Raise error in wait_output if stdout is already being consumed (#998)

Error on already used stdout in wait_output

ops/pebble.py

@@ -1331,6 +1331,12 @@ def wait_output(self) -> Tuple[AnyStr, Optional[AnyStr]]:
             ChangeError: if there was an error starting or running the process.
             ExecError: if the process exits with a non-zero exit code.
         """
+        if self.stdout is None:
+            raise TypeError(
+                "can't use wait_output() when exec was called with the stdout argument; "
+                "use wait() instead"
+            )
+
         if self._encoding is not None:
             out = io.StringIO()
             err = io.StringIO() if self.stderr is not None else None

test/test_pebble.py

@@ -2925,6 +2925,15 @@ def test_wait_output_send_stdin_bytes(self):
             ('TXT', '{"command":"end"}'),
         ])
 
+    def test_wait_output_no_stdout(self):
+        stdio, stderr, _ = self.add_responses('123', 0)
+        stdio.receives.append('{"command":"end"}')
+        stderr.receives.append('{"command":"end"}')
+        stdout_buffer = io.BytesIO()
+        process = self.client.exec(["echo", "FOOBAR"], stdout=stdout_buffer, encoding=None)
+        with self.assertRaises(TypeError):
+            process.wait_output()
+
     def test_wait_output_bad_command(self):
         stdio, stderr, _ = self.add_responses('123', 0)
         stdio.receives.append(b'Python 3.8.10\n')