upload: Keep a review stacked on its relative branch's tip

revup/topic_stack.py

@@ -921,11 +921,18 @@ async def mark_rebases(self, skip_rebase: bool) -> None:
                 if topic.relative_topic is None:
                     if not review.base_ref:
                         raise RuntimeError("Review doesn't have a base ref!")
-                    # For non-relative reviews, the base is correct if the remote base commit is a
-                    # first-parent ancestor of the local remote base.
-                    is_on_correct_base = await self.git_ctx.is_ancestor(
-                        review.remote_commits[0].parents[0], review.base_ref
-                    )
+                    remote_parent = review.remote_commits[0].parents[0]
+                    if review.relative_branch:
+                        # A relative branch is someone else's work that this review is stacked
+                        # on, so the review has to sit exactly on it, same as a relative series.
+                        # Anything else makes the forge show their commits as part of this review.
+                        is_on_correct_base = remote_parent == review.base_ref
+                    else:
+                        # For non-relative reviews, the base is correct if the remote base commit
+                        # is a first-parent ancestor of the local remote base.
+                        is_on_correct_base = await self.git_ctx.is_ancestor(
+                            remote_parent, review.base_ref
+                        )
                 else:
                     # For a relative series of reviews, revup will only ever upload them directly
                     # on top of each other. If this relationship is ever broken, we always reupload

tests/test_upload.py

@@ -1439,6 +1439,59 @@ async def test_base_derived_by_walking_back_from_head(self):
             assert g_review.is_pure_rebase
             assert g_review.remote_commits[0].parents[0] == b_review.remote_commits[-1].commit_id
 
+    @async_test
+    async def test_relative_branch_moving_forward_is_pushed(self):
+        """A review stacked on a relative branch follows that branch's tip."""
+        async with GitTestEnvironment() as env:
+            await setup_repo(env)
+            root = await env.get_commit_hash()
+            await env.git_ctx.git("branch", "origin/staging", root)
+            await env.commit("feat\n\nTopic: alpha\nRelative-Branch: staging", {"a.txt": "a"})
+
+            first = await run_upload_pipeline(env)
+            first_review = first.topics["alpha"].reviews["origin/main"]
+            remote_head = first_review.new_commits[-1]
+            remote_num_commits = len(first_review.new_commits)
+
+            await env.git_ctx.git("checkout", root)
+            await env.commit("staging moves on", {"s.txt": "s"})
+            await env.git_ctx.git("branch", "origin/staging", "HEAD", "-f")
+            await env.git_ctx.git("checkout", "main")
+
+            topics = await run_upload_pipeline(env)
+            review = topics.topics["alpha"].reviews["origin/main"]
+            review.pr_info = PrInfo(
+                baseRef="staging",
+                headRef=review.remote_head,
+                headRefOid=remote_head,
+                numCommits=remote_num_commits,
+                body="",
+                title="",
+                state="OPEN",
+            )
+
+            await topics.mark_rebases(skip_rebase=True)
+
+            assert review.is_pure_rebase
+            assert review.remote_commits[0].parents[0] == root
+            assert review.push_status == PushStatus.PUSHED
+
+    @async_test
+    async def test_relative_branch_staying_put_is_nochange(self):
+        """A review already on the relative branch's tip isn't pushed again."""
+        async with GitTestEnvironment() as env:
+            await setup_repo(env)
+            await env.git_ctx.git("branch", "origin/staging", "HEAD")
+            await env.commit("feat\n\nTopic: alpha\nRelative-Branch: staging", {"a.txt": "a"})
+
+            topics = await run_upload_pipeline(env)
+            review = topics.topics["alpha"].reviews["origin/main"]
+            review.pr_info = make_pr_info(review, base_branch="staging")
+
+            await topics.mark_rebases(skip_rebase=True)
+
+            assert review.push_status == PushStatus.NOCHANGE
+
 
 class TestSkipEmptyFirstCommit:
     @async_test