python: backport 'avoid creating additional event loops per thread'

This commit is two backports squashed into one to avoid regressions.

python: *really* remove get_event_loop

A prior commit, aa1ff990, switched away from using get_event_loop *by
default*, but this is not good enough to avoid deprecation warnings as
`asyncio.get_event_loop_policy().get_event_loop()` is *also*
deprecated. Replace this mechanism with explicit calls to
asyncio.get_new_loop() and revise the cleanup mechanisms in __del__ to
match.

python: avoid creating additional event loops per thread

"Too hasty by far!", commit 21ce2ee4 attempted to avoid deprecated
behavior altogether by calling new_event_loop() directly if there was no
loop currently running, but this has the unfortunate side effect of
potentially creating multiple event loops per thread if tests
instantiate multiple QMP connections in a single thread. This behavior
is apparently not well-defined and causes problems in some, but not all,
combinations of Python interpreter version and platform environment.

Partially revert to Daniel Berrange's original patch, which calls
get_event_loop and simply suppresses the deprecation warning in
Python<=3.13. This time, however, additionally register new loops
created with new_event_loop() so that future calls to get_event_loop()
will return the loop already created.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@21ce2ee4f2df87efe84a27b9c5112487f4670622
cherry picked from commit python-qemu-qmp@c08fb82b38212956ccffc03fc6d015c3979f42fe
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

1 files changed, 28 insertions, 18 deletions

diff --git a/python/qemu/qmp/legacy.py b/python/qemu/qmp/legacy.py
index 735d42971e..e46695ae2c 100644
--- a/python/qemu/qmp/legacy.py
+++ b/python/qemu/qmp/legacy.py
@@ -38,6 +38,7 @@ from typing import (
 from .error import QMPError
 from .protocol import Runstate, SocketAddrT
 from .qmp_client import QMPClient
+from .util import get_or_create_event_loop
 
 
 #: QMPMessage is an entire QMP message of any kind.
@@ -86,17 +87,13 @@ class QEMUMonitorProtocol:
                 "server argument should be False when passing a socket")
 
         self._qmp = QMPClient(nickname)
-
-        try:
-            self._aloop = asyncio.get_running_loop()
-        except RuntimeError:
-            # No running loop; since this is a sync shim likely to be
-            # used in fully sync programs, create one if neccessary.
-            self._aloop = asyncio.get_event_loop_policy().get_event_loop()
-
         self._address = address
         self._timeout: Optional[float] = None
 
+        # This is a sync shim intended for use in fully synchronous
+        # programs. Create and set an event loop if necessary.
+        self._aloop = get_or_create_event_loop()
+
         if server:
             assert not isinstance(self._address, socket.socket)
             self._sync(self._qmp.start_server(self._address))
@@ -313,17 +310,30 @@ class QEMUMonitorProtocol:
         self._qmp.send_fd_scm(fd)
 
     def __del__(self) -> None:
-        if self._qmp.runstate == Runstate.IDLE:
-            return
+        if self._qmp.runstate != Runstate.IDLE:
+            self._qmp.logger.warning(
+                "QEMUMonitorProtocol object garbage collected without a prior "
+                "call to close()"
+            )
 
         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.
+            if self._qmp.runstate != Runstate.IDLE:
+                # If the user neglected to close the QMP session and we
+                # are not currently running in an asyncio context, we
+                # have the opportunity to close the QMP session. If we
+                # do not do this, the error messages presented over
+                # dangling async resources may not make any sense to the
+                # user.
+                self.close()
+
+        if self._qmp.runstate != Runstate.IDLE:
+            # If QMP is still not quiesced, it means that the garbage
+            # collector ran from a context within the event loop and we
+            # are simply too late to take any corrective action. Raise
+            # our own error to give meaningful feedback to the user in
+            # order to prevent pages of asyncio stacktrace jargon.
             raise QMPError(
-                "QEMUMonitorProtocol.close()"
-                " was not called before object was garbage collected"
+                "QEMUMonitorProtocol.close() was not called before object was "
+                "garbage collected, and could not be closed due to GC running "
+                "in the event loop"
             )