Implement a constant for an invalid system. Whenever valid_systems is…

… empty, explicitely set it equal to this constant. That way we know (and can test in other hooks) that it was explicitely filtered out as an invalid test by one of our hooks

eessi/testsuite/constants.py

@@ -60,3 +60,6 @@
     '8_nodes': {'num_nodes': 8, 'node_part': 1},
     '16_nodes': {'num_nodes': 16, 'node_part': 1},
 }
+
+# When tests are filtered by the hooks, the valid_systems is set to this system name:
+INVALID_SYSTEM="INVALID_SYSTEM"

eessi/testsuite/hooks.py

@@ -291,7 +291,7 @@ def filter_supported_scales(test: rfm.RegressionTest):
     # test.valid_systems wasn't set yet, so set it
     if len(test.valid_systems) == 0:
         test.valid_systems = [valid_systems]
-    # test.valid_systems likely contains requested features and extras set in another hook, so append
+    # test.valid_systems was already set. Append the current valid_systems
     elif len(test.valid_systems) == 1:
         test.valid_systems[0] = f'{test.valid_systems[0]} {valid_systems}'
 
@@ -301,6 +301,10 @@ def filter_valid_systems_by_device_type(test: rfm.RegressionTest, required_devic
     """
     Filter valid_systems by required device type and by whether the module supports CUDA,
     unless valid_systems is specified with --setvar valid_systems=<comma-separated-list>.
+
+    Any invalid combination (e.g. a non-CUDA module with a required_device_type GPU) will
+    cause the valid_systems to be set to an empty string, and consequently the
+    test.valid_systems to an invalid system name (eessi.testsuite.constants.INVALID_SYSTEM).
     """
     is_cuda_module = is_cuda_required_module(test.module_name)
 
@@ -322,12 +326,14 @@ def filter_valid_systems_by_device_type(test: rfm.RegressionTest, required_devic
         # test.valid_systems wasn't set yet, so set it
         if len(test.valid_systems) == 0:
             test.valid_systems = [valid_systems]
+        # test.valid_systems was already set. Append the current valid_systems
         elif len(test.valid_systems) == 1:
             test.valid_systems[0] = f'{test.valid_systems[0]} {valid_systems}'
-    # Explicitely set to empty because of invalid combination of module type and device type
-    # (even if other filters such as filter_valid_test_scales have already set it)
+    # Explicitely set to an invalid system name. The combination of module type and device type is invalid,
+    # so this test should never be generated.
+    # Note that this does (and should) overwrite any test.valid_systems that was potentially set before
     else:
-        test.valid_systems = []
+        test.valid_systems = [eessi.testsuite.constants.INVALID_SYSTEM]
 
     log(f'valid_systems set to {test.valid_systems}')