bpo-32309: Add support for contextvars in asyncio.to_thread() (GH-20278)

Allows contextvars from the main thread to be accessed in the separate thread used in `asyncio.to_thread()`. See the [discussion](https://github.com/python/cpython/pull/20143#discussion_r427808225) in GH-20143 for context.

Automerge-Triggered-By: @aeros

Lib/asyncio/threads.py

@@ -1,6 +1,7 @@
 """High-level support for working with threads in asyncio"""
 
 import functools
+import contextvars
 
 from . import events
 
@@ -12,10 +13,13 @@ async def to_thread(func, /, *args, **kwargs):
     """Asynchronously run function *func* in a separate thread.
 
     Any *args and **kwargs supplied for this function are directly passed
-    to *func*.
+    to *func*. Also, the current :class:`contextvars.Context` is propogated,
+    allowing context variables from the main thread to be accessed in the
+    separate thread.
 
     Return an asyncio.Future which represents the eventual result of *func*.
     """
     loop = events.get_running_loop()
-    func_call = functools.partial(func, *args, **kwargs)
+    ctx = contextvars.copy_context()
+    func_call = functools.partial(ctx.run, func, *args, **kwargs)
     return await loop.run_in_executor(None, func_call)