fix: Add workaround for grpc/grpc#25364 (#213)

Author: dpcollins-google

google/cloud/pubsublite/cloudpubsub/internal/managed_event_loop.py

@@ -14,24 +14,76 @@
 
 from asyncio import AbstractEventLoop, new_event_loop, run_coroutine_threadsafe
 from concurrent.futures import Future
-from threading import Thread
-from typing import ContextManager
+from threading import Thread, Lock
+from typing import ContextManager, Generic, TypeVar, Optional, Callable
 
+_T = TypeVar("_T")
 
-class ManagedEventLoop(ContextManager):
+
+class _Lazy(Generic[_T]):
+    _Factory = Callable[[], _T]
+
+    _lock: Lock
+    _factory: _Factory
+    _impl: Optional[_T]
+
+    def __init__(self, factory: _Factory):
+        self._lock = Lock()
+        self._factory = factory
+        self._impl = None
+
+    def get(self) -> _T:
+        with self._lock:
+            if self._impl is None:
+                self._impl = self._factory()
+            return self._impl
+
+
+class _ManagedEventLoopImpl(ContextManager):
     _loop: AbstractEventLoop
     _thread: Thread
 
     def __init__(self, name=None):
         self._loop = new_event_loop()
-        self._thread = Thread(target=lambda: self._loop.run_forever(), name=name)
+        self._thread = Thread(
+            target=lambda: self._loop.run_forever(), name=name, daemon=True
+        )
 
     def __enter__(self):
         self._thread.start()
+        return self
 
     def __exit__(self, exc_type, exc_value, traceback):
         self._loop.call_soon_threadsafe(self._loop.stop)
         self._thread.join()
 
     def submit(self, coro) -> Future:
         return run_coroutine_threadsafe(coro, self._loop)
+
+
+# TODO(dpcollins): Remove when underlying issue is fixed.
+# This is a workaround for https://github.com/grpc/grpc/issues/25364, a grpc
+# issue which prevents grpc-asyncio working with multiple event loops in the
+# same process. This workaround enables multiple topic publishing as well as
+# publish/subscribe from the same process, but does not enable use with other
+# grpc-asyncio clients. Once this issue is fixed, roll back the PR which
+# introduced this to return to a single event loop per client for isolation.
+_global_event_loop: _Lazy[_ManagedEventLoopImpl] = _Lazy(
+    lambda: _ManagedEventLoopImpl(name="PubSubLiteEventLoopThread").__enter__()
+)
+
+
+class ManagedEventLoop(ContextManager):
+    _loop: _ManagedEventLoopImpl
+
+    def __init__(self, name=None):
+        self._loop = _global_event_loop.get()
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        pass
+
+    def submit(self, coro) -> Future:
+        return self._loop.submit(coro)