huskies: merge 519_story_mergemaster_should_detect_no_commits_ahead_of_master_and_fail_loudly_instead_of_exiting_silently

server/src/agents/pool/pipeline/advance.rs

@@ -69,13 +69,8 @@ impl AgentPool {
                                 slog_error!(
                                     "[pipeline] Failed to move '{story_id}' to 4_merge/: {e}"
                                 );
-                            } else if let Err(e) = self
+                            } else {
-                                .start_agent(&project_root, story_id, Some("mergemaster"), None)
+                                self.start_mergemaster_or_block(&project_root, story_id).await;
-                                .await
-                            {
-                                slog_error!(
-                                    "[pipeline] Failed to start mergemaster for '{story_id}': {e}"
-                                );
                             }
                         }
                         crate::io::story_metadata::QaMode::Agent => {
@@ -186,13 +181,8 @@ impl AgentPool {
                                 slog_error!(
                                     "[pipeline] Failed to move '{story_id}' to 4_merge/: {e}"
                                 );
-                            } else if let Err(e) = self
+                            } else {
-                                .start_agent(&project_root, story_id, Some("mergemaster"), None)
+                                self.start_mergemaster_or_block(&project_root, story_id).await;
-                                .await
-                            {
-                                slog_error!(
-                                    "[pipeline] Failed to start mergemaster for '{story_id}': {e}"
-                                );
                             }
                         }
                     } else if let Some(reason) = should_block_story(story_id, config.max_retries, "qa-coverage") {
@@ -333,6 +323,41 @@ impl AgentPool {
         // become available (bug 295).
         self.auto_assign_available_work(&project_root).await;
     }
+
+    /// Start the mergemaster agent for `story_id`, but only if the feature
+    /// branch has commits that are not yet on master.
+    ///
+    /// If the branch has zero commits ahead of master, this logs an error and
+    /// sends a [`WatcherEvent::StoryBlocked`] instead of spawning a Claude
+    /// session.  A no-op merge session was observed spending $0.82 in the
+    /// 2026-04-09 incident (story 519).
+    async fn start_mergemaster_or_block(&self, project_root: &Path, story_id: &str) {
+        let branch = format!("feature/story-{story_id}");
+        if !crate::agents::lifecycle::feature_branch_has_unmerged_changes(project_root, story_id) {
+            slog_error!(
+                "[mergemaster] Branch '{branch}' has no commits ahead of master — \
+                 refusing to spawn merge session. \
+                 Likely cause: the worktree was reset to master after the feature \
+                 branch's commits were created. Investigate the worktree's git state \
+                 before retrying. Story '{story_id}' stays in 4_merge/ for human review."
+            );
+            let _ = self.watcher_tx.send(WatcherEvent::StoryBlocked {
+                story_id: story_id.to_string(),
+                reason: format!(
+                    "Feature branch '{branch}' has no commits ahead of master — nothing to merge. \
+                     The worktree may have been reset to master. \
+                     Check the worktree's git state and retry manually."
+                ),
+            });
+            return;
+        }
+        if let Err(e) = self
+            .start_agent(project_root, story_id, Some("mergemaster"), None)
+            .await
+        {
+            slog_error!("[pipeline] Failed to start mergemaster for '{story_id}': {e}");
+        }
+    }
 }
 
 /// Spawn pipeline advancement as a background task.
@@ -693,6 +718,91 @@ stage = "qa"
         );
     }
 
+    // ── story 519: mergemaster pre-flight blocks when no commits ahead ──
+
+    /// Regression test for story 519: when the feature branch has zero commits
+    /// ahead of master, mergemaster must not spawn a Claude session.  A no-op
+    /// session spent $0.82 in the 2026-04-09 incident because the worktree was
+    /// reset to master before mergemaster ran.
+    #[tokio::test]
+    async fn mergemaster_blocks_and_sends_story_blocked_when_no_commits_ahead() {
+        use std::fs;
+        use std::process::Command;
+
+        let tmp = tempfile::tempdir().unwrap();
+        let root = tmp.path();
+
+        // Init a bare git repo on master with one empty commit.
+        Command::new("git").args(["init"]).current_dir(root).output().unwrap();
+        Command::new("git").args(["config", "user.email", "test@test.com"]).current_dir(root).output().unwrap();
+        Command::new("git").args(["config", "user.name", "Test"]).current_dir(root).output().unwrap();
+        Command::new("git").args(["commit", "--allow-empty", "-m", "init"]).current_dir(root).output().unwrap();
+
+        // Create a feature branch that points at master HEAD (zero commits ahead).
+        // This replicates the incident where the worktree was reset to master.
+        Command::new("git")
+            .args(["branch", "feature/story-9919_story_no_commits"])
+            .current_dir(root)
+            .output()
+            .unwrap();
+
+        // Set up pipeline dirs and story file.
+        let current = root.join(".huskies/work/2_current");
+        fs::create_dir_all(&current).unwrap();
+        fs::create_dir_all(root.join(".huskies/work/4_merge")).unwrap();
+        fs::write(current.join("9919_story_no_commits.md"), "---\nname: Test\n---\n").unwrap();
+
+        let pool = AgentPool::new_test(3001);
+        let mut rx = pool.watcher_tx.subscribe();
+
+        // Simulate coder completing with gates passed (qa: server → goes to merge).
+        pool.run_pipeline_advance(
+            "9919_story_no_commits",
+            "coder-1",
+            CompletionReport {
+                summary: "done".to_string(),
+                gates_passed: true,
+                gate_output: String::new(),
+            },
+            Some(root.to_path_buf()),
+            None,
+            false,
+        )
+        .await;
+
+        // Story should be in 4_merge/ (pipeline moved it there before the block).
+        assert!(
+            root.join(".huskies/work/4_merge/9919_story_no_commits.md").exists(),
+            "story should remain in 4_merge/ — not moved to done"
+        );
+
+        // A StoryBlocked event must have been emitted (triggers chat failure notice,
+        // not the success 🎉 emoji).
+        let mut got_blocked = false;
+        while let Ok(evt) = rx.try_recv() {
+            if let WatcherEvent::StoryBlocked { story_id, .. } = &evt
+                && story_id == "9919_story_no_commits"
+            {
+                got_blocked = true;
+                break;
+            }
+        }
+        assert!(
+            got_blocked,
+            "StoryBlocked event must be sent when feature branch has no commits ahead of master"
+        );
+
+        // No mergemaster agent should have been started.
+        let agents = pool.agents.lock().unwrap();
+        let mergemaster_started = agents
+            .values()
+            .any(|a| a.agent_name.contains("mergemaster"));
+        assert!(
+            !mergemaster_started,
+            "mergemaster agent must NOT be started when no commits ahead of master"
+        );
+    }
+
     // ── bug 295: pipeline advance picks up waiting QA stories ──────────
 
     #[tokio::test]