feat(coding-agent): detect linked git worktrees (stack 1/2)

packages/coding-agent/src/core/footer-data-provider.ts

@@ -2,6 +2,7 @@ import { type ExecFileException, execFile, spawnSync } from "child_process";
 import { existsSync, type FSWatcher, readFileSync, type Stats, statSync, unwatchFile, watchFile } from "fs";
 import { dirname, join, resolve } from "path";
 import { closeWatcher, FS_WATCH_RETRY_DELAY_MS, watchWithErrorHandler } from "../utils/fs-watch.ts";
+import { linkedWorktreeName } from "./git-worktree.ts";
 
 export type GitPaths = {
 	repoDir: string;
@@ -131,6 +132,11 @@ export class FooterDataProvider {
 		return this.cachedBranch;
 	}
 
+	/** Linked worktree name, null in the main checkout or outside a repo */
+	getWorktreeName(): string | null {
+		return this.gitPaths ? linkedWorktreeName(this.gitPaths) : null;
+	}
+
 	/** Extension status texts set via ctx.ui.setStatus() */
 	getExtensionStatuses(): ReadonlyMap<string, string> {
 		return this.extensionStatuses;
@@ -384,5 +390,5 @@ export class FooterDataProvider {
 /** Read-only view for extensions - excludes setExtensionStatus, setAvailableProviderCount and dispose */
 export type ReadonlyFooterDataProvider = Pick<
 	FooterDataProvider,
-	"getGitBranch" | "getExtensionStatuses" | "getAvailableProviderCount" | "onBranchChange"
+	"getGitBranch" | "getWorktreeName" | "getExtensionStatuses" | "getAvailableProviderCount" | "onBranchChange"
 >;

packages/coding-agent/src/core/git-worktree.ts

@@ -0,0 +1,12 @@
+import { basename, dirname, resolve } from "path";
+import type { GitPaths } from "./footer-data-provider.ts";
+
+/**
+ * Name of the linked worktree that `paths` belongs to, or null for the main checkout.
+ * A linked worktree's HEAD lives under `<common>/worktrees/<name>/`, apart from the shared git dir.
+ */
+export function linkedWorktreeName(paths: GitPaths): string | null {
+	const gitDir = resolve(dirname(paths.headPath));
+	if (gitDir === resolve(paths.commonGitDir)) return null;
+	return basename(paths.repoDir);
+}

packages/coding-agent/test/footer-width.test.ts

@@ -92,9 +92,10 @@ function createSession(options: {
 	return session as unknown as AgentSession;
 }
 
-function createFooterData(providerCount: number): ReadonlyFooterDataProvider {
+function createFooterData(providerCount: number, worktree: string | null = null): ReadonlyFooterDataProvider {
 	const provider = {
 		getGitBranch: () => "main",
+		getWorktreeName: () => worktree,
 		getExtensionStatuses: () => new Map<string, string>(),
 		getAvailableProviderCount: () => providerCount,
 		onBranchChange: (callback: () => void) => {

packages/coding-agent/test/git-worktree.test.ts

@@ -0,0 +1,17 @@
+import { join } from "path";
+import { describe, expect, it } from "vitest";
+import { linkedWorktreeName } from "../src/core/git-worktree.ts";
+
+describe("linkedWorktreeName", () => {
+	it("returns null for the main checkout", () => {
+		const repoDir = join("/repo", "app");
+		const commonGitDir = join(repoDir, ".git");
+		expect(linkedWorktreeName({ repoDir, commonGitDir, headPath: join(commonGitDir, "HEAD") })).toBeNull();
+	});
+
+	it("names a linked worktree after its directory", () => {
+		const commonGitDir = join("/repo", "app", ".git");
+		const headPath = join(commonGitDir, "worktrees", "feature-x", "HEAD");
+		expect(linkedWorktreeName({ repoDir: join("/repo", "feature-x"), commonGitDir, headPath })).toBe("feature-x");
+	});
+});