feat(browse): Xvfb auto-spawn with PID + start-time validation

Adds browse/src/xvfb.ts: a Linux-only Xvfb auto-spawn module for
running headed Chromium in containers without DISPLAY. The module
walks a display range to pick a free one (never hardcodes :99) and
validates orphan PIDs by BOTH /proc/<pid>/cmdline matching 'Xvfb' AND
start-time matching the recorded value before sending any signal.
Defends against PID reuse — refuses to kill anything that doesn't
match both checks.

- shouldSpawnXvfb(env, platform) — pure decision: skip on macOS/Windows,
  on Linux skip when DISPLAY or WAYLAND_DISPLAY is set (codex F2)
- pickFreeDisplay(99..120) — probes via xdpyinfo
- spawnXvfb(display) — returns { pid, startTime, display } handle
- isOurXvfb(pid, startTime) — both-checks validator
- cleanupXvfb(state) — best-effort, validates ownership before SIGTERM

Wired into server.ts startup: when shouldSpawnXvfb says yes, picks a
free display, spawns Xvfb, sets DISPLAY for chromium.launchHeaded, and
records xvfbPid/xvfbStartTime/xvfbDisplay in the state file. Cleanup
runs on process.on('exit'). The CLI's disconnect path also runs
cleanupXvfb() in the force-cleanup branch when the server is dead.

Disconnect now applies to any non-default daemon (headed mode OR
configHash-tagged daemon — i.e. one started with --proxy/--headed),
not just headed mode.

Adds xvfb + x11-utils to .github/docker/Dockerfile.ci so CI exercises
the Linux container --headed path on every run. Without it the most
common production path would go untested.

Tests: 17 new across decision logic, PID validation defenses
(cmdline mismatch, start-time mismatch), no-op safety on bad inputs,
and a Linux+Xvfb-installed gate for the spawn → validate → cleanup
round trip. Tests skip on macOS/Windows automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

browse/src/cli.ts

@@ -1056,8 +1056,12 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
   // guard blocks all commands when the server is unresponsive.
   if (command === 'disconnect') {
     const existingState = readState();
-    if (!existingState || existingState.mode !== 'headed') {
-      console.log('Not in headed mode — nothing to disconnect.');
+    // disconnect applies when there's a non-default daemon — headed mode OR
+    // any custom config (--proxy/--headed) recorded as configHash. Plain
+    // headless daemons should use 'stop' instead.
+    const hasCustomConfig = existingState && (existingState.mode === 'headed' || existingState.configHash);
+    if (!existingState || !hasCustomConfig) {
+      console.log('Not in headed/custom-config mode — nothing to disconnect.');
       process.exit(0);
     }
     // Try graceful shutdown via server
@@ -1093,6 +1097,22 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
     for (const lockFile of ['SingletonLock', 'SingletonSocket', 'SingletonCookie']) {
       safeUnlinkQuiet(path.join(profileDir, lockFile));
     }
+    // Xvfb orphan cleanup: if the recorded PID still matches our Xvfb (by
+    // cmdline AND start-time), kill it. PID-only would risk killing a
+    // recycled PID belonging to an unrelated process.
+    if (existingState.xvfbPid && existingState.xvfbStartTime) {
+      try {
+        const { cleanupXvfb } = await import('./xvfb');
+        cleanupXvfb({
+          pid: existingState.xvfbPid,
+          startTime: existingState.xvfbStartTime,
+          display: existingState.xvfbDisplay || ':99',
+        });
+      } catch {
+        // Best effort — Linux-only module on a non-Linux disconnect may
+        // not load; cleanup is best-effort anyway.
+      }
+    }
     safeUnlinkQuiet(config.stateFile);
     console.log('Disconnected (server was unresponsive — force cleaned).');
     process.exit(0);

