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

[Amador Pahim]

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().


>
>>          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.