[Qemu-devel] [PATCH v3 2/3] 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 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 <apahim@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
---
 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()
         except:
+            self._load_io_log()
             if self.is_running():
                 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
 
-- 
2.13.3