fix(mcp): fail fast on a second direct-mode writer per project (#1740) (#1744)

Concurrent CODEGRAPH_NO_DAEMON / in-process fallback serve --mcp instances
each started a FileWatcher and contended on codegraph.lock until auto-sync
degraded. Add an exclusive .codegraph/writer.pid lock held by the daemon or
the single direct writer; a second writer exits with actionable guidance.
Daemon mode still multiplexes N proxies onto one writer.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

__tests__/mcp-writer-lock.test.ts

@@ -0,0 +1,128 @@
+/**
+ * Issue #1740 — concurrent direct-mode serve --mcp must fail fast on the
+ * second writer instead of silently degrading auto-sync.
+ */
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { ChildProcessWithoutNullStreams, spawn } from 'child_process';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { CodeGraph } from '../src';
+import { getWriterPidPath } from '../src/mcp/writer-lock';
+
+const BIN = path.resolve(__dirname, '../dist/bin/codegraph.js');
+
+function sleep(ms: number): Promise<void> {
+  return new Promise((r) => setTimeout(r, ms));
+}
+
+function spawnMcp(
+  cwd: string,
+  env: NodeJS.ProcessEnv,
+): { child: ChildProcessWithoutNullStreams; getStderr: () => string } {
+  const child = spawn(process.execPath, [BIN, 'serve', '--mcp'], {
+    cwd,
+    stdio: ['pipe', 'pipe', 'pipe'],
+    env: { ...process.env, ...env },
+  }) as ChildProcessWithoutNullStreams;
+  child.on('error', () => {});
+  child.stdin.on('error', () => {});
+  let stderr = '';
+  child.stderr.on('data', (c: Buffer) => { stderr += c.toString('utf8'); });
+  child.stdout.on('data', () => {});
+  return { child, getStderr: () => stderr };
+}
+
+describe('issue #1740 — direct-mode writer lock', () => {
+  let tempDir: string;
+  let realRoot: string;
+  const children: ChildProcessWithoutNullStreams[] = [];
+
+  beforeEach(async () => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'cg1740-mcp-'));
+    realRoot = fs.realpathSync(tempDir);
+    fs.mkdirSync(path.join(realRoot, 'src'));
+    fs.writeFileSync(path.join(realRoot, 'src/a.ts'), 'export function a() { return 1; }\n');
+    const cg = await CodeGraph.init(realRoot);
+    await cg.indexAll();
+    cg.close();
+  });
+
+  afterEach(async () => {
+    for (const c of children) {
+      try { c.kill('SIGTERM'); } catch { /* ignore */ }
+    }
+    children.length = 0;
+    await sleep(300);
+    try { fs.rmSync(tempDir, { recursive: true, force: true }); } catch { /* ignore */ }
+  });
+
+  it('second CODEGRAPH_NO_DAEMON serve --mcp exits with writer-lock error', async () => {
+    const env = {
+      CODEGRAPH_NO_DAEMON: '1',
+      CODEGRAPH_MCP_DEBUG: '1',
+      CODEGRAPH_NO_WATCHDOG: '1',
+      CODEGRAPH_STARTUP_HANDSHAKE_TIMEOUT_MS: '0',
+      // Avoid wasm --liftoff-only re-exec so lock.pid matches the spawned pid.
+      CODEGRAPH_NO_RELAUNCH: '1',
+      CODEGRAPH_WASM_RELAUNCHED: '1',
+    };
+    const first = spawnMcp(realRoot, env);
+    children.push(first.child);
+
+    const lockPath = getWriterPidPath(realRoot);
+    const deadline = Date.now() + 10000;
+    while (Date.now() < deadline && !fs.existsSync(lockPath)) {
+      await sleep(50);
+    }
+    expect(fs.existsSync(lockPath)).toBe(true);
+    expect(first.child.exitCode).toBeNull();
+
+    const second = spawnMcp(realRoot, env);
+    children.push(second.child);
+
+    const code = await new Promise<number | null>((resolve) => {
+      const timer = setTimeout(() => resolve(second.child.exitCode), 10000);
+      second.child.on('close', (c) => {
+        clearTimeout(timer);
+        resolve(c);
+      });
+    });
+
+    expect(code).toBe(1);
+    expect(second.getStderr()).toMatch(/writer lock held/i);
+    expect(second.getStderr()).toMatch(/CODEGRAPH_NO_DAEMON/);
+    expect(first.child.exitCode).toBeNull();
+    const lock = JSON.parse(fs.readFileSync(lockPath, 'utf8')) as { pid: number };
+    expect(lock.pid).toBe(first.child.pid);
+  }, 20000);
+
+  it('default daemon mode still allows two proxies to share one writer', async () => {
+    const env = {
+      CODEGRAPH_MCP_LOG_ATTACH: '1',
+      CODEGRAPH_NO_WATCHDOG: '1',
+      CODEGRAPH_STARTUP_HANDSHAKE_TIMEOUT_MS: '0',
+      CODEGRAPH_NO_RELAUNCH: '1',
+      CODEGRAPH_WASM_RELAUNCHED: '1',
+    };
+    const a = spawnMcp(realRoot, env);
+    const b = spawnMcp(realRoot, env);
+    children.push(a.child, b.child);
+
+    const lockPath = getWriterPidPath(realRoot);
+    const deadline = Date.now() + 15000;
+    while (Date.now() < deadline && !fs.existsSync(lockPath)) {
+      await sleep(50);
+    }
+    expect(fs.existsSync(lockPath)).toBe(true);
+    await sleep(1000);
+    expect(a.child.exitCode).toBeNull();
+    expect(b.child.exitCode).toBeNull();
+
+    const lock = JSON.parse(fs.readFileSync(lockPath, 'utf8')) as { pid: number; mode: string };
+    expect(lock.mode).toBe('daemon');
+    expect(lock.pid).not.toBe(a.child.pid);
+    expect(lock.pid).not.toBe(b.child.pid);
+  }, 25000);
+});