Fix race condition in ThreadedHistory.

The Lock in `ThreadedHistory` was not always properly released, and because of
that, in situations where the user was pasting enormous amounts of text, the
application could freeze at the point where lines were added to the history.

prompt_toolkit/history.py

@@ -158,8 +158,13 @@ async def load(self) -> AsyncGenerator[str, None]:
                     continue
 
                 # Read new items (in lock).
-                await loop.run_in_executor(None, self._lock.acquire)
+                # (Important: acquiring the lock should happen *in* the try
+                # block. Otherwise it's not guaranteed it will ever be
+                # released. This can happen when this coroutine is cancelled at
+                # an "await" point. This did actually happen when continuously
+                # pasting huge amounts of text in ptpython.)
                 try:
+                    await loop.run_in_executor(None, self._lock.acquire)
                     new_items = self._loaded_strings[items_yielded:]
                     done = self._loaded
                     event.clear()