utils/asyn: Use nest_asyncio when possible

nest_asyncio only supports the event loop implementation from the
standard library, but allows seemingly blocking calls to actually be
non-blocking in any situation, as opposed to the greenlet implementation
that sometimes is forced to run the coroutine in a separate thread.

devlib/utils/asyn.py

@@ -20,6 +20,7 @@
 
 import abc
 import asyncio
+import asyncio.events
 import functools
 import itertools
 import contextlib
@@ -28,9 +29,67 @@
 import inspect
 import threading
 from concurrent.futures import ThreadPoolExecutor
-from weakref import WeakSet
+from weakref import WeakSet, WeakKeyDictionary
 
 from greenlet import greenlet
+import nest_asyncio
+
+
+
+def _apply_nest_asyncio():
+    try:
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        loop = asyncio.new_event_loop()
+
+    policy = asyncio.get_event_loop_policy()
+
+    # Only apply nest_asyncio if the current event loop policy provides a
+    # BaseEventLoop from the standard library, as once nest_asyncio.apply() has
+    # been called, the policy itself is patched and there is no coming back
+    # from that. If the loop turns out to be non-patchable, every
+    # loop.run_until_complete()/asyncio.run() will just raise.
+    if isinstance(loop, asyncio.BaseEventLoop) and isinstance(policy, asyncio.events.BaseDefaultEventLoopPolicy):
+        try:
+            nest_asyncio.apply()
+        except Exception:
+            pass
+
+_apply_nest_asyncio()
+
+
+_USE_NEST_ASYNCIO_LOCK = threading.Lock()
+_USE_NEST_ASYNCIO_LOOP = WeakKeyDictionary()
+def _use_nest_asyncio(loop=None):
+    if loop is None:
+        try:
+            loop = asyncio.get_running_loop()
+        except RuntimeError:
+            loop = asyncio.new_event_loop()
+            asyncio.set_event_loop(loop)
+
+    def _run(coro):
+        return loop.run_until_complete(coro)
+
+    async def test_nested():
+        # Smoke-test that should trigger an exception if the event loop
+        # in use is not supported by asyncio or it did not get patched
+        # somehow.
+        _run(asyncio.sleep(0))
+
+    with _USE_NEST_ASYNCIO_LOCK:
+        try:
+            return _USE_NEST_ASYNCIO_LOOP[loop]
+        except KeyError:
+            try:
+                _run(test_nested())
+            except Exception:
+                ok = False
+            else:
+                ok = True
+
+            _USE_NEST_ASYNCIO_LOOP[loop] = ok
+            return ok
 
 
 def create_task(awaitable, name=None):
@@ -373,6 +432,17 @@ def allow_nested_run(coro):
     .. warning:: The coroutine needs to be consumed in the same OS thread it
         was created in.
     """
+    return _allow_nested_run(coro)
+
+
+def _allow_nested_run(coro, loop=None):
+    if _use_nest_asyncio(loop):
+        return coro
+    else:
+        return _do_allow_nested_run(coro)
+
+
+def _do_allow_nested_run(coro):
     return _AwaitableGen.wrap_coro(coro)
 
 
@@ -385,7 +455,7 @@ def _coro_thread_f(coro):
         asyncio.set_event_loop(loop)
 
         # The coroutine needs to be wrapped in the same thread that will consume it,
-    coro = allow_nested_run(coro)
+    coro = _allow_nested_run(coro, loop)
     return loop.run_until_complete(coro)
 
 
@@ -425,32 +495,35 @@ def run(coro):
     """
     assert inspect.getcoroutinestate(coro) == inspect.CORO_CREATED
 
-    try:
-        loop = asyncio.get_running_loop()
-    except RuntimeError:
-        # We are not currently running an event loop, so it's ok to just use
-        # asyncio.run() and let it create one.
-        # Once the coroutine is wrapped, we will be able to yield across
-        # blocking function boundaries thanks to _Genlet
-        return asyncio.run(allow_nested_run(coro))
+    if _use_nest_asyncio():
+        return asyncio.run(coro)
     else:
-        # Increase the odds that in the future, we have a wrapped coroutine in
-        # our callstack to avoid the _run_in_thread() path.
-        _install_task_factory(loop)
-
-        if loop.is_running():
-            g = _Genlet.get_enclosing()
-            if g is None:
-                # If we are not running under a wrapped coroutine, we don't
-                # have a choice and we need to run in a separate event loop.
-                return _run_in_thread(coro)
-            else:
-                # This requires that we have an coroutine wrapped with
-                # allow_nested_run() higher in the callstack, that we will be
-                # able to use as a conduit to yield the futures.
-                return g.consume_coro(coro, None)
+        try:
+            loop = asyncio.get_running_loop()
+        except RuntimeError:
+            # We are not currently running an event loop, so it's ok to just use
+            # asyncio.run() and let it create one.
+            # Once the coroutine is wrapped, we will be able to yield across
+            # blocking function boundaries thanks to _Genlet
+            return asyncio.run(_do_allow_nested_run(coro))
         else:
-            return loop.run_until_complete(coro)
+            # Increase the odds that in the future, we have a wrapped coroutine in
+            # our callstack to avoid the _run_in_thread() path.
+            _install_task_factory(loop)
+
+            if loop.is_running():
+                g = _Genlet.get_enclosing()
+                if g is None:
+                    # If we are not running under a wrapped coroutine, we don't
+                    # have a choice and we need to run in a separate event loop.
+                    return _run_in_thread(coro)
+                else:
+                    # This requires that we have an coroutine wrapped with
+                    # allow_nested_run() higher in the callstack, that we will be
+                    # able to use as a conduit to yield the futures.
+                    return g.consume_coro(coro, None)
+            else:
+                return loop.run_until_complete(coro)
 
 
 def asyncf(f):

setup.py

@@ -104,6 +104,7 @@ def _load_path(filepath):
         'pandas',
         'pytest',
         'lxml', # More robust xml parsing
+        'nest_asyncio', # Allows running nested asyncio loops
         'greenlet', # Allows running nested asyncio loops
         'future', # for the "past" Python package
         'ruamel.yaml >= 0.15.72', # YAML formatted config parsing