browse/src/server.ts

@@ -44,6 +44,7 @@ import { safeUnlink, safeUnlinkQuiet, safeKill } from './error-handling';
 import { startSocksBridge, testUpstream, type BridgeHandle } from './socks-bridge';
 import { parseProxyConfig, toUpstreamConfig, ProxyConfigError } from './proxy-config';
 import { redactProxyUrl } from './proxy-redact';
+import { shouldSpawnXvfb, pickFreeDisplay, spawnXvfb, xvfbInstallHint, type XvfbHandle } from './xvfb';
 import { logTunnelDenial } from './tunnel-denial-log';
 import {
   mintSseSessionToken, validateSseSessionToken, extractSseCookie,
@@ -1087,6 +1088,33 @@ async function start() {
     });
   }
 
+  // ─── Xvfb auto-spawn (Linux + headed + no DISPLAY) ─────────────
+  // codex F2: walk display range to pick a free one (never hardcode :99);
+  // record start-time alongside PID so cleanup can validate ownership and
+  // not kill a recycled PID.
+  let xvfb: XvfbHandle | null = null;
+  const xvfbDecision = shouldSpawnXvfb(process.env, process.platform);
+  if (xvfbDecision.spawn) {
+    const displayNum = pickFreeDisplay();
+    if (displayNum == null) {
+      console.error('[browse] no free X display in range :99-:120 — refusing to clobber existing X servers');
+      process.exit(1);
+    }
+    try {
+      xvfb = await spawnXvfb(displayNum);
+      process.env.DISPLAY = xvfb.display;
+      console.log(`[browse] [xvfb] spawned on ${xvfb.display} (pid ${xvfb.pid})`);
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : String(err);
+      console.error(`[browse] [xvfb] FAILED: ${msg}`);
+      console.error(`[browse] [xvfb] hint: ${xvfbInstallHint()}`);
+      process.exit(1);
+    }
+    process.on('exit', () => { try { xvfb?.close(); } catch { /* shutting down */ } });
+  } else if (process.env.BROWSE_HEADED === '1') {
+    console.log(`[browse] [xvfb] skipped: ${xvfbDecision.reason}`);
+  }
+
   // Launch browser (headless or headed with extension)
   // BROWSE_HEADLESS_SKIP=1 skips browser launch entirely (for HTTP-only testing)
   const skipBrowser = process.env.BROWSE_HEADLESS_SKIP === '1';
@@ -2068,6 +2096,10 @@ async function start() {
     // D2 daemon-mismatch detection: CLI computes the same hash from its
     // resolved flags and refuses if it differs from this stored value.
     ...(process.env.BROWSE_CONFIG_HASH ? { configHash: process.env.BROWSE_CONFIG_HASH } : {}),
+    // Xvfb child PID + start-time + display so disconnect (or a future
+    // daemon launch on this state file) can validate-then-cleanup orphans
+    // without clobbering a recycled PID.
+    ...(xvfb ? { xvfbPid: xvfb.pid, xvfbStartTime: xvfb.startTime, xvfbDisplay: xvfb.display } : {}),
   };
   const tmpFile = config.stateFile + '.tmp';
   fs.writeFileSync(tmpFile, JSON.stringify(state, null, 2), { mode: 0o600 });

