python/machine: Handle QMP errors on close more meticulously

To use the AQMP backend, Machine just needs to be a little more diligent
about what happens when closing a QMP connection. The operation is no
longer a freebie in the async world; it may return errors encountered in
the async bottom half on incoming message receipt, etc.

(AQMP's disconnect, ultimately, serves as the quiescence point where all
async contexts are gathered together, and any final errors reported at
that point.)

Because async QMP continues to check for messages asynchronously, it's
almost certainly likely that the loop will have exited due to EOF after
issuing the last 'quit' command. That error will ultimately be bubbled
up when attempting to close the QMP connection. The manager class here
then is free to discard it -- if it was expected.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20211026175612.4127598-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine/machine.py

@@ -342,9 +342,15 @@ class QEMUMachine:
         # Comprehensive reset for the failed launch case:
         self._early_cleanup()
 
-        if self._qmp_connection:
-            self._qmp.close()
-            self._qmp_connection = None
+        try:
+            self._close_qmp_connection()
+        except Exception as err:  # pylint: disable=broad-except
+            LOG.warning(
+                "Exception closing QMP connection: %s",
+                str(err) if str(err) else type(err).__name__
+            )
+        finally:
+            assert self._qmp_connection is None
 
         self._close_qemu_log_file()
 
@@ -420,6 +426,31 @@ class QEMUMachine:
                                        close_fds=False)
         self._post_launch()
 
+    def _close_qmp_connection(self) -> None:
+        """
+        Close the underlying QMP connection, if any.
+
+        Dutifully report errors that occurred while closing, but assume
+        that any error encountered indicates an abnormal termination
+        process and not a failure to close.
+        """
+        if self._qmp_connection is None:
+            return
+
+        try:
+            self._qmp.close()
+        except EOFError:
+            # EOF can occur as an Exception here when using the Async
+            # QMP backend. It indicates that the server closed the
+            # stream. If we successfully issued 'quit' at any point,
+            # then this was expected. If the remote went away without
+            # our permission, it's worth reporting that as an abnormal
+            # shutdown case.
+            if not (self._user_killed or self._quit_issued):
+                raise
+        finally:
+            self._qmp_connection = None
+
     def _early_cleanup(self) -> None:
         """
         Perform any cleanup that needs to happen before the VM exits.
@@ -460,9 +491,14 @@ class QEMUMachine:
         self._early_cleanup()
 
         if self._qmp_connection:
-            if not self._quit_issued:
-                # Might raise ConnectionReset
-                self.qmp('quit')
+            try:
+                if not self._quit_issued:
+                    # May raise ExecInterruptedError or StateError if the
+                    # connection dies or has *already* died.
+                    self.qmp('quit')
+            finally:
+                # Regardless, we want to quiesce the connection.
+                self._close_qmp_connection()
 
         # May raise subprocess.TimeoutExpired
         self._subp.wait(timeout=timeout)