terminal-agent watchdog flashes a console window every tick on Windows — isProcessAlive() spawns tasklist instead of signal-0 #1952

TL;DR

On Windows, a connected headed browse session blinks a console window
(conhost.exe) on a fixed interval for the entire session — no CLI command, no
user action. The terminal-agent watchdog calls isProcessAlive() once per
setInterval tick to check the agent PID, and on Windows isProcessAlive()
shells out to tasklist, which Windows gives its own console window.
macOS/Linux stay silent because their path is a process.kill(pid, 0) syscall
that spawns nothing. It is purely cosmetic — each window self-closes — but it
blinks on a fixed interval for the whole session with no indication of what it
is, which is confusing and distracting.

Root Cause

isProcessAlive() (browse/src/error-handling.ts) has a Windows-only branch
that spawns tasklist:

export function isProcessAlive(pid: number): boolean {
  if (IS_WINDOWS) {
    try {
      const result = Bun.spawnSync(
        ['tasklist', '/FI', `PID eq ${pid}`, '/NH', '/FO', 'CSV'],
        { stdout: 'pipe', stderr: 'pipe', timeout: 3000 }
      );
      return result.stdout.toString().includes(`"${pid}"`);
    } catch { return false; }
  }
  try { process.kill(pid, 0); return true; } catch { return false; }
}

The terminal-agent watchdog (browse/src/server.ts) registers a setInterval
that fires every AGENT_WATCHDOG_TICK_MS (default 60000) on the normal headed
connect path (ownsTerminalAgent === true) and calls isProcessAlive() on
every tick:

agentWatchdogInterval = setInterval(() => {
  if (isShuttingDown) return;
  if (agentRespawnGuardTripped) return;
  const record = readAgentRecord(stateDir);
  if (record && isProcessAlive(record.pid)) return;   // ← spawns tasklist on Windows, every tick
  // ...respawn dead agent...
}, AGENT_WATCHDOG_TICK_MS);

Each tick → Bun.spawnSync(['tasklist', …]) → a console window. The parent
watchdog two functions away already avoids exactly this: it calls
process.kill(BROWSE_PARENT_PID, 0) directly
(// signal 0 = existence check only, no signal sent). Only isProcessAlive()
takes the spawn path.

Evidence

Reproduced on Windows 11, gstack 1.57.6.0:

1. browse connect (headed), then issue no further commands.
2. A console window flashes open and immediately closes on a fixed interval,
   indefinitely, for as long as the session stays connected. Each one
   self-closes — it does not pile up — but the steady periodic blink is the
   problem.
3. The only process-spawning call on the watchdog tick is isProcessAlive() →
   tasklist; macOS shows no window for the same watchdog (its branch is
   process.kill(pid, 0)).

Proposed Fix

Use signal-0 on all platforms — what the parent watchdog already does. Node/Bun
implement process.kill(pid, 0) on Windows via OpenProcess: a pure existence
check, no child process, no window.

 export function isProcessAlive(pid: number): boolean {
-  if (IS_WINDOWS) {
-    try {
-      const result = Bun.spawnSync(
-        ['tasklist', '/FI', `PID eq ${pid}`, '/NH', '/FO', 'CSV'],
-        { stdout: 'pipe', stderr: 'pipe', timeout: 3000 }
-      );
-      return result.stdout.toString().includes(`"${pid}"`);
-    } catch {
-      return false;
-    }
-  }
-  try {
-    process.kill(pid, 0);
-    return true;
-  } catch {
-    return false;
-  }
+  try {
+    process.kill(pid, 0);
+    return true;
+  } catch (err: any) {
+    // ESRCH → gone; EPERM → exists but not signallable → still alive.
+    return err?.code === 'EPERM';
+  }
 }

Why EPERM ⇒ alive

process.kill(pid, 0) throws ESRCH when the process is gone (→ false) and
EPERM when it exists but can't be signalled (→ alive, → true). The old Unix
branch collapsed every error to false; unifying on signal-0 corrects that edge
for free.

Verification

Applied the patch and restarted the server:

• Patched isProcessAlive() to the signal-0 form in src/error-handling.ts
  and in the running dist/server-node.mjs — on Windows the server is
  launched from the bundle, not src/ (resolveNodeServerScript() in
  cli.ts), so a src/-only edit has no runtime effect until re-bundled.
• After restart, the periodic console-window flashes stopped completely —
  observed over several minutes on Windows 11.
• The watchdog's alive/dead branch is unchanged; signal-0 returns the same
  boolean it already keys on, so dead-agent detection and respawn still work.

The same isProcessAlive is also compiled into the browse CLI binary, which
spawns tasklist on every stale-detection call — the one-line fix covers it on
rebuild.

Windows 11. gstack 1.57.6.0. Bun.