Test for future persisting past the lifetime of the executor

Motivated the `wait=False` in executor __del__

tests/test_future.py

@@ -27,3 +27,51 @@ def test_pool_serial_multi_core(self):
             sleep(1)
         self.assertTrue(output.done())
         self.assertEqual(output.result(), [np.array(4), np.array(4)])
+
+    def test_independence_from_executor(self):
+        """
+        Ensure that futures are able to live on after the executor gets garbage
+        collected.
+        """
+
+        with self.subTest("From the main process"):
+            mutable = []
+
+            def slow_callable():
+                from time import sleep
+                sleep(1)
+                return True
+
+            def callback(future):
+                mutable.append("Called back")
+
+            def submit():
+                # Executor only exists in this scope and can get garbage collected after
+                # this function is exits
+                future = PyMPISingleTaskExecutor().submit(slow_callable)
+                future.add_done_callback(callback)
+                return future
+
+            self.assertListEqual(
+                [],
+                mutable,
+                msg="Sanity check that test is starting in the expected condition"
+            )
+            future = submit()
+
+            self.assertFalse(
+                future.done(),
+                msg="The submit function is slow, it should be running still"
+            )
+            self.assertListEqual(
+                [],
+                mutable,
+                msg="While running, the mutable should not have been impacted by the "
+                    "callback"
+            )
+            future.result()  # Wait for the calculation to finish
+            self.assertListEqual(
+                ["Called back"],
+                mutable,
+                msg="After completion, the callback should modify the mutable data"
+            )