[PULL 13/19] python: backport 'avoid creating additional event loops per thread'

[John Snow <jsnow@redhat.com>]

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>
---
 python/qemu/qmp/legacy.py  | 46 +++++++++++++++++++++++---------------
 python/qemu/qmp/qmp_tui.py | 10 ++-------
 python/qemu/qmp/util.py    | 27 ++++++++++++++++++++++
 3 files changed, 57 insertions(+), 26 deletions(-)

diff --git a/python/qemu/qmp/legacy.py b/python/qemu/qmp/legacy.py
index 735d42971e9..e46695ae2c8 100644
--- a/python/qemu/qmp/legacy.py
+++ b/python/qemu/qmp/legacy.py
@@ -38,6 +38,7 @@
 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 @@ def __init__(self,
                 "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 @@ def send_fd_scm(self, fd: int) -> None:
         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"
             )
diff --git a/python/qemu/qmp/qmp_tui.py b/python/qemu/qmp/qmp_tui.py
index 12bdc17c99e..d946c205131 100644
--- a/python/qemu/qmp/qmp_tui.py
+++ b/python/qemu/qmp/qmp_tui.py
@@ -51,7 +51,7 @@
 from .message import DeserializationError, Message, UnexpectedTypeError
 from .protocol import ConnectError, Runstate
 from .qmp_client import ExecInterruptedError, QMPClient
-from .util import pretty_traceback
+from .util import get_or_create_event_loop, pretty_traceback
 
 
 # The name of the signal that is used to update the history list
@@ -387,13 +387,7 @@ def run(self, debug: bool = False) -> None:
         """
         screen = urwid.raw_display.Screen()
         screen.set_terminal_properties(256)
-
-        try:
-            self.aloop = asyncio.get_running_loop()
-        except RuntimeError:
-            # No running asyncio event loop. Create one if necessary.
-            self.aloop = asyncio.get_event_loop_policy().get_event_loop()
-
+        self.aloop = get_or_create_event_loop()
         self.aloop.set_debug(debug)
 
         # Gracefully handle SIGTERM and SIGINT signals
diff --git a/python/qemu/qmp/util.py b/python/qemu/qmp/util.py
index 0b3e781373d..47ec39a8b5e 100644
--- a/python/qemu/qmp/util.py
+++ b/python/qemu/qmp/util.py
@@ -10,6 +10,7 @@
 import sys
 import traceback
 from typing import TypeVar, cast
+import warnings
 
 
 T = TypeVar('T')
@@ -20,6 +21,32 @@
 # --------------------------
 
 
+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.
-- 
2.51.0