in _post_launch
>>         self._qmp.accept()
>>       File "qmp/qmp.py", line 145, in accept
>>         self.__sock, _ = self.__sock.accept()
>>       File "/usr/lib64/python2.7/socket.py", line 206, in accept
>>         sock, addr = self._sock.accept()
>>     socket.timeout: timed out
>>     >>>
>>
>> After this patch:
>>
>>     >>> import qemu
>>     >>> vm = qemu.QEMUMachine('qemu-system-aarch64', debug=True)
>>     >>> vm.launch()
>>     DEBUG:qemu:Error launching VM. Command: 'qemu-system-aarch64
>>     -chardev socket,id=mon,path=/var/tmp/qemu-5298-monitor.sock
>>     -mon chardev=mon,mode=control -display none -vga none'.
>>     Output: 'qemu-system-aarch64: No machine specified, and there
>>     is no default\nUse -machine help to list supported machines\n'.
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>       File "qemu.py", line 175, in launch
>>         self._launch()
>>       File "qemu.py", line 189, in _launch
>>         self._post_launch()
>>       File "qemu.py", line 154, in _post_launch
>>         self._qmp.accept()
>>       File "qmp/qmp.py", line 145, in accept
>>         self.__sock, _ = self.__sock.accept()
>>       File "/usr/lib64/python2.7/socket.py", line 206, in accept
>>         sock, addr = self._sock.accept()
>>     socket.timeout: timed out
>>     >>>
>>
>> Signed-off-by: Amador Pahim <apahim@redhat.com>
>> ---
>>  scripts/qemu.py | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/scripts/qemu.py b/scripts/qemu.py
>> index e9a3a96d13..43fd0b072c 100644
>> --- a/scripts/qemu.py
>> +++ b/scripts/qemu.py
>> @@ -158,7 +158,10 @@ class QEMUMachine(object):
>>              self._remove_if_exists(self._created_files.pop())
>>
>>      def launch(self):
>> -        '''Launch the VM and establish a QMP connection'''
>> +        '''
>> +        Try to launch the VM and make sure we cleanup and expose the
>> +        command line/output in case of exception.
>> +        '''
>>
>>          if self.is_running():
>>              raise QEMULaunchError('VM already running.')
>> @@ -169,6 +172,7 @@ class QEMUMachine(object):
>>                                    'before launching again.')
>>
>>          try:
>> +            self._iolog = None
>>              self._qemu_full_args = None
>>              self._qemu_full_args = (self._wrapper + [self._binary] +
>>                                      self._base_args() + self._args)
>> @@ -176,9 +180,15 @@ class QEMUMachine(object):
>>              self._pending_shutdown = True
>>          except:
>>              self.shutdown()
>> +            LOG.debug('Error launching VM.%s%s',
>> +                      ' Command: %r.' % ' '.join(self._qemu_full_args)
>> +                      if self._qemu_full_args else '',
>> +                      ' Output: %r.' % self._iolog
>> +                      if self._iolog else '')
>
> This nested interpolation is too ugly to live :)

I find it beautiful :)
Killing it anyway.

>
> Nesting is easy to avoid:
>
>                LOG.debug('Error launching VM\n. ' Command: %r. Output: %r.'
>                          % (' '.join(self._qemu_full_args)
>                             if self._qemu_full_args else '',
>                             self._iolog
>                             if self._iolog else ''))

In this case we will have the strings "Command: " and "Output:" even
with no content to expose.

>
> Why %r and not %s?

It's a matter of preference. I tend to avoid allowing messages to
break the log line. Anyway, I'm improving this.

>
> Are you sure '\n' is appropriate in the argument of LOG.debug()?
>
> Why cram everything into a single LOG.debug()?
>
>                LOG.debug('Error launching VM')
>                LOG.debug('Command: %s' % (' '.join(self._qemu_full_args)
>                                           if self._qemu_full_args else ''))
>                LOG.debug('Output: %s' % self._iolog if self._iolog else ''))

Sure, I'm ok with both ways.

>
>>              raise
>>
>>      def _launch(self):
>> +        '''Launch the VM and establish a QMP connection.'''
>>          self._pre_launch()
>>          devnull = open(os.path.devnull, 'rb')
>>          self._popen = subprocess.Popen(self._qemu_full_args,