fix(audio): use monotonic time for the JS queue

bend2/effs/audio_open.js

@@ -6,7 +6,7 @@ function audio_open(rate) {
   if (rate < 8000 || rate > 192000) {
     return io_fail(22);
   }
-  return io_done({ rate, queued: 0, at: Date.now() });
+  return io_done({ rate, queued: 0, at: performance.now() });
 }
 
 io_eff(CID(Audio.open), audio_open);

bend2/effs/audio_write.js

@@ -6,7 +6,7 @@ function audio_write(audio, samples) {
   for (let xs = samples; xs.$ === CID(Con); xs = xs.tail) {
     n += 1;
   }
-  const now = Date.now();
+  const now = performance.now();
   audio.queued = Math.max(0,
     audio.queued - (now - audio.at) * audio.rate / 1000);
   audio.at = now;

tests/io/audio_monotonic_clock.bend

@@ -0,0 +1,22 @@
+# A backward wall clock must not add frames to the silent audio ring.
+# JS only: the native ring drains by the device, without a wall clock.
+import Base
+
+def wall_clock_backwards(reset: Bool) -> IO(Unit):
+  import "./audio_monotonic_clock.js"
+
+def report(pair: Audio & U32) -> IO(Unit):
+  (audio, queued) = pair
+  do IO<Unit>:
+    Unit <- Audio.close(audio)
+    IO.print(U32.show(queued))
+
+def main() -> IO(Unit):
+  do IO<Unit>:
+    Unit <- wall_clock_backwards(False{})
+    a : Audio <- IO.try(Audio, Audio.open(48000))
+    pair : Audio & U32 <- Audio.write(a, [])
+    Unit <- wall_clock_backwards(True{})
+    report(pair)
+
+#|0

tests/io/audio_monotonic_clock.js

@@ -0,0 +1,20 @@
+// One second backwards between opening the ring and its empty write.
+const wall_now = Date.now;
+
+function wall_clock_backwards(reset) {
+  if (reset) {
+    Date.now = wall_now;
+  } else {
+    let first = true;
+    Date.now = () => {
+      if (first) {
+        first = false;
+        return 1000;
+      }
+      return 0;
+    };
+  }
+  return { $: CID(Unit) };
+}
+
+io_eff(CID(wall_clock_backwards), wall_clock_backwards);