More explanation :P

bharel · bharel · commit d41e9e095239 · 2018-02-02T23:46:10.000+02:00
diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py
@@ -224,6 +224,9 @@ def _wake_up_first(self):
         except StopIteration:
             return
 
+        # .done() necessarily means that a waiter will wake up later on and
+        # either take the lock, or, if it was cancelled and lock wasn't
+        # taken already, will hit this again and wake up a new waiter.
         if not fut.done():
             fut.set_result(True)
 
