fix(spa): wake the download worker before handing it a stream

Reported from Chrome: a download started while an upload was running took
thirty seconds to begin, every time. The console named it exactly —

    /_mbdl/mtty5btz-sbmgdegx  404 ()
    [MeshBay] the worker did not answer the download within 15s (attempt 1)

A 404 from the hub means the request reached the *network*: the worker looked,
found no entry for that id and let it through. So the worker was alive and
controlling the page, and the message handing it the stream had simply never
been processed.

`pending` lives in the worker's memory, and a worker with nothing to do is
terminated within tens of seconds. A WebRTC upload gives it no events at all,
so minutes of uploading leave it dead; the stream posted to it is lost,
silently, and the iframe then wakes it with nothing to find.

`mbdl-ping` already existed for this exact reason -- sent every ten seconds
*while* writing, because a streaming response does not count as activity.
Nothing sent one before *starting*. So a download now wakes the worker and waits
for the pong, and `sw.js` answers `mbdl-ready` once it has actually stored the
entry, which the page waits for before navigating: confirmed rather than
assumed. A worker that predates the ack sends nothing and the page navigates
anyway, which is what it did before.

This cause was measured and wrongly dismissed hours earlier, with an idle probe
that made the worker work between its own attempts -- it never actually slept.
A measurement that does not reproduce the conditions refutes nothing. The
harness now models a worker that is asleep: a ping wakes it, and anything else
posted while it sleeps is lost, which is what made the failure silent.

`test_backpressure_is_real` read the first `worker.postMessage` in the function
to check that the readable half is transferred rather than copied. The wake-up
put a ping in front of it, so it began inspecting a call that carries only a
port -- and kept passing. It now checks every post, each bounded by its own
call, since the keep-alive ping transfers nothing at all. Same shape as the
upload-seal contract this morning: a guard that reads "the first" stops
guarding the moment something is inserted before it.

Hub suite 851 passed. Both new cases checked against the unfixed source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

1 files changed, 59 insertions, 2 deletions

diff --git a/packages/meshbay-hub/tests/test_streamed_download_reliability.py b/packages/meshbay-hub/tests/test_streamed_download_reliability.py
index bfd4a89..e1b3800 100644
--- a/packages/meshbay-hub/tests/test_streamed_download_reliability.py
+++ b/packages/meshbay-hub/tests/test_streamed_download_reliability.py
@@ -50,15 +50,37 @@ globalThis.localStorage = {
 };
 const PLAN = %(plan)s;
 const log = { registers: 0, claims: 0, navigations: 0, served: 0,
-              unregisters: 0 };
+              unregisters: 0, wakes: 0 };
 
 // The worker as the page sees it: something with postMessage. It answers a
 // navigation by posting mbdl-serving back on the port it was handed, which is
 // exactly the confirmation the real sw.js sends from its fetch handler.
 let controller = null;
 const pendingByFrame = new Map();
+// Set before the controller exists, because the declaration below is what
+// the temporal dead zone protects.
+let asleep = PLAN.workerAsleep;
 const makeController = () => ({
   postMessage: (msg, transfer) => {
+    // A worker with nothing to do is terminated, and `pending` goes with it.
+    // A ping wakes it; anything else posted while it sleeps is simply lost,
+    // which is what makes this failure silent.
+    if (asleep) {
+      if (msg.type === 'mbdl-ping') {
+        asleep = false;
+        log.wakes += 1;
+        if (msg.ports || (transfer && transfer[0])) {
+          const port = (transfer && transfer[0]) || null;
+          if (port) setTimeout(() => port.postMessage({type: 'mbdl-pong'}), 0);
+        }
+      }
+      return;
+    }
+    if (msg.type === 'mbdl-ping') {
+      const port = (transfer && transfer[0]) || null;
+      if (port) setTimeout(() => port.postMessage({type: 'mbdl-pong'}), 0);
+      return;
+    }
     if (msg.type === 'mbdl-claim') {
       log.claims += 1;
       // A worker that actually claims when asked, which is what sw.js does.
@@ -70,6 +92,9 @@ const makeController = () => ({
     }
     if (msg.type !== 'mbdl') return;
     pendingByFrame.set('/_mbdl/' + msg.id, msg.port);
+    // The worker says it has it, which is what the page waits for.
+    if (msg.port) setTimeout(() => msg.port.postMessage({type: 'mbdl-ready',
+                                                         id: msg.id}), 0);
   },
 });
 
@@ -167,7 +192,8 @@ def _run(tmp_path, body, *, control_after_ms=0, active=True,
          serve="always", register_throws=False, control_budget_ms=800,
          ready_settles=True, register_hangs=False,
          active_after_unregister=False, control_on_claim=False,
-         controlled_at_load=False, registered_at_load=False):
+         controlled_at_load=False, registered_at_load=False,
+         worker_asleep=False):
     module = tmp_path / "downloads.mjs"
     module.write_text(DOWNLOADS.read_text())
     (tmp_path / "package.json").write_text('{"type":"module"}')
@@ -181,6 +207,7 @@ def _run(tmp_path, body, *, control_after_ms=0, active=True,
         "activeAfterUnregister": active_after_unregister,
         "controlOnClaim": control_on_claim,
         "registeredAtLoad": registered_at_load,
+        "workerAsleep": worker_asleep,
     }
     script = tmp_path / "case.mjs"
     script.write_text(
@@ -530,3 +557,33 @@ def test_the_streamed_target_says_it_cannot_be_paused(tmp_path):
       if (target) await target.writable.close();
     """)
     assert out["pausable"] is False
+
+
+# ── a worker that was asleep when we posted ─────────────────────────────────
+
+def test_a_sleeping_worker_is_woken_before_it_is_handed_a_stream(tmp_path):
+    """Reported from Chrome: a download started while an upload was running took
+    thirty seconds to begin, every time.
+
+    `pending` lives in the worker's memory and a worker with nothing to do is
+    terminated — which is what a long upload leaves it, for minutes, since a
+    WebRTC transfer gives it no events at all. The stream posted to it was lost;
+    the iframe then woke it with nothing to find and the request fell through to
+    the network, measured in the console as a 404 from the hub and fifteen
+    seconds of silence, twice.
+
+    `mbdl-ping` already existed — sent every ten seconds *while* writing, for
+    the same reason. Nothing sent one before *starting*.
+    """
+    out = _run(tmp_path, """
+      const target = await M.openStreamedDownload('film.mkv', 20e9, FAST);
+      out.target = target !== null;
+      out.wakes = log.wakes;
+      out.navigations = log.navigations;
+      if (target) await target.writable.close();
+    """, worker_asleep=True)
+    assert out["target"] is True, "the download never started"
+    assert out["wakes"] == 1, "the worker was handed a stream while asleep"
+    assert out["navigations"] == 1, (
+        f"took {out['navigations']} attempts — the first one was wasted on a "
+        "worker that had not been woken")