Skip the buffer join in respond() when no watchers are attached

invoke/runners.py

@@ -944,6 +944,11 @@ def respond(self, buffer_: List[str]) -> None:
 
         .. versionadded:: 1.0
         """
+        # Nothing to do unless at least one watcher is attached; bail early so
+        # the (potentially O(n^2)) join below never runs in the common case of
+        # running without watchers while a subprocess emits lots of output.
+        if not self.watchers:
+            return
         # Join buffer contents into a single string; without this,
         # StreamWatcher subclasses can't do things like iteratively scan for
         # pattern matches.

tests/runners.py

@@ -982,6 +982,16 @@ def nothing_is_written_to_stdin_by_default(self):
             self._runner(klass=klass).run(_)
             assert not klass.write_proc_stdin.called
 
+        def no_watchers_skips_the_buffer_join(self):
+            # Regression test for #1079: respond() used to join the whole
+            # accumulated capture buffer on every chunk even with no watchers
+            # attached, an O(n^2) cost that made runs with large outputs
+            # (hundreds of MB) take hours. With no watchers it must return
+            # without touching the buffer -- the canary here is a non-string
+            # element, which raises TypeError if a join ever runs.
+            runner = self._runner()
+            runner.respond(["chunk", 42])
+
         def _expect_response(self, **kwargs):
             """
             Execute a run() w/ ``watchers`` set from ``responses``.