upload: Add topological_topics as a generator

Add recursive helper generator functions to iterate
topologically through topics, and use it in some key
places. Also detect cycles in the topic graph.

revup/topic_stack.py

@@ -179,6 +179,20 @@ class Topic:
     # Reviews for this topic, keyed by base branch
     reviews: Dict[str, Review] = field(default_factory=dict)
 
+    def depth_first_iter(self) -> Iterator[Topic]:
+        def depth_first_iter_helper(topic: Topic, seen: Set[str]) -> Iterator[Topic]:
+            if topic.name in seen:
+                raise RevupUsageException(
+                    f'Unexpected relative topic cycle at "{topic.name}" chain includes {seen}'
+                )
+            seen.add(topic.name)
+            if topic.relative_topic:
+                for t in depth_first_iter_helper(topic.relative_topic, seen):
+                    yield t
+            yield topic
+
+        return depth_first_iter_helper(self, set())
+
 
 @dataclass
 class TopicStack:
@@ -234,10 +248,23 @@ def all_reviews_iter(self) -> Iterator[Tuple[str, Topic, str, Review]]:
         """
         One liner for common iteration pattern to reduce indentation a bit.
         """
-        for name, topic in self.topics.items():
+        for name, topic in self.topological_topics():
             for base_branch, review in topic.reviews.items():
                 yield name, topic, base_branch, review
 
+    def topological_topics(self) -> Iterator[Tuple[str, Topic]]:
+        """
+        Iterate through all topics one at a time with the requirement that any topic must always
+        come after a topic it is relative to.
+        """
+        seen: Set[str] = set()
+        for topic in self.topics.values():
+            for t in topic.depth_first_iter():
+                if t.name in seen:
+                    continue
+                seen.add(t.name)
+                yield t.name, t
+
     def parse_commit_tags(self, commit_msg: str) -> Tuple[Dict[str, Set[str]], str]:
         """
         Parse all commit tags in the commit message and return them in a dict, as well as
@@ -548,7 +575,7 @@ async def populate_reviews(
         await self.populate_relative_reviews(uploader)
 
     async def populate_relative_reviews(self, uploader: str) -> None:
-        for name, topic in list(self.topics.items()):
+        for name, topic in self.topological_topics():
             if topic.relative_topic:
                 if len(topic.tags[TAG_BRANCH]) == 0:
                     topic.tags[TAG_BRANCH].update(topic.relative_topic.tags[TAG_BRANCH])