[PATCH v3 2/4] python/machine: raise VMLaunchFailure exception from launch()

[John Snow <jsnow@redhat.com>]

This allows us to pack in some extra information about the failure,
which guarantees that if the caller did not *intentionally* cause a
failure (by capturing this Exception), some pretty good clues will be
printed at the bottom of the traceback information.

This will help make failures in the event of a non-negative return code
more obvious when they go unhandled; the current behavior is to print a
warning message only in the event of signal-based terminations (for
negative return codes).

(Note: In Python, catching BaseException instead of Exception is like
installing a signal handler that will run as long as Python itself
doesn't crash. KeyboardInterrupt and several other "strong" events in
Python are a BaseException. These events should generally not be
suppressed, but occasionally we want to perform some cleanup in response
to one.)

Signed-off-by: John Snow <jsnow@redhat.com>
---
 python/qemu/machine/machine.py            | 45 ++++++++++++++++++++---
 tests/qemu-iotests/tests/mirror-top-perms |  3 +-
 2 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/python/qemu/machine/machine.py b/python/qemu/machine/machine.py
index 67ab06ca2b..a5972fab4d 100644
--- a/python/qemu/machine/machine.py
+++ b/python/qemu/machine/machine.py
@@ -74,6 +74,35 @@ class QEMUMachineAddDeviceError(QEMUMachineError):
     """
 
 
+class VMLaunchFailure(QEMUMachineError):
+    """
+    Exception raised when a VM launch was attempted, but failed.
+    """
+    def __init__(self, exitcode: Optional[int],
+                 command: str, output: Optional[str]):
+        super().__init__(exitcode, command, output)
+        self.exitcode = exitcode
+        self.command = command
+        self.output = output
+
+    def __str__(self) -> str:
+        ret = ''
+        if self.__cause__ is not None:
+            name = type(self.__cause__).__name__
+            reason = str(self.__cause__)
+            if reason:
+                ret += f"{name}: {reason}"
+            else:
+                ret += f"{name}"
+        ret += '\n'
+
+        if self.exitcode is not None:
+            ret += f"\tExit code: {self.exitcode}\n"
+        ret += f"\tCommand: {self.command}\n"
+        ret += f"\tOutput: {self.output}\n"
+        return ret
+
+
 class AbnormalShutdown(QEMUMachineError):
     """
     Exception raised when a graceful shutdown was requested, but not performed.
@@ -397,7 +426,7 @@ def launch(self) -> None:
 
         try:
             self._launch()
-        except:
+        except BaseException as exc:
             # We may have launched the process but it may
             # have exited before we could connect via QMP.
             # Assume the VM didn't launch or is exiting.
@@ -408,11 +437,15 @@ def launch(self) -> None:
             else:
                 self._post_shutdown()
 
-            LOG.debug('Error launching VM')
-            if self._qemu_full_args:
-                LOG.debug('Command: %r', ' '.join(self._qemu_full_args))
-            if self._iolog:
-                LOG.debug('Output: %r', self._iolog)
+            if isinstance(exc, Exception):
+                raise VMLaunchFailure(
+                    exitcode=self.exitcode(),
+                    command=' '.join(self._qemu_full_args),
+                    output=self._iolog
+                ) from exc
+
+            # Don't wrap 'BaseException'; doing so would downgrade
+            # that exception. However, we still want to clean up.
             raise
 
     def _launch(self) -> None:
diff --git a/tests/qemu-iotests/tests/mirror-top-perms b/tests/qemu-iotests/tests/mirror-top-perms
index 0a51a613f3..b5849978c4 100755
--- a/tests/qemu-iotests/tests/mirror-top-perms
+++ b/tests/qemu-iotests/tests/mirror-top-perms
@@ -21,7 +21,6 @@
 
 import os
 
-from qemu.aqmp import ConnectError
 from qemu.machine import machine
 from qemu.qmp import QMPConnectError
 
@@ -107,7 +106,7 @@ class TestMirrorTopPerms(iotests.QMPTestCase):
                 self.vm_b.launch()
                 print('ERROR: VM B launched successfully, '
                       'this should not have happened')
-        except (QMPConnectError, ConnectError):
+        except (QMPConnectError, machine.VMLaunchFailure):
             assert 'Is another process using the image' in self.vm_b.get_log()
 
         result = self.vm.qmp('block-job-cancel',
-- 
2.31.1