remote/client: rework event loop handling in start_session()

Calling asyncio.get_event_loop() with no current event loop is deprecated
since Python 3.10 and will be an error in some future Python release
[1]. Using it in labgrid.remote.client.start_session() causes errors in
IPython when using a RemotePlace:

  In [1]: from labgrid.resource.remote import RemotePlace
     ...: from labgrid import Target
     ...:
     ...: target = Target("example")
     ...: RemotePlace(target, name="example-place")
  [...]
  RuntimeError: There is no current event loop in thread 'MainThread'.

For labgrid.remote.client.start_session() there is no reliable way of
retrieving the thread's event loop without being called from an async
context (which we cannot assume here). Instead of using
asyncio.get_event_loop(), use a new helper function ensure_event_loop()
that returns the first available loop instance from:

- externally provided event loop
- stashed event loop
- OS thread's running event loop (when called from async code)
- new event loop

The returned loop is stashed for future calls. See also [2] for a
similar approach.

start_session() now accepts a new optional argument "loop" for providing
an external event loop.

[1] https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.get_event_loop
[2] jupyter/jupyter_core#387

Signed-off-by: Bastian Krause <bst@pengutronix.de>

labgrid/remote/client.py

@@ -4,6 +4,7 @@
 import argparse
 import asyncio
 import contextlib
+from contextvars import ContextVar
 import enum
 import os
 import pathlib
@@ -1529,8 +1530,45 @@ def print_version(self):
         print(labgrid_version())
 
 
-def start_session(address, extra, debug=False):
-    loop = asyncio.get_event_loop()
+_loop: ContextVar["asyncio.AbstractEventLoop | None"] = ContextVar("_loop", default=None)
+
+
+def ensure_event_loop(external_loop=None):
+    """Get the event loop for this thread, or create a new event loop."""
+    # get stashed loop
+    loop = _loop.get()
+
+    # ignore closed stashed loop
+    if loop and loop.is_closed():
+        loop = None
+
+    if external_loop:
+        # if a loop is stashed, expect it to be the same as the external one
+        if loop:
+            assert loop is external_loop
+        _loop.set(external_loop)
+        return external_loop
+
+    # return stashed loop
+    if loop:
+        return loop
+
+    try:
+        # if called from async code, try to get current's thread loop
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        # no previous, external or running loop found, create a new one
+        loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(loop)
+
+    # stash it
+    _loop.set(loop)
+    return loop
+
+
+def start_session(address, extra, debug=False, loop=None):
+    loop = ensure_event_loop(loop)
+
     if debug:
         loop.set_debug(True)
 
@@ -2040,7 +2078,9 @@ def main():
                 coordinator_address = os.environ.get("LG_COORDINATOR", "127.0.0.1:20408")
 
             logging.debug('Starting session with "%s"', coordinator_address)
-            session = start_session(coordinator_address, extra, args.debug)
+            loop = asyncio.new_event_loop()
+            asyncio.set_event_loop(loop)
+            session = start_session(coordinator_address, extra=extra, debug=args.debug, loop=loop)
             logging.debug("Started session")
 
             try: