Make selection of a task type to create take account of the potential full inheritance tree rather than just the next level, and prevent creation of abstract tasks

wagtail/admin/views/workflows.py

@@ -17,6 +17,7 @@ from wagtail.admin.views.generic import CreateView, DeleteView, EditView, IndexV
 from wagtail.admin.views.pages import get_valid_next_url_from_request
 from wagtail.core.models import Page, Task, TaskState, WorkflowState
 from wagtail.core.permissions import task_permission_policy, workflow_permission_policy
+from wagtail.core.workflows import get_task_types
 
 
 class Index(IndexView):
@@ -290,7 +291,7 @@ def select_task_type(request):
 
     task_types = [
         (model.get_verbose_name(), model._meta.app_label, model._meta.model_name)
-        for model in Task.__subclasses__()
+        for model in get_task_types()
     ]
     # sort by lower-cased version of verbose name
     task_types.sort(key=lambda task_type: task_type[0].lower())

wagtail/core/workflows.py

@@ -1,3 +1,27 @@
+from wagtail.core.models import Task
+
+
+TASK_TYPES = []
+
+def get_concrete_descendants(model_class, inclusive=True):
+    """Retrieves non-abstract descendants of the given model class. If `inclusive` is set to
+    True, includes model_class"""
+    subclasses = model_class.__subclasses__()
+    if subclasses:
+        for subclass in subclasses:
+            yield from get_concrete_descendants(subclass)
+    if inclusive and not model_class._meta.abstract:
+        yield model_class
+
+
+def get_task_types(task_class=None):
+    global TASK_TYPES
+    if TASK_TYPES:
+        return TASK_TYPES
+    TASK_TYPES = list(get_concrete_descendants(Task, inclusive=False))
+    return TASK_TYPES
+
+
 def publish_workflow_state(workflow_state):
     # publish the Page associated with a WorkflowState
     if workflow_state.current_task_state: