Fix IndexError when submitting buffered futures

core.py

@@ -33,17 +33,21 @@ def __init__(self, submit_size, *args, **kwargs):
     def submit(self, fn, *args, **kwargs):
         self._buffer.append((fn, args, kwargs))
 
-    def __submit_from_buffer(self):
-        fn, args, kwargs = self._buffer.pop(0)
-        return self._executor.submit(fn, *args, **kwargs)
+    def __submit_from_buffer(self, count=1):
+        futures = list()
+        while count and self._buffer:
+            fn, args, kwargs = self._buffer.pop(0)
+            futures.append(self._executor.submit(fn, *args, **kwargs))
+            count -= 1
+        return futures
 
     def as_completed(self):
-        submitted = [self.__submit_from_buffer() for _ in range(self._submit_size)]
+        submitted = self.__submit_from_buffer(self._submit_size)
         while self._buffer and not self._shutdown:
             done, _ = concurrent.futures.wait(submitted, return_when=concurrent.futures.FIRST_COMPLETED)
             for future in done:
                 submitted.remove(future)
-                submitted.append(self.__submit_from_buffer())
+                submitted += self.__submit_from_buffer()
                 yield future
 
     def shutdown(self, wait=True):