feat(FN-2490): merge fusion/fn-2490

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

@@ -111,6 +111,7 @@ import {
   shouldSyncDependenciesForMerge,
   summarizeVerificationOutput,
   inferDefaultTestCommand,
+  resolveTaskDiffBaseRef,
   type ConflictCategory,
 } from "../merger.js";
 import { mergerLog } from "../logger.js";
@@ -3789,6 +3790,108 @@ describe("validateDiffScope", () => {
   });
 });
 
+describe("resolveTaskDiffBaseRef", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("prefers merge-base when it differs from head", async () => {
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === 'git merge-base "HEAD" "main"') return "merge-base-123" as any;
+      if (cmdStr === 'git rev-parse "HEAD"') return "head-456" as any;
+      throw new Error(`Unexpected command: ${cmdStr}`);
+    });
+
+    const diffBase = await resolveTaskDiffBaseRef({
+      cwd: "/tmp/root",
+      headRef: "HEAD",
+      baseBranch: "main",
+      baseCommitSha: "task-base-789",
+    });
+
+    expect(diffBase).toBe("merge-base-123");
+    expect(
+      mockedExecSync.mock.calls.some(([cmd]) =>
+        String(cmd).includes("merge-base --is-ancestor"),
+      ),
+    ).toBe(false);
+  });
+
+  it("uses baseCommitSha when merge-base equals head", async () => {
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === 'git merge-base "HEAD" "main"') return "head-456" as any;
+      if (cmdStr === 'git rev-parse "HEAD"') return "head-456" as any;
+      if (cmdStr === 'git merge-base --is-ancestor "task-base-789" "HEAD"') return "" as any;
+      throw new Error(`Unexpected command: ${cmdStr}`);
+    });
+
+    const diffBase = await resolveTaskDiffBaseRef({
+      cwd: "/tmp/root",
+      headRef: "HEAD",
+      baseBranch: "main",
+      baseCommitSha: "task-base-789",
+    });
+
+    expect(diffBase).toBe("task-base-789");
+  });
+
+  it("falls back to HEAD~1 when merge-base is unavailable and baseCommitSha is stale", async () => {
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === 'git merge-base "HEAD" "main"') {
+        throw new Error("missing local main");
+      }
+      if (cmdStr === 'git merge-base "HEAD" "origin/main"') {
+        throw new Error("missing remote main");
+      }
+      if (cmdStr === 'git merge-base --is-ancestor "stale-base" "HEAD"') {
+        throw new Error("stale base sha");
+      }
+      if (cmdStr === 'git rev-parse "HEAD~1"') return "parent-123" as any;
+      throw new Error(`Unexpected command: ${cmdStr}`);
+    });
+
+    const diffBase = await resolveTaskDiffBaseRef({
+      cwd: "/tmp/root",
+      headRef: "HEAD",
+      baseBranch: "main",
+      baseCommitSha: "stale-base",
+    });
+
+    expect(diffBase).toBe("parent-123");
+  });
+
+  it("returns undefined when no merge base or fallback refs are available", async () => {
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === 'git merge-base "HEAD" "main"') {
+        throw new Error("missing local main");
+      }
+      if (cmdStr === 'git merge-base "HEAD" "origin/main"') {
+        throw new Error("missing remote main");
+      }
+      if (cmdStr === 'git merge-base --is-ancestor "stale-base" "HEAD"') {
+        throw new Error("stale base sha");
+      }
+      if (cmdStr === 'git rev-parse "HEAD~1"') {
+        throw new Error("single commit repo");
+      }
+      throw new Error(`Unexpected command: ${cmdStr}`);
+    });
+
+    const diffBase = await resolveTaskDiffBaseRef({
+      cwd: "/tmp/root",
+      headRef: "HEAD",
+      baseBranch: "main",
+      baseCommitSha: "stale-base",
+    });
+
+    expect(diffBase).toBeUndefined();
+  });
+});
+
 describe("aiMergeTask — post-merge workflow steps", () => {
   beforeEach(() => {
     vi.clearAllMocks();

packages/engine/src/merger.ts

@@ -1010,6 +1010,92 @@ export async function validateDiffScope(
   return result;
 }
 
