Re: [Qemu-devel] [PATCH v3 2/3] qemu.py: include debug information on launch error

[Markus Armbruster]

Amador Pahim <address@hidden> writes:

> On Thu, Jul 20, 2017 at 1:58 PM, Markus Armbruster <address@hidden> wrote:
>> Amador Pahim <address@hidden> 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 the patch:
>>>
>>>     >>> VM = qemu.QEMUMachine('../aarch64-softmmu/qemu-system-aarch64')
>>>     >>> VM.launch()
>>>     Traceback (most recent call last):
>>>       File "<stdin>", line 1, in <module>
>>>       File "qemu.py", line 137, in launch
>>>         self._post_launch()
>>>       File "qemu.py", line 121, 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 the patch:
>>>
>>>     >>> VM = qemu.QEMUMachine('../aarch64-softmmu/qemu-system-aarch64')
>>>     >>> VM.launch()
>>>     Traceback (most recent call last):
>>>       File "<stdin>", line 1, in <module>
>>>       File "qemu.py", line 156, in launch
>>>         raise RuntimeError(msg)
>>>     RuntimeError: Error launching VM.
>>>     Original Exception:
>>>     Traceback (most recent call last):
>>>       File "qemu.py", line 138, in launch
>>>         self._post_launch()
>>>       File "qemu.py", line 122, 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()
>>>     timeout: timed out
>>>     Command:
>>>     /usr/bin/qemu-system-aarch64 -chardev socket,id=mon,
>>>     path=/var/tmp/qemu-23958-monitor.sock -mon chardev=mon,mode=control
>>>     -display none -vga none
>>>     Output:
>>>     qemu-system-aarch64: No machine specified, and there is no default
>>>     Use -machine help to list supported machines
>>>
>>> Also, if the launch() faces an exception, the 'except' now will use args
>>> to fill the debug information. So this patch assigns 'args' earlier,
>>> assuring it will be available for the 'except'.
>>>
>>> Signed-off-by: Amador Pahim <address@hidden>
>>> Reviewed-by: Fam Zheng <address@hidden>
>>> ---
>>>  scripts/qemu.py | 18 ++++++++++++++++--
>>>  1 file changed, 16 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/scripts/qemu.py b/scripts/qemu.py
>>> index f0fade32bd..2707ae7f75 100644
>>> --- a/scripts/qemu.py
>>> +++ b/scripts/qemu.py
>>> @@ -18,6 +18,7 @@ import os
>>>  import sys
>>>  import subprocess
>>>  import qmp.qmp
>>> +import traceback
>>>
>>>
>>>  class QEMUMachine(object):
>>> @@ -129,17 +130,30 @@ class QEMUMachine(object):
>>>          '''Launch the VM and establish a QMP connection'''
>>>          devnull = open('/dev/null', 'rb')
>>>          qemulog = open(self._qemu_log_path, 'wb')
>>> +        args = self._wrapper + [self._binary] + self._base_args() + 
>>> self.args
>>>          try:
>>>              self._pre_launch()
>>> -            args = self._wrapper + [self._binary] + self._base_args() + 
>>> self._args
>>>              self._popen = subprocess.Popen(args, stdin=devnull, 
>>> stdout=qemulog,
>>>                                             stderr=subprocess.STDOUT, 
>>> shell=False)
>>>              self._post_launch()
>>
>> Lifting the assignment out of the try makes sense, because exceptions
>> there are programming errors.
>>
>> Why are ._pre_launch() and ._post_launch() inside the try?
>
> The _pre_launch()/_post_launch() are probably the main source of
> exceptions here and I assume we want to handle the exceptions coming
> from them.
>
> The call to subprocess.Popen() is unlikely to raise exceptions. I
> mean, it's easy to trigger an 'OSError' exception providing a
> non-existent command to Popen, but once we provide something
> executable, we will not see an exception here if the command fails.
> The CalledProcessError is only raised by subprocess.check_call() or
> subprocess.check_output().

Okay.

Catching all exceptions is too sloppy for my taste.  But it's not your
patch's fault.

>>>          except:
>>
>> Why do we catch all exceptions, and not just
>> subprocess.CalledProcessError?
>>
>>> +            self._load_io_log()
>>>              if self.is_running():
>>
>> I suspect we can't get here only for "odd" exceptions, not for
>> subprocess.CalledProcessError.
>>
>>>                  self._popen.kill()
>>>                  self._popen.wait()
>>> -            self._load_io_log()
>>> +            else:
>>> +                exc_type, exc_value, exc_traceback = sys.exc_info()
>>> +                msg = ('Error launching VM.\n'
>>> +                       'Original Exception: \n%s'
>>> +                       'Command:\n%s\n'
>>> +                       'Output:\n%s\n' %
>>> +                       (''.join(traceback.format_exception(exc_type,
>>> +                                                           exc_value,
>>> +                                                           exc_traceback)),
>>> +                        ' '.join(args),
>>> +                        self._iolog))
>>> +                self._post_shutdown()
>>> +                raise RuntimeError(msg)
>>>              self._post_shutdown()
>>>              raise
>>
>> Why do we print a traceback when subprocess.CalledProcessError?
>
> subprocess.CalledProcessError will not happen with subprocess.Popen,
> even if the command fails. But if the virtual machine does not start,
> we will, for example, see a 'socket.timeout' exception coming from
> _post_launch() when it tries to connect to the qemu monitor.

A traceback is a inexcusably lazy way to report such errors.  You're
putting lipstick on a pig.

When putting on lipstick is all we can do, it can be better than
nothing.  Is it all we can do?