Engine: Improve error message when submitting without broker

The `aiida.engine.launch.submit` method was just raising a vague
`AssertionError` in case the runner did not have a communicator, which
is the case if it was constructed without a communicator which in turn
happens for profiles that do not configure a broker.

Since profiles without brokers are now supported and users are bound to
try to submit anyway, the error message should be clearer.

Author: sphuber

Diff for: src/aiida/engine/launch.py

@@ -111,8 +111,14 @@ def submit(
         raise InvalidOperation('Cannot use top-level `submit` from within another process, use `self.submit` instead')
 
     runner = manager.get_manager().get_runner()
+
+    if runner.controller is None:
+        raise InvalidOperation(
+            'Cannot submit because the runner does not have a process controller. Does the current profile define a '
+            'broker? Submitting processes requires a broker like RabbitMQ.'
+        )
+
     assert runner.persister is not None, 'runner does not have a persister'
-    assert runner.controller is not None, 'runner does not have a controller'
 
     process_inited = instantiate_process(runner, process, **inputs)
 

Diff for: tests/engine/test_launch.py

@@ -20,6 +20,16 @@
 ArithmeticAddCalculation = CalculationFactory('core.arithmetic.add')
 
 
+@pytest.fixture
+def arithmetic_add_builder(aiida_code_installed):
+    builder = ArithmeticAddCalculation.get_builder()
+    builder.code = aiida_code_installed(default_calc_job_plugin='core.arithmetic.add', filepath_executable='/bin/bash')
+    builder.x = orm.Int(1)
+    builder.y = orm.Int(1)
+    builder.metadata = {'options': {'resources': {'num_machines': 1, 'num_mpiprocs_per_machine': 1}}}
+    return builder
+
+
 @calcfunction
 def add(term_a, term_b):
     return term_a + term_b
@@ -69,18 +79,28 @@ def add(self):
 
 
 @pytest.mark.usefixtures('started_daemon_client')
-def test_submit_wait(aiida_code_installed):
+def test_submit_wait(arithmetic_add_builder):
     """Test the ``wait`` argument of :meth:`aiida.engine.launch.submit`."""
-    builder = ArithmeticAddCalculation.get_builder()
-    builder.code = aiida_code_installed(default_calc_job_plugin='core.arithmetic.add', filepath_executable='/bin/bash')
-    builder.x = orm.Int(1)
-    builder.y = orm.Int(1)
-    builder.metadata = {'options': {'resources': {'num_machines': 1, 'num_mpiprocs_per_machine': 1}}}
-    node = launch.submit(builder, wait=True, wait_interval=0.1)
+    node = launch.submit(arithmetic_add_builder, wait=True, wait_interval=0.1)
     assert node.is_finished, node.process_state
     assert node.is_finished_ok, node.exit_code
 
 
+def test_submit_no_broker(arithmetic_add_builder, monkeypatch, manager):
+    """Test that ``submit`` raises ``InvalidOperation`` if the runner does not have a controller.
+
+    The runner does not have a controller if the runner was not provided a communicator which is the case for profiles
+    that do not define a broker.
+    """
+    runner = manager.get_runner()
+    monkeypatch.setattr(runner, '_controller', None)
+
+    with pytest.raises(
+        exceptions.InvalidOperation, match=r'Cannot submit because the runner does not have a process controller.*'
+    ):
+        launch.submit(arithmetic_add_builder)
+
+
 def test_await_processes_invalid():
     """Test :func:`aiida.engine.launch.await_processes` for invalid inputs."""
     with pytest.raises(TypeError):