python/aqmp: add __del__ method to legacy interface

asyncio can complain *very* loudly if you forget to back out of things
gracefully before the garbage collector starts destroying objects that
contain live references to asyncio Tasks.

The usual fix is just to remember to call aqmp.disconnect(), but for the
sake of the legacy wrapper and quick, one-off scripts where a graceful
shutdown is not necessarily of paramount imporance, add a courtesy
cleanup that will trigger prior to seeing screenfuls of confusing
asyncio tracebacks.

Note that we can't *always* save you from yourself; depending on when
the GC runs, you might just seriously be out of luck. The best we can do
in this case is to gently remind you to clean up after yourself.

(Still much better than multiple pages of incomprehensible python
warnings for the crime of forgetting to put your toys away.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>

python/qemu/aqmp/legacy.py

@@ -16,6 +16,8 @@ from typing import (
 import qemu.qmp
 from qemu.qmp import QMPMessage, QMPReturnValue, SocketAddrT
 
+from .error import AQMPError
+from .protocol import Runstate
 from .qmp_client import QMPClient
 
 
@@ -136,3 +138,19 @@ class QEMUMonitorProtocol(qemu.qmp.QEMUMonitorProtocol):
 
     def send_fd_scm(self, fd: int) -> None:
         self._aqmp.send_fd_scm(fd)
+
+    def __del__(self) -> None:
+        if self._aqmp.runstate == Runstate.IDLE:
+            return
+
+        if not self._aloop.is_running():
+            self.close()
+        else:
+            # Garbage collection ran while the event loop was running.
+            # Nothing we can do about it now, but if we don't raise our
+            # own error, the user will be treated to a lot of traceback
+            # they might not understand.
+            raise AQMPError(
+                "QEMUMonitorProtocol.close()"
+                " was not called before object was garbage collected"
+            )