browse/src/xvfb.ts

@@ -0,0 +1,193 @@
+/**
+ * Xvfb (X virtual framebuffer) auto-spawn for headed Chromium on Linux
+ * containers without DISPLAY.
+ *
+ * The motivating use case: a headless container needs to run Chromium in
+ * "headed" mode (visible window) — for example, to run with the
+ * AutomationControlled flag off and pass anti-bot fingerprint checks. Xvfb
+ * provides an off-screen X server that Chromium can render into.
+ *
+ * Design notes:
+ *   - Pick a free display dynamically (try :99, :100, :101...). NEVER unlink
+ *     /tmp/.X<n>-lock for displays we didn't create — that would steal an
+ *     active X server from another process or user.
+ *   - Validate orphan Xvfb processes by BOTH /proc/<pid>/cmdline matching
+ *     'Xvfb' AND start-time matching the recorded value. PID reuse is real;
+ *     a one-field check would let us send SIGTERM to an unrelated process
+ *     that happened to inherit a recycled PID.
+ *   - Skip spawn entirely on macOS/Windows (native windowing) and on Linux
+ *     when DISPLAY or WAYLAND_DISPLAY is already set (codex F2).
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import { safeKill, isProcessAlive } from './error-handling';
+
+export interface XvfbHandle {
+  pid: number;
+  startTime: string;
+  display: string; // e.g. ":99"
+  /** Best-effort cleanup. Validates ownership before kill. */
+  close: () => void;
+}
+
+export interface ShouldSpawnDecision {
+  spawn: boolean;
+  reason: string;
+}
+
+const DISPLAY_RANGE_START = 99;
+const DISPLAY_RANGE_END = 120;
+
+/**
+ * Decide whether the daemon should auto-spawn an Xvfb. Pure: takes env +
+ * platform and returns a decision. Easy to unit test.
+ */
+export function shouldSpawnXvfb(env: NodeJS.ProcessEnv, platform: NodeJS.Platform): ShouldSpawnDecision {
+  if (env.BROWSE_HEADED !== '1') return { spawn: false, reason: 'not headed mode' };
+  if (platform !== 'linux') return { spawn: false, reason: `platform ${platform} uses native windowing` };
+  if (env.DISPLAY) return { spawn: false, reason: `DISPLAY=${env.DISPLAY} already set` };
+  if (env.WAYLAND_DISPLAY) return { spawn: false, reason: `WAYLAND_DISPLAY=${env.WAYLAND_DISPLAY} set; Chromium uses Wayland natively` };
+  return { spawn: true, reason: 'linux headed without DISPLAY/WAYLAND_DISPLAY' };
+}
+
+/**
+ * Probe a display number — return true if no X server is currently listening
+ * on it (i.e., we can safely spawn a new Xvfb there).
+ */
+export function isDisplayFree(displayNum: number): boolean {
+  // xdpyinfo exits 0 if a display is reachable. Exit non-zero means no
+  // server, which is what we want.
+  const result = Bun.spawnSync(['xdpyinfo', '-display', `:${displayNum}`], {
+    stdout: 'ignore', stderr: 'ignore', timeout: 2000,
+  });
+  return result.exitCode !== 0;
+}
+
+/**
+ * Walk the display range and return the first free one, or null if all
+ * displays in the range are taken.
+ */
+export function pickFreeDisplay(
+  rangeStart: number = DISPLAY_RANGE_START,
+  rangeEnd: number = DISPLAY_RANGE_END,
+): number | null {
+  for (let n = rangeStart; n <= rangeEnd; n++) {
+    if (isDisplayFree(n)) return n;
+  }
+  return null;
+}
+
+/**
+ * Read the wall-clock start time of a PID via `ps -o lstart=`. Stable across
+ * reads (unlike /proc/stat field 22 which reports jiffies since boot in a
+ * format that's harder to compare). Returns an empty string if the process
+ * is gone or ps fails.
+ */
+export function readPidStartTime(pid: number): string {
+  if (!isProcessAlive(pid)) return '';
+  const result = Bun.spawnSync(['ps', '-p', String(pid), '-o', 'lstart='], {
+    stdout: 'pipe', stderr: 'pipe', timeout: 2000,
+  });
+  if (result.exitCode !== 0) return '';
+  return result.stdout.toString().trim();
