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

[Amador Pahim]

On Thu, Jul 20, 2017 at 5:01 PM, Markus Armbruster <address@hidden> wrote:
> 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?

Ok, so let me take this opportunity to cleanup this code at once.