Fixes grpc#26322 (grpc#26340)

* Fixes grpc#26322

* Support runtimes without contextvars

* Hoist member declaration higher

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

@@ -57,7 +57,11 @@ cdef int _get_metadata(void *state,
       with nogil:
         cb(user_data, NULL, 0, status, c_error_details)
   args = context.service_url, context.method_name, callback,
-  _spawn_callback_async(<object>state, args)
+  plugin = <object>state
+  if plugin._stored_ctx is not None:
+    plugin._stored_ctx.run(_spawn_callback_async, plugin, args)
+  else:
+    _spawn_callback_async(<object>state, args)
   return 0  # Asynchronous return
 
 

src/python/grpcio/grpc/_plugin_wrapping.py

@@ -68,6 +68,17 @@ class _Plugin(object):
 
     def __init__(self, metadata_plugin):
         self._metadata_plugin = metadata_plugin
+        self._stored_ctx = None
+
+        try:
+            import contextvars  # pylint: disable=wrong-import-position
+            # The plugin may be invoked on a thread created by Core, which will not
+            # have the context propagated. This context is stored and installed in
+            # the thread invoking the plugin.
+            self._stored_ctx = contextvars.copy_context()
+        except ImportError:
+            # Support versions predating contextvars.
+            pass
 
     def __call__(self, service_url, method_name, callback):
         context = _AuthMetadataContext(_common.decode(service_url),