feat(FN-4499): complete Step 3 — add reanchor primitive

Fusion-Task-Id: FN-4499
Fusion-Task-Lineage: 3e9fee75-5c3d-4ce5-aa8b-3f2bb94e48eb

packages/engine/src/__tests__/branch-conflicts-recovery.test.ts

@@ -6,6 +6,7 @@ import { exec } from "node:child_process";
 import { promisify } from "node:util";
 import {
   autoRecoverCrossContamination,
+  reanchorBranchToBase,
   classifyBootstrapMisbinding,
   classifyForeignCommits,
   type BranchCrossContaminationCommit,
@@ -183,6 +184,27 @@ describe("branch contamination recovery classification", () => {
     });
   });
 
+  it("reanchors branch to base and clears bootstrap foreign history", async () => {
+    const { repoDir, baseSha } = await setupRepo();
+    const foreign = await makeCommit(repoDir, "foreign-reanchor", "feat(FN-4367): dependency change", "FN-4367");
+    const before = await run("git rev-parse feature", repoDir);
+
+    const result = await reanchorBranchToBase({
+      repoDir,
+      worktreePath: repoDir,
+      branchName: "feature",
+      baseSha,
+      taskId: "FN-4488",
+    });
+
+    const after = await run("git rev-parse feature", repoDir);
+    const range = await run(`git rev-list --count ${baseSha}..feature`, repoDir);
+    expect(result.previousTipSha).toBe(before);
+    expect(result.newTipSha).toBe(after);
+    expect(result.previousTipSha).toBe(foreign.sha);
+    expect(range).toBe("0");
+  });
+
   it("auto-recovers by dropping already-upstream foreign commits and preserving remaining branch work", async () => {
     const { repoDir, baseSha } = await setupRepo();
     await writeFile(path.join(repoDir, "foreign.txt"), "", "utf-8");
@@ -212,3 +234,4 @@ describe("branch contamination recovery classification", () => {
     expect(result.newTipSha).not.toEqual(originalTip);
   });
 });
+

packages/engine/src/branch-conflicts.ts

@@ -507,6 +507,52 @@ export async function classifyForeignCommits(
   }
 }
 
+export interface ReanchorBranchToBaseInput {
+  repoDir: string;
+  worktreePath: string;
+  branchName: string;
+  baseSha: string;
+  taskId: string;
+}
+
+export interface ReanchorBranchToBaseResult {
+  previousTipSha: string;
+  newTipSha: string;
+}
+
+export async function reanchorBranchToBase(
+  input: ReanchorBranchToBaseInput,
+): Promise<ReanchorBranchToBaseResult> {
+  const { repoDir, worktreePath, branchName, baseSha, taskId } = input;
+  const previousTipSha = await revParse(repoDir, branchName);
+
+  try {
+    await execAsync("git checkout -- .", {
+      cwd: worktreePath,
+      encoding: "utf-8",
+      timeout: GIT_TIMEOUT_MS,
+      maxBuffer: GIT_MAX_BUFFER,
+    });
+  } catch {
+    // best-effort: worktree may already be clean
+  }
+
+  await execAsync("git clean -fd", {
+    cwd: worktreePath,
+    encoding: "utf-8",
+    timeout: GIT_TIMEOUT_MS,
+    maxBuffer: GIT_MAX_BUFFER,
+  });
+  await runGit(worktreePath, `git checkout --detach ${quoteShellArg(baseSha)}`);
+  await runGit(worktreePath, `git checkout -B ${quoteShellArg(branchName)} ${quoteShellArg(baseSha)}`);
+  await assertCleanBranchAtBase(repoDir, branchName, baseSha, taskId);
+
+  return {
+    previousTipSha,
+    newTipSha: await revParse(repoDir, branchName),
+  };
+}
+
 export interface AutoRecoverCrossContaminationInput {
   repoDir: string;
   branchName: string;