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, 27 insertions, 0 deletions

diff --git a/python/qemu/qmp/util.py b/python/qemu/qmp/util.py
index 0b3e781373..47ec39a8b5 100644
--- a/python/qemu/qmp/util.py
+++ b/python/qemu/qmp/util.py
@@ -10,6 +10,7 @@ import asyncio
 import sys
 import traceback
 from typing import TypeVar, cast
+import warnings
 
 
 T = TypeVar('T')
@@ -20,6 +21,32 @@ T = TypeVar('T')
 # --------------------------
 
 
+def get_or_create_event_loop() -> asyncio.AbstractEventLoop:
+    """
+    Return this thread's current event loop, or create a new one.
+
+    This function behaves similarly to asyncio.get_event_loop() in
+    Python<=3.13, where if there is no event loop currently associated
+    with the current context, it will create and register one. It should
+    generally not be used in any asyncio-native applications.
+    """
+    try:
+        with warnings.catch_warnings():
+            # Python <= 3.13 will trigger deprecation warnings if no
+            # event loop is set, but will create and set a new loop.
+            warnings.simplefilter("ignore")
+            loop = asyncio.get_event_loop()
+    except RuntimeError:
+        # Python 3.14+: No event loop set for this thread,
+        # create and set one.
+        loop = asyncio.new_event_loop()
+        # Set this loop as the current thread's loop, to be returned
+        # by calls to get_event_loop() in the future.
+        asyncio.set_event_loop(loop)
+
+    return loop
+
+
 async def flush(writer: asyncio.StreamWriter) -> None:
     """
     Utility function to ensure a StreamWriter is *fully* drained.