Support Windows

src/python/grpcio/grpc/_cython/_cygrpc/fork_posix.pyx.pxi

@@ -94,28 +94,6 @@ def fork_handlers_and_grpc_init():
                 _fork_state.fork_handler_registered = True
 
 
-def _contextvars_supported():
-    try:
-        import contextvars
-        return True
-    except ImportError:
-        return False
-
-
-if _contextvars_supported():
-    import contextvars
-    def _run_with_context(target):
-        ctx = contextvars.copy_context()
-        def _run(*args):
-            ctx.run(target, *args)
-        return _run
-else:
-    # NOTE(rbellevi): `contextvars` was not introduced until 3.7. On earlier
-    # interpreters, we simply do not propagate contextvars between threads.
-    def _run_with_context(target):
-        def _run(*args):
-            target(*args)
-        return _run
 
 
 class ForkManagedThread(object):

src/python/grpcio/grpc/_cython/_cygrpc/fork_windows.pyx.pxi

@@ -19,10 +19,9 @@ def fork_handlers_and_grpc_init():
     grpc_init()
 
 
-# TODO: Propagate contextvars.
 class ForkManagedThread(object):
     def __init__(self, target, args=()):
-        self._thread = threading.Thread(target=target, args=args)
+        self._thread = threading.Thread(target=_tun_with_context(target), args=args)
 
     def setDaemon(self, daemonic):
         self._thread.daemon = daemonic

src/python/grpcio/grpc/_cython/_cygrpc/thread.pyx.pxi

@@ -0,0 +1,60 @@
+# Copyright 2020 The gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+def _contextvars_supported():
+    """Determines if the contextvars module is supported.
+
+    We use a 'try it and see if it works approach' here rather than predicting
+    based on interpreter version in order to support older interpreters that
+    may have a backported module based on, e.g. `threading.local`.
+
+    Returns:
+      A bool indicating whether `contextvars` are supported in the current
+      environment.
+    """
+    try:
+        import contextvars
+        return True
+    except ImportError:
+        return False
+
+
+def _run_with_context(target):
+    """Runs a callable with contextvars propagated.
+
+    If contextvars are supported, the calling thread's context will be copied
+    and propagated. If they are not supported, this function is equivalent
+    to the identity function.
+
+    Args:
+      target: A callable object to wrap.
+    Returns:
+      A callable object with the same signature as `target` but with
+        contextvars propagated.
+    """
+    pass
+
+
+if _contextvars_supported():
+    import contextvars
+    def _run_with_context(target):
+        ctx = contextvars.copy_context()
+        def _run(*args):
+            ctx.run(target, *args)
+        return _run
+else:
+    def _run_with_context(target):
+        def _run(*args):
+            target(*args)
+        return _run

src/python/grpcio/grpc/_cython/cygrpc.pyx

@@ -59,6 +59,8 @@ include "_cygrpc/iomgr.pyx.pxi"
 
 include "_cygrpc/grpc_gevent.pyx.pxi"
 
+include "_cygrpc/thread.pyx.pxi"
+
 IF UNAME_SYSNAME == "Windows":
     include "_cygrpc/fork_windows.pyx.pxi"
 ELSE:

src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-"""Test of propagation of contextvars between threads."""
+"""Test of propagation of contextvars to AuthMetadataPlugin threads.."""
 
 import contextlib
 import logging
@@ -30,9 +30,6 @@ def _unary_unary_handler(request, context):
     return request
 
 
-# TODO: Test for <3.7 and 3.7+.
-
-
 def contextvars_supported():
     try:
         import contextvars
@@ -110,8 +107,6 @@ class ContextVarsPropagationTest(unittest.TestCase):
                 self.assertEqual(_REQUEST, response)
                 test_call_credentials.assert_called(self)
 
-    # TODO: Test simple unary-unary.
-
 
 if __name__ == '__main__':
     logging.basicConfig()