+}
+
+/**
+ * Read the cmdline of a PID via /proc/<pid>/cmdline. Returns empty string
+ * if the process is gone or the cmdline isn't readable.
+ */
+export function readPidCmdline(pid: number): string {
+  try {
+    return fs.readFileSync(`/proc/${pid}/cmdline`, 'utf-8').replace(/\0/g, ' ').trim();
+  } catch {
+    return '';
+  }
+}
+
+/**
+ * Validate that PID is still our Xvfb child. Both checks must pass:
+ *   1. /proc/<pid>/cmdline contains 'Xvfb' (string match — Xvfb's argv[0] is
+ *      always 'Xvfb' or a full path ending in /Xvfb)
+ *   2. Start time matches the recorded value (PID reuse defense)
+ */
+export function isOurXvfb(pid: number, recordedStartTime: string): boolean {
+  if (!pid || !recordedStartTime) return false;
+  const cmdline = readPidCmdline(pid);
+  if (!cmdline.toLowerCase().includes('xvfb')) return false;
+  const currentStart = readPidStartTime(pid);
+  if (!currentStart) return false;
+  return currentStart === recordedStartTime;
+}
+
+/**
+ * Spawn Xvfb on the given display. Returns a handle including the validated
+ * start-time so future cleanup can confirm ownership.
+ *
+ * Throws if Xvfb isn't installed (caller should print a platform-specific
+ * install hint).
+ */
+export async function spawnXvfb(displayNum: number): Promise<XvfbHandle> {
+  const display = `:${displayNum}`;
+
+  // Spawn detached: Xvfb's lifetime is tied to whether we've explicitly
+  // killed it via the handle's close() method, not to the parent process.
+  const proc = Bun.spawn(['Xvfb', display, '-screen', '0', '1920x1080x24', '-ac'], {
+    stdio: ['ignore', 'ignore', 'ignore'],
+  });
+  proc.unref();
+
+  // Wait for the X server to become reachable — Xvfb takes a few hundred ms
+  // to bind. Probe via xdpyinfo with retries.
+  const deadline = Date.now() + 3000;
+  let ready = false;
+  while (Date.now() < deadline) {
+    await Bun.sleep(100);
+    if (!isDisplayFree(displayNum)) { ready = true; break; }
+    // If Xvfb crashed during startup, fail fast.
+    if (proc.exitCode != null) {
+      throw new Error(`Xvfb on ${display} exited during startup (code ${proc.exitCode}). Hint: install xvfb (apt-get install xvfb / yum install xorg-x11-server-Xvfb).`);
+    }
+  }
+  if (!ready) {
+    try { proc.kill('SIGKILL'); } catch { /* ignore */ }
+    throw new Error(`Xvfb on ${display} never became reachable within 3s timeout`);
+  }
+
+  const startTime = readPidStartTime(proc.pid);
+  return {
+    pid: proc.pid,
+    startTime,
+    display,
+    close: () => cleanupXvfb({ pid: proc.pid, startTime, display }),
+  };
+}
+
+/**
+ * Cleanup an Xvfb child if it's still ours. Validates ownership first; if
+ * the PID has been recycled or the cmdline doesn't match, leave it alone.
+ *
+ * Best-effort: never throws.
+ */
+export function cleanupXvfb(state: { pid: number; startTime: string; display: string }): void {
+  if (!state.pid) return;
+  if (!isOurXvfb(state.pid, state.startTime)) return;
+  try { safeKill(state.pid, 'SIGTERM'); } catch { /* swallow */ }
+  // Wait briefly for Xvfb to exit, then SIGKILL if still alive.
+  const deadline = Date.now() + 1000;
+  while (Date.now() < deadline) {
+    if (!isProcessAlive(state.pid)) break;
+  }
+  if (isProcessAlive(state.pid)) {
+    try { safeKill(state.pid, 'SIGKILL'); } catch { /* swallow */ }
+  }
+}
+
+/**
+ * Print a platform-specific install hint and return the message string.
+ * Used by server.ts when Xvfb isn't installed.
+ */
+export function xvfbInstallHint(): string {
+  return 'Xvfb not installed. apt-get install xvfb (Debian/Ubuntu) or yum install xorg-x11-server-Xvfb (RHEL/CentOS). Note: minimal containers (alpine, distroless) may also need fonts, dbus, gtk libs for headed Chromium to render.';
+}