fix(windows): give the bun-polyfill spawn shim a real exited promise

Bun.spawn exposes `proc.exited` as a Promise resolving to the exit code.
The Node fallback shim (dist/bun-polyfill.cjs) returned no such field, so
every `await proc.exited` on the Windows path resolved instantly to
undefined — the Windows cookie picker (cookie-import-browser.ts races
proc.exited at three sites) read stdout before the child produced it and
silent-failed; browser-skill-commands and terminal-agent hit the same
class.

The shim now:
- drains stdout/stderr eagerly into capped in-memory buffers (Node's
  Readables are pull-based; without draining, a child writing past the
  OS pipe buffer blocks in write() and 'exit' never fires), replaying
  them as fresh single-shot Web ReadableStreams so reads work before or
  after awaiting exit;
- caps the buffer at 16 MB (GSTACK_SPAWN_MAX_BUFFER to override), still
  draining past the cap so a runaway child can't wedge or OOM;
- resolves `exited` with Bun-matching codes (exit code, 128+signal, 1 on
  spawn error) after both pipes finish, and resolves on 'error' too —
  Node fires 'error' without 'exit' when the binary is missing, which
  otherwise hangs the await forever.

Six tests pin exit codes, the read-after-exit ordering, spawn-failure
resolution, the buffer cap, and the large-output drain. Adapted to the
current test file (require path goes through the requirePath variable
from the windowsHide commit), and the 1 MB drain test's child now exits
in the write callback — on modern Node a pipe write past the OS buffer
is async and process.exit() straight after write() truncates at ~64 KB
even with a live reader, which fails the test for reasons unrelated to
the shim.

Contributed by @punksterlabs (PR #1743).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/bun-polyfill.cjs

@@ -102,11 +102,103 @@ globalThis.Bun = {
       windowsHide: options.windowsHide !== false,
     });
 
+    // Drain stdout/stderr eagerly into in-memory buffers. Bun's spawn buffers
+    // these for the consumer; Node's Readables are pull-based, so if the caller
+    // awaits `proc.exited` before reading, anything past the OS pipe buffer
+    // (~16-64 KB) back-pressures the child until it blocks in write() and
+    // `exit` never fires. Eager draining keeps the pipes flowing regardless
+    // of read order; replay below is via fresh Web ReadableStreams.
+    //
+    // Cap the buffer so a runaway child can't OOM the server. 16 MB is
+    // generous: DPAPI outputs are tiny, tasklist is <1 KB, and the
+    // browser-skill consumer has its own 1 MB readCapped. Once the cap is
+    // reached we keep draining the pipe (so the child never blocks) but
+    // discard further bytes. Override via GSTACK_SPAWN_MAX_BUFFER (bytes).
+    const MAX_BUFFER = Math.max(
+      0,
+      parseInt(process.env.GSTACK_SPAWN_MAX_BUFFER || '', 10) || 16 * 1024 * 1024,
+    );
+    const drain = (stream) => {
+      if (!stream) return { done: Promise.resolve(), chunks: [], truncated: false };
+      const state = { chunks: [], bytes: 0, truncated: false };
+      const done = new Promise((resolve) => {
+        stream.on('data', (chunk) => {
+          if (state.bytes >= MAX_BUFFER) { state.truncated = true; return; }
+          if (state.bytes + chunk.length <= MAX_BUFFER) {
+            state.chunks.push(chunk);
+            state.bytes += chunk.length;
+          } else {
+            const remaining = MAX_BUFFER - state.bytes;
+            state.chunks.push(chunk.subarray(0, remaining));
+            state.bytes = MAX_BUFFER;
+            state.truncated = true;
+          }
+        });
+        // Any terminal event resolves: 'end' on normal close, 'error' on a
+        // stream-level error, 'close' as the belt-and-suspenders for spawn
+        // failures where Node fires 'close' but neither 'end' nor 'error'.
+        stream.once('end', resolve);
+        stream.once('error', resolve);
+        stream.once('close', resolve);
+      });
+      return { done, chunks: state.chunks };
+    };
+    const stdoutDrain = drain(proc.stdout);
+    const stderrDrain = drain(proc.stderr);
+
+    // Bun's spawn exposes `proc.exited` as a Promise resolving to the exit
+    // code; several call sites — DPAPI decryption, isBrowserRunning,
+    // browser-skill-commands — `await proc.exited` directly or via
+    // Promise.race with a timeout. Without this, those awaits resolve to
+    // `undefined` immediately and the operation looks like a silent failure.
+    // Resolve only after both pipes have finished draining so consumers that
+    // read stdout AFTER awaiting exit see the full output, not a partial buffer.
+    const exited = new Promise((resolveExited) => {
+      let exitStatus;
+      proc.once('exit', (code, signal) => {
+        // Match Bun: exit code on normal exit; 128 + signal number on signal;
+        // 0 if neither was reported.
+        if (code !== null) exitStatus = code;
+        else if (signal) exitStatus = 128 + (require('os').constants.signals[signal] || 0);
+        else exitStatus = 0;
+      });
+      proc.once('error', () => {
+        if (exitStatus === undefined) exitStatus = 1;
+      });
+      // Wait for either 'exit' (normal child lifecycle) or 'error' (spawn
+      // failure — Node fires error without exit when the binary is missing).
+      // Either path resolves the lifecycle promise; without listening to both
+      // a spawn error hangs `await proc.exited` until the consumer's own
+      // timeout fires.
+      const lifecycle = new Promise((r) => {
+        proc.once('exit', r);
+        proc.once('error', r);
+      });
+      Promise.all([lifecycle, stdoutDrain.done, stderrDrain.done])
+        .then(() => resolveExited(exitStatus !== undefined ? exitStatus : 0));
+    });
+
+    // Replay buffered output as a fresh Web ReadableStream. `start()` awaits
+    // the drain before enqueueing so `new Response(proc.stdout).text()` yields
+    // the complete output regardless of whether the consumer reads before or
+    // after awaiting `proc.exited`. Stream is single-shot (locked after one
+    // read), matching Bun's behavior.
+    const replay = (d) => new ReadableStream({
+      async start(controller) {
+        await d.done;
+        for (const chunk of d.chunks) {
+          controller.enqueue(chunk instanceof Uint8Array ? chunk : new Uint8Array(chunk));
+        }
+        controller.close();
+      },
+    });
+
     return {
       pid: proc.pid,
-      stdout: proc.stdout,
-      stderr: proc.stderr,
+      stdout: replay(stdoutDrain),
+      stderr: replay(stderrDrain),
       stdin: proc.stdin,
+      exited,
       unref() { proc.unref(); },
       kill(signal) { proc.kill(signal); },
     };