+interface DiffBaseResolutionInput {
+  cwd: string;
+  headRef: string;
+  baseBranch?: string;
+  baseCommitSha?: string;
+}
+
+/**
+ * Resolve the commit ref used as diff base for task-scoped changed-file views.
+ *
+ * IMPORTANT: This ordering must stay in lockstep with dashboard `resolveDiffBase`
+ * so merge-time scope warnings evaluate the exact same change set operators see.
+ *
+ * Strategy (priority order):
+ * 1. Live merge-base between `headRef` and `{baseBranch}` (fallback to
+ *    `origin/{baseBranch}` when local ref is missing).
+ * 2. `baseCommitSha` when merge-base is unavailable or equals `headRef`, and
+ *    the SHA is still an ancestor of `headRef`.
+ * 3. `headRef~1` as last resort.
+ */
+export async function resolveTaskDiffBaseRef({
+  cwd,
+  headRef,
+  baseBranch,
+  baseCommitSha,
+}: DiffBaseResolutionInput): Promise<string | undefined> {
+  const resolvedBaseBranch = baseBranch?.trim() || "main";
+  const quotedHeadRef = quoteArg(headRef);
+  let mergeBase: string | undefined;
+
+  try {
+    try {
+      const { stdout } = await execAsync(`git merge-base ${quotedHeadRef} ${quoteArg(resolvedBaseBranch)}`, {
+        cwd,
+        encoding: "utf-8",
+      });
+      mergeBase = stdout.trim() || undefined;
+    } catch {
+      const { stdout } = await execAsync(`git merge-base ${quotedHeadRef} ${quoteArg(`origin/${resolvedBaseBranch}`)}`, {
+        cwd,
+        encoding: "utf-8",
+      });
+      mergeBase = stdout.trim() || undefined;
+    }
+  } catch {
+    // Base branch may not exist locally/remotely.
+  }
+
+  // Same guard as dashboard routes: when merge-base === headRef, the range
+  // would be empty, so prefer a still-valid task-scoped baseCommitSha.
+  if (mergeBase) {
+    try {
+      const { stdout } = await execAsync(`git rev-parse ${quotedHeadRef}`, {
+        cwd,
+        encoding: "utf-8",
+      });
+      const headSha = stdout.trim();
+      if (headSha && headSha !== mergeBase) return mergeBase;
+    } catch {
+      return mergeBase;
+    }
+  }
+
+  if (baseCommitSha) {
+    try {
+      await execAsync(`git merge-base --is-ancestor ${quoteArg(baseCommitSha)} ${quotedHeadRef}`, {
+        cwd,
+        encoding: "utf-8",
+      });
+      return baseCommitSha;
+    } catch {
+      // stale or unreachable — fall through
+    }
+  }
+
+  try {
+    const { stdout } = await execAsync(`git rev-parse ${quoteArg(`${headRef}~1`)}`, {
+      cwd,
+      encoding: "utf-8",
+    });
+    return stdout.trim() || undefined;
+  } catch {
+    return undefined;
+  }
+}
+
 /**
  * Get list of conflicted files from git.
  * Runs `git diff --name-only --diff-filter=U` and returns array of file paths.
@@ -1961,10 +2047,19 @@ export async function aiMergeTask(
   }
 
   // 4. Gather context for the agent (used in all attempts)
+  // Keep this range strategy aligned with dashboard changed-files endpoints.
+  const diffBaseRef = await resolveTaskDiffBaseRef({
+    cwd: rootDir,
+    headRef: branch,
+    baseBranch: task.baseBranch,
+    baseCommitSha: task.baseCommitSha,
+  });
+  const contextDiffRange = diffBaseRef ? `${diffBaseRef}..${branch}` : `HEAD..${branch}`;
+
   let commitLog = "";
   let diffStat = "";
   try {
-    const { stdout: logOutput } = await execAsync(`git log HEAD..${branch} --format="- %s"`, {
+    const { stdout: logOutput } = await execAsync(`git log ${contextDiffRange} --format="- %s"`, {
       cwd: rootDir,
       encoding: "utf-8",
     });
@@ -1973,12 +2068,7 @@ export async function aiMergeTask(
     commitLog = "(unable to read commit log)";
   }
   try {
-    const { stdout: mergeBaseOutput } = await execAsync(`git merge-base HEAD ${branch}`, {
-      cwd: rootDir,
-      encoding: "utf-8",
-    });
-    const mergeBase = mergeBaseOutput.trim();
-    const { stdout: diffOutput } = await execAsync(`git diff ${mergeBase}..${branch} --stat`, {
+    const { stdout: diffOutput } = await execAsync(`git diff ${contextDiffRange} --stat`, {
       cwd: rootDir,
       encoding: "utf-8",
     });