Stopped spinning in CurrentThreadExecutor.run_until_future()

Rather that spinning on
- check for work, non-blocking
- check the future
- sleep briefly

We can register a done callback on the future, which will be called
with the future itself as an arugment. Registering our task queue's
`put()` method lets us use the future itself as a sentinal value.

Now we can use `queue.get()` in blocking fashion. If we find the job
is the future we're waiting on, we're done getting tasks and we
return, otherwise we run the job and go back to blocking on
`queue.get()` for more tasks.

This noticeably cuts down on CPU usage in run_until_future().

asgiref/current_thread_executor.py

@@ -1,6 +1,5 @@
 import queue
 import threading
-import time
 from concurrent.futures import Executor, Future
 
 
@@ -51,21 +50,17 @@ def run_until_future(self, future):
             raise RuntimeError(
                 "You cannot run CurrentThreadExecutor from a different thread"
             )
-        # Keep getting work items and checking the future
+        future.add_done_callback(self._work_queue.put)
+        # Keep getting and running work items until we get the future we're waiting for
+        # back via the future's done callback.
         try:
             while True:
                 # Get a work item and run it
-                try:
-                    work_item = self._work_queue.get(block=False)
-                except queue.Empty:
-                    # See if the future is done (we only exit if the work queue is empty)
-                    if future.done():
-                        return
-                    # Prevent hot-looping on nothing
-                    time.sleep(0.001)
-                else:
-                    work_item.run()
-                    del work_item
+                work_item = self._work_queue.get()
+                if work_item is future:
+                    return
+                work_item.run()
+                del work_item
         finally:
             self._broken = True