From 00438b2bb4142432b177b8096c95c54482aaa282 Mon Sep 17 00:00:00 2001
From: Jeremy McSpadden <jeremy@fluxlabs.net>
Date: Mon, 16 Mar 2026 20:24:24 -0500
Subject: [PATCH] fix: skip redundant checkout in worktree merge when main
 already current (#757)

When mergeMilestoneToMain runs from a worktree context, main is already
checked out at the project root. The unconditional git checkout main
fails with "already used by worktree" because git refuses to checkout a
branch that is active in another worktree.

Skip the checkout when the integration branch is already current at the
project root, which is always the case in worktree-mode merges.
---
 src/resources/extensions/gsd/auto-worktree.ts |  8 +++--
 .../auto-worktree-milestone-merge.test.ts     | 34 +++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/src/resources/extensions/gsd/auto-worktree.ts b/src/resources/extensions/gsd/auto-worktree.ts
index 672b6bb93..f411e50e1 100644
--- a/src/resources/extensions/gsd/auto-worktree.ts
+++ b/src/resources/extensions/gsd/auto-worktree.ts
@@ -429,8 +429,12 @@ export function mergeMilestoneToMain(
   const integrationBranch = readIntegrationBranch(originalBasePath_, milestoneId);
   const mainBranch = integrationBranch ?? prefs.main_branch ?? "main";
 
-  // 5. Checkout integration branch
-  nativeCheckoutBranch(originalBasePath_, mainBranch);
+  // 5. Checkout integration branch (skip if already current — avoids git error
+  //    when main is already checked out in the project-root worktree, #757)
+  const currentBranchAtBase = nativeGetCurrentBranch(originalBasePath_);
+  if (currentBranchAtBase !== mainBranch) {
+    nativeCheckoutBranch(originalBasePath_, mainBranch);
+  }
 
   // 6. Build rich commit message
   const milestoneTitle = roadmap.title.replace(/^M\d+:\s*/, "").trim() || milestoneId;
diff --git a/src/resources/extensions/gsd/tests/auto-worktree-milestone-merge.test.ts b/src/resources/extensions/gsd/tests/auto-worktree-milestone-merge.test.ts
index df78b49d8..806f56097 100644
--- a/src/resources/extensions/gsd/tests/auto-worktree-milestone-merge.test.ts
+++ b/src/resources/extensions/gsd/tests/auto-worktree-milestone-merge.test.ts
@@ -290,6 +290,40 @@ async function main(): Promise<void> {
       assertTrue(existsSync(join(repo, "feature.ts")), "feature.ts merged to main");
     }
 
+    // ─── Test 6: Skip checkout when main already current (#757) ───────
+    console.log("\n=== skip checkout when main already current (#757) ===");
+    {
+      const repo = freshRepo();
+      const wtPath = createAutoWorktree(repo, "M060");
+
+      addSliceToMilestone(repo, wtPath, "M060", "S01", "Skip checkout test", [
+        { file: "skip-checkout.ts", content: "export const skip = true;\n", message: "add skip-checkout" },
+      ]);
+
+      const roadmap = makeRoadmap("M060", "Skip checkout verification", [
+        { id: "S01", title: "Skip checkout test" },
+      ]);
+
+      // Verify main is already checked out at repo root (worktree default)
+      const branchAtRoot = run("git rev-parse --abbrev-ref HEAD", repo);
+      assertEq(branchAtRoot, "main", "main is already checked out at project root");
+
+      // mergeMilestoneToMain should succeed without attempting to checkout main
+      // (which would fail with "already used by worktree" error)
+      let threw = false;
+      try {
+        const result = mergeMilestoneToMain(repo, "M060", roadmap);
+        assertTrue(result.commitMessage.includes("feat(M060)"), "merge commit created");
+      } catch (err) {
+        threw = true;
+        console.error("Unexpected error:", err);
+      }
+      assertTrue(!threw, "does not fail when main is already checked out at project root");
+
+      // Verify the merge actually happened
+      assertTrue(existsSync(join(repo, "skip-checkout.ts")), "skip-checkout.ts merged to main");
+    }
+
   } finally {
     process.chdir(savedCwd);
     for (const d of tempDirs) {