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

[Amador Pahim <apahim@redhat.com>]

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 '')
             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,
-- 
2.13.3