A closed PR's head can be superseded again

src/agent_comms/head_review.py

@@ -85,17 +85,21 @@ def among(cls, repositories: tuple[RepositoryName, ...], pr: int, head: str) ->
             raise RecordProblems(problems)
         return cls(held[0].repo, pr, head)
 
-    @classmethod
-    def at(cls, repo: RepositoryName, pr: int) -> OpenPullRequestHead:
-        """Read GitHub's head, its base branch and draft state in one request."""
+    @staticmethod
+    def forge_read(repo: RepositoryName, pr: int) -> str:
+        """GitHub's head, base branch, draft state and state in one request."""
         read = repo.rest(
             f"pulls/{pr}",
             "--jq",
             '.head.sha + " " + .base.ref + " " + (.draft | tostring) + " " + .state',
         )
         if read.returncode:
             raise ToolRefusedError(read.stderr.strip() or f"#{pr} could not be read.")
-        return OpenPullRequestHead.from_forge(repo, pr, read.stdout)
+        return read.stdout
+
+    @classmethod
+    def at(cls, repo: RepositoryName, pr: int) -> OpenPullRequestHead:
+        return OpenPullRequestHead.from_forge(repo, pr, cls.forge_read(repo, pr))
 
     def change_identity(self, reader: WorkBranches, base: str) -> PullRequestChange:
         """Compare the PR's patch, never the unrelated changes in its base tree.
@@ -118,6 +122,18 @@ def same_pull_request(self, other: PullRequestHead) -> bool:
     def current(self) -> OpenPullRequestHead:
         """GitHub's read of the PR, refused unless the reviewed head is still its head."""
         current = self.at(self.repo, self.pr)
+        if not current.holds(self.head):
+            raise ToolRefusedError(
+                f"#{self.pr} moved to {current.head[:9]} since {self.head} was reviewed; "
+                "review again."
+            )
+        return current
+
+    def closed_or_open(self) -> ReadPullRequestHead:
+        """GitHub's read in whatever state the PR is in, refused unless this is still its head:
+        a closed PR's head can only be dropped, so nothing here may arm or integrate it."""
+        head, base, *_ = self.forge_read(self.repo, self.pr).split()
+        current = ReadPullRequestHead(self.repo, self.pr, head, base=base)
         if not current.holds(self.head):
             raise ToolRefusedError(
                 f"#{self.pr} moved to {current.head[:9]} since {self.head} was reviewed; review again."
@@ -768,7 +784,7 @@ def record_review(
 class PreparedHeadReview:
     """The current forge head and its checked status text, read once before recording."""
 
-    current: OpenPullRequestHead
+    current: ReadPullRequestHead
     subject: str
 
 

src/agent_comms/order_tools.py

@@ -227,7 +227,7 @@ def apply(self, comms: Comms, caller: ThreadCaller) -> HeadReviewed:
                 actors.append(caller.head_review_actor(comms, self.request, order))
         heads = []
         with RecordProblems.collecting(problems, '--head', 'supply the current PR head SHA with --head'):
-            heads.append(self.request.current())
+            heads.append(self.operation.current_head(self.request))
         preparations = []
         for current in heads:
             with RecordProblems.collecting(problems, self.operation.preparation_field,

src/agent_comms/todos.py

@@ -1,3 +1,3 @@
 """A durable todo owns its one assignment; it does not start an agent.
 
 This is an opt-in post-PR1 primitive. An ACP/fork adapter must verify current
@@ -4273,6 +4273,10 @@
     preparation_field: ClassVar[str] = '--head'
     preparation_fix: ClassVar[str] = 'supply the current PR head SHA with --head'
 
+    def current_head(self, request: PullRequestHead) -> ReadPullRequestHead:
+        """The read this decision stands on: an open PR at the reviewed head."""
+        return request.current()
+
     @classmethod
     def require_ok(cls, request: PullRequestHead) -> None:
         """The commander's admin merge needs this exact head's recorded ok, nothing more."""
@@ -4305,7 +4309,7 @@
             self.after_record(goals, recorded)
         return HeadReviewed(recorded, self.landing(prepared.current))
 
-    def landing(self, current: OpenPullRequestHead) -> MergeArm:
+    def landing(self, current: ReadPullRequestHead) -> MergeArm:
         """Source verdicts never request merge; native explicit promotion owns enqueueing."""
         return UntouchedMergeArm()
 
@@ -4488,6 +4492,10 @@
     def supersedes(self) -> bool:
         return True
 
+    def current_head(self, request: PullRequestHead) -> ReadPullRequestHead:
+        """Dropping a head is how a closed PR's lost commits get their recorded reason."""
+        return request.closed_or_open()
+
     def description(self, reviewer: str, subject: str) -> str:
         return f"{super().description(reviewer, subject)} (replaced by {self.replacement})"
 

tests/test_head_review.py

@@ -1413,3 +1413,16 @@ def test_another_commanders_linked_request_stays_off_the_orderless_board(head_bu
                       held.generation).record(comms.goals, todo.id)
     assert ready.pr not in {rule.request.pr for rule in
                             OrderlessPullRequest.scan_orderless(comms.goals, ProgressRead(()))}
+
+
+def test_only_superseded_may_decide_a_closed_head(head_bus):
+    comms, github, request = head_bus
+    stamp = datetime.fromtimestamp(time.time() - 601, UTC).strftime('%Y-%m-%dT%H:%M:%SZ')
+    github.put(request.repo, ClosedListedPullRequest(
+        request.pr, 'feature', request.head, closed_at=stamp, updated=stamp,
+        base='main', reviewable_times=(stamp,)))
+    with pytest.raises(Exception, match='no longer an open source'):
+        invoke(comms, request, ReviewedHeadVerdict(ACCEPTED))
+    invoke(comms, request, SupersededHeadVerdict(ACCEPTED, 'P2'))
+    reviewed, = StoredHeadReview.head_reviews(comms.goals.todos, request)
+    assert reviewed.verdict.supersedes()