python/machine: handle "fast" QEMU terminations

In the case that the QEMU process actually launches -- but then dies so
quickly that we can't establish a QMP connection to it -- QEMUMachine
currently calls _post_shutdown() assuming that it never launched the VM
process.

This isn't true, though: it "merely" may have failed to establish a QMP
connection and the process is in the middle of its own exit path.

If we don't wait for the subprocess, the caller may get a bogus `None`
return for .exitcode(). This behavior was observed from
device-crash-test; after the switch to Async QMP, the timings were
changed such that it was now seemingly possible to witness the failure
of "vm.launch()" *prior* to the exitcode becoming available.

The semantic of the `_launched` property is changed in this
patch. Instead of representing the condition "launch() executed
successfully", it will now represent "has forked a child process
successfully". This way, wait() when called in the exit path won't
become a no-op.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-id: 20211118204620.1897674-6-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine/machine.py

@@ -349,9 +349,6 @@ def _post_shutdown(self) -> None:
         Called to cleanup the VM instance after the process has exited.
         May also be called after a failed launch.
         """
-        # Comprehensive reset for the failed launch case:
-        self._early_cleanup()
-
         try:
             self._close_qmp_connection()
         except Exception as err:  # pylint: disable=broad-except
@@ -400,9 +397,16 @@ def launch(self) -> None:
 
         try:
             self._launch()
-            self._launched = True
         except:
-            self._post_shutdown()
+            # 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.
+            # If we don't wait for the process, exitcode() may still be
+            # 'None' by the time control is ceded back to the caller.
+            if self._launched:
+                self.wait()
+            else:
+                self._post_shutdown()
 
             LOG.debug('Error launching VM')
             if self._qemu_full_args:
@@ -426,6 +430,7 @@ def _launch(self) -> None:
                                        stderr=subprocess.STDOUT,
                                        shell=False,
                                        close_fds=False)
+        self._launched = True
         self._post_launch()
 
     def _close_qmp_connection(self) -> None:
@@ -457,8 +462,8 @@ def _early_cleanup(self) -> None:
         """
         Perform any cleanup that needs to happen before the VM exits.
 
-        May be invoked by both soft and hard shutdown in failover scenarios.
-        Called additionally by _post_shutdown for comprehensive cleanup.
+        This method may be called twice upon shutdown, once each by soft
+        and hard shutdown in failover scenarios.
         """
         # If we keep the console socket open, we may deadlock waiting
         # for QEMU to exit, while QEMU is waiting for the socket to