Take the terminal back before retrying a shell command in cmd2's group

When a terminal pipeline exits before a shell command can join its
group, do_shell() retries in cmd2's own group. The terminal was still
lent to the dead pipeline, so the command stopped with SIGTTIN on its
first terminal read and the wait for it never returned. End the lend
before retrying.

Author: tleonhardt

cmd2/cmd2.py

@@ -5317,6 +5317,10 @@ def do_shell(self, args: argparse.Namespace) -> None:
                     # The pipeline exited before the command could join its group.
                     if kwargs.pop("process_group", None) is None:
                         raise
+                    # The retry runs in our own group, so take the terminal back from the dead
+                    # pipeline first. Its watcher left it lent, and the command would otherwise
+                    # stop with SIGTTIN on its first terminal read, with nothing to resume it.
+                    terminal_stack.close()
 
             # A command that joined the pipeline's job is waited for in short polls. Only the
             # main thread runs Python signal handlers, and the job-control stop the pipeline's

tests/test_cmd2.py

@@ -438,13 +438,33 @@ def test_shell_falls_back_to_own_group_when_pipeline_exited(base_app, tmp_path)
     # pipeline can exit between the check and the spawn, like `shell sleep 1 | true`.
     leader = subprocess.Popen([sys.executable, "-c", "pass"], process_group=0)
     leader.wait()
-    base_app._cur_pipe_proc_reader = mock.Mock(terminal_group=leader.pid, lend_terminal=contextlib.nullcontext)
-    with (tmp_path / "output").open("w+") as output:
+    lent = []
+
+    @contextlib.contextmanager
+    def lend_terminal():
+        lent.append(True)
+        try:
+            yield
+        finally:
+            lent.pop()
+
+    # The retry runs in our own group, so the terminal has to come back from the dead
+    # pipeline first. Otherwise the command stops with SIGTTIN on its first terminal read.
+    spawned_while_lent = []
+    real_popen = subprocess.Popen
+
+    def popen(*args, **kwargs):
+        spawned_while_lent.append(bool(lent))
+        return real_popen(*args, **kwargs)
+
+    base_app._cur_pipe_proc_reader = mock.Mock(terminal_group=leader.pid, lend_terminal=lend_terminal)
+    with (tmp_path / "output").open("w+") as output, mock.patch("subprocess.Popen", popen):
         base_app.stdout = output
         base_app.do_shell("echo joined")
         output.seek(0)
         assert output.read() == "joined\n"
     assert base_app.last_result == 0
+    assert spawned_while_lent == [True, False]
 
 
 @pytest.mark.skipif(sys.platform == "win32", reason="POSIX shell executable")