From 80700b2445774589277afac4d52ef294bbb9d1eb Mon Sep 17 00:00:00 2001
From: Jason Carver <ut96caarrs@snkmail.com>
Date: Fri, 28 Sep 2018 16:43:44 -0700
Subject: [PATCH] Use 0-prereqs, in prep for skeleton sync

---
 .../utils/test_ordered_task_preparation.py    | 39 +++++++++++++++++--
 trinity/utils/datastructures.py               |  8 ++--
 2 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/tests/trinity/utils/test_ordered_task_preparation.py b/tests/trinity/utils/test_ordered_task_preparation.py
index 6679d1a21c..fa7c409c16 100644
--- a/tests/trinity/utils/test_ordered_task_preparation.py
+++ b/tests/trinity/utils/test_ordered_task_preparation.py
@@ -20,6 +20,10 @@ async def wait(coro, timeout=DEFAULT_TIMEOUT):
     return await asyncio.wait_for(coro, timeout=timeout)
 
 
+class NoPrerequisites(Enum):
+    pass
+
+
 class OnePrereq(Enum):
     one = auto()
 
@@ -187,6 +191,17 @@ def test_reregister_duplicates():
         ti.register_tasks((2, ))
 
 
+@pytest.mark.asyncio
+async def test_no_prereq_tasks():
+    ti = OrderedTaskPreparation(NoPrerequisites, identity, lambda x: x - 1)
+    ti.set_finished_dependency(1)
+    ti.register_tasks((2, 3))
+
+    # with no prerequisites, tasks are *immediately* finished, as long as they are in order
+    finished = await wait(ti.ready_tasks())
+    assert finished == (2, 3)
+
+
 @pytest.mark.asyncio
 async def test_register_out_of_order():
     ti = OrderedTaskPreparation(OnePrereq, identity, lambda x: x - 1, accept_dangling_tasks=True)
@@ -207,13 +222,29 @@ async def test_register_out_of_order():
     assert finished == (2, 3, 4, 5)
 
 
-def test_empty_enum():
+@pytest.mark.asyncio
+async def test_no_prereq_tasks_out_of_order():
+    ti = OrderedTaskPreparation(
+        NoPrerequisites,
+        identity,
+        lambda x: x - 1,
+        accept_dangling_tasks=True,
+    )
+    ti.set_finished_dependency(1)
+    ti.register_tasks((4, 5))
 
-    class NoPrerequisites(Enum):
+    try:
+        finished = await wait(ti.ready_tasks())
+    except asyncio.TimeoutError:
         pass
+    else:
+        assert False, f"No steps should be ready, but got {finished!r}"
 
-    with pytest.raises(ValidationError):
-        OrderedTaskPreparation(NoPrerequisites, identity, lambda x: x - 1)
+    ti.register_tasks((2, 3))
+
+    # with no prerequisites, tasks are *immediately* finished, as long as they are in order
+    finished = await wait(ti.ready_tasks())
+    assert finished == (2, 3, 4, 5)
 
 
 @pytest.mark.asyncio
diff --git a/trinity/utils/datastructures.py b/trinity/utils/datastructures.py
index 91a26db0fe..21e05fd281 100644
--- a/trinity/utils/datastructures.py
+++ b/trinity/utils/datastructures.py
@@ -301,10 +301,6 @@ class BaseTaskPrerequisites(Generic[TTask, TPrerequisite]):
 
     @classmethod
     def from_enum(cls, prereqs: Type[TPrerequisite]) -> 'Type[BaseTaskPrerequisites[Any, Any]]':
-
-        if len(prereqs) < 1:
-            raise ValidationError("There must be at least one prerequisite to track completions")
-
         return type('CompletionFor' + prereqs.__name__, (cls, ), dict(_prereqs=prereqs))
 
     def __init__(self, task: TTask) -> None:
@@ -540,6 +536,10 @@ def register_tasks(self, tasks: Tuple[TTask, ...]) -> None:
                 self._unready.add(task_id)
                 self._dependencies[dependency_id].add(task_id)
 
+                if prereq_tracker.is_complete and self._is_ready(prereq_tracker.task):
+                    # this is possible for tasks with 0 prerequisites (useful for pure ordering)
+                    self._mark_complete(task_id)
+
     def finish_prereq(self, prereq: TPrerequisite, tasks: Tuple[TTask, ...]) -> None:
         """For every task in tasks, mark the given prerequisite as completed"""
         if len(self._tasks) == 0: