fix(FN-3842): treat already-merged branch as finalize success

packages/engine/src/__tests__/merger.test.ts

@@ -143,6 +143,7 @@ import {
   summarizeVerificationOutput,
   inferDefaultTestCommand,
   resolveTaskDiffBaseRef,
+  commitOrAmendMergeWithFixes,
   MergeAbortedError,
   type ConflictCategory,
 } from "../merger.js";
@@ -7627,3 +7628,45 @@ describe("aiMergeTask — in-merge verification fix", () => {
     expect(fixAttempts[4][1]).toContain("attempt 3/3");
   });
 });
+
+describe("commitOrAmendMergeWithFixes", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("returns already-merged success when branch tip is ancestor of integration target and finalize has no staged content", async () => {
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr.includes("git diff --cached --name-only")) return "" as any;
+      if (cmdStr === "git diff --name-only") return "" as any;
+      if (cmdStr.includes("git status -z --porcelain")) return "" as any;
+      if (cmdStr === "git rev-parse HEAD") return "abc123" as any;
+      if (cmdStr === "git rev-parse fusion/fn-9999") return "def456" as any;
+      if (cmdStr === "git merge-base def456 abc123") return "abc123" as any;
+      if (cmdStr === "git diff --stat abc123..fusion/fn-9999") return "" as any;
+      if (cmdStr === "git ls-files --others --exclude-standard") return "" as any;
+      if (cmdStr.includes("git log -1 --pretty=%B HEAD")) return "commit message without trailer" as any;
+      if (cmdStr === "git merge-base --is-ancestor def456 abc123") return "" as any;
+      return "" as any;
+    });
+
+    const result = await commitOrAmendMergeWithFixes(
+      "/tmp/root",
+      "FN-9999",
+      "fusion/fn-9999",
+      "",
+      true,
+      "abc123",
+      "",
+      undefined,
+      DEFAULT_SETTINGS,
+      undefined,
+      null,
+      null,
+      new Set(),
+    );
+
+    expect(result).toEqual({ ok: true, reason: "branch-already-merged" });
+    expect(mockedExecSync.mock.calls.some((call) => String(call[0]) === "git merge-base --is-ancestor def456 abc123")).toBe(true);
+  });
+});

packages/engine/src/merger.ts

@@ -2733,6 +2733,9 @@ export async function commitOrAmendMergeWithFixes(
         `  diffStat(preAttemptHeadSha..branch)\n${diffStatSummary || "  <empty>"}`;
       mergerLog.warn(diagnostics);
 
+      // FN-3842 ordering: trailer short-circuit first (this task already on
+      // HEAD), then ancestor short-circuit (branch already reachable from
+      // integration target via a different commit path), then squash-restore.
       if (trailerOnHead) {
         mergerLog.log(
           `${taskId}: HEAD already carries Fusion-Task-Id trailer — treating in-merge fix finalize as no-op success`,
@@ -2740,6 +2743,23 @@ export async function commitOrAmendMergeWithFixes(
         return { ok: true, reason: "head-task-trailer" };
       }
 
+      let branchAlreadyOnIntegrationTarget = false;
+      try {
+        await execAsync(`git merge-base --is-ancestor ${branchTip} ${preAttemptHeadSha}`, {
+          cwd: rootDir,
+          encoding: "utf-8",
+        });
+        branchAlreadyOnIntegrationTarget = true;
+      } catch {
+        branchAlreadyOnIntegrationTarget = false;
+      }
+      if (branchAlreadyOnIntegrationTarget) {
+        mergerLog.log(
+          `${taskId}: branch tip ${branchTip} is already ancestor of integration target ${preAttemptHeadSha} — treating finalize as already-merged success`,
+        );
+        return { ok: true, reason: "branch-already-merged" };
+      }
+
       // No commit and no staged content can still be recoverable when the
       // in-merge fix path cleared the previous squash index state. Rebuild the
       // squash from branch -> preAttemptHeadSha and continue normally.