fix(server): worktree threads get their generated branch name when a branch named t3 exists

apps/server/src/vcs/GitVcsDriverCore.test.ts

@@ -3641,6 +3641,47 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
       }),
     );
 
+    it.effect("flattens a generated name when a branch at its parent path blocks it", () =>
+      Effect.gen(function* () {
+        const cwd = yield* makeTmpDir();
+        yield* initRepoWithCommit(cwd);
+        const driver = yield* GitVcsDriver.GitVcsDriver;
+        // A local `t3` branch is why launches start on the flat `t3-<hex>` temporary branch.
+        yield* git(cwd, ["branch", "t3"]);
+        yield* git(cwd, ["checkout", "-b", "t3-abcd1234"]);
+        yield* git(cwd, ["branch", "t3-fix-login"]);
+
+        const renamed = yield* driver.renameBranch({
+          cwd,
+          oldBranch: "t3-abcd1234",
+          newBranch: "t3/fix-login",
+        });
+
+        assert.equal(renamed.branch, "t3-fix-login-1");
+        assert.equal(yield* git(cwd, ["branch", "--show-current"]), "t3-fix-login-1");
+      }),
+    );
+
+    it.effect("skips a flattened name that a nested branch blocks", () =>
+      Effect.gen(function* () {
+        const cwd = yield* makeTmpDir();
+        yield* initRepoWithCommit(cwd);
+        const driver = yield* GitVcsDriver.GitVcsDriver;
+        yield* git(cwd, ["branch", "t3"]);
+        yield* git(cwd, ["checkout", "-b", "t3-abcd1234"]);
+        yield* git(cwd, ["branch", "t3-fix-login/child"]);
+
+        const renamed = yield* driver.renameBranch({
+          cwd,
+          oldBranch: "t3-abcd1234",
+          newBranch: "t3/fix-login",
+        });
+
+        assert.equal(renamed.branch, "t3-fix-login-1");
+        assert.equal(yield* git(cwd, ["branch", "--show-current"]), "t3-fix-login-1");
+      }),
+    );
+
     it.effect("rejects a missing branch without restoring a matching dirty file", () =>
       Effect.gen(function* () {
         const cwd = yield* makeTmpDir();

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -1247,16 +1247,37 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
 
   const resolveAvailableBranchName = Effect.fn("resolveAvailableBranchName")(function* (
     cwd: string,
-    desiredBranch: string,
+    requestedBranch: string,
   ) {
-    const isDesiredTaken = yield* branchExists(cwd, desiredBranch);
+    // A branch at a parent path (such as `t3`) blocks every ref nested under it, so no suffix
+    // would help; flatten to a sibling the way temporary worktree branches already do.
+    const segments = requestedBranch.split("/");
+    let blockedByParent = false;
+    for (let index = 1; index < segments.length && !blockedByParent; index += 1) {
+      blockedByParent = yield* branchExists(cwd, segments.slice(0, index).join("/"));
+    }
+    const desiredBranch = blockedByParent ? segments.join("-") : requestedBranch;
+    // A branch nested under the name (`t3-fix/child`) blocks it as surely as one with the name.
+    const isBranchNameTaken = (name: string) =>
+      Effect.zipWith(
+        branchExists(cwd, name),
+        runGitStdout(
+          "GitVcsDriver.branchExists",
+          cwd,
+          ["for-each-ref", "--count=1", "--format=%(refname)", `refs/heads/${name}/`],
+          true,
+        ),
+        (exists, nested) => exists || nested.trim().length > 0,
+      );
+
+    const isDesiredTaken = yield* isBranchNameTaken(desiredBranch);
     if (!isDesiredTaken) {
       return desiredBranch;
     }
 
     for (let suffix = 1; suffix <= 100; suffix += 1) {
       const candidate = `${desiredBranch}-${suffix}`;
-      const isCandidateTaken = yield* branchExists(cwd, candidate);
+      const isCandidateTaken = yield* isBranchNameTaken(candidate);
       if (!isCandidateTaken) {
         return candidate;
       }