Try to fix background glitching over long periods of time

tests/VoiceCat.Tests/AudioEngineTests.cs

@@ -446,6 +446,22 @@ public class AudioEngineTests
         Assert.Contains("public void Resynchronize()", buffer);
     }
 
+    [Fact]
+    public void ResynchronizeInputsDropsStalledBacklogToTheBufferTarget()
+    {
+        using var send = new AudioEngine((_, _, _, _) => true, false);
+        send.DeviceBufferMilliseconds = 40; send.AddLocalStream(Stream());
+        short[] mono = Tone();
+        for (int i = 0; i < 40; i++) send.FeedPcm(1, mono, 1);
+        Assert.True(send.GetLocalDiagnostics(1).BufferedFrames > 1920 * 2);
+
+        send.ResynchronizeInputs();
+
+        Assert.Equal(1920, send.GetLocalDiagnostics(1).BufferedFrames);
+        send.ProcessCycle();
+        Assert.Equal(0, send.GetLocalDiagnostics(1).StarvedCycles);
+    }
+
     [Fact]
     public void RemotePlaybackSettingsAreIndependentForEachUserStream()
     {

tests/VoiceCat.Tests/PublishServerScriptTests.cs

@@ -164,6 +164,38 @@ public class PublishServerScriptTests
         Assert.Contains("deviceClockedAudio: true", model);
     }
 
+    [Fact]
+    public async Task IosRecoversBackgroundAudioWithoutForegrounding()
+    {
+        string root = FindRoot();
+        string microphone = await File.ReadAllTextAsync(Path.Combine(
+            root, "clients", "apple", "VoiceCat.iOS", "IosAudioEngine.cs"));
+        string router = await File.ReadAllTextAsync(Path.Combine(
+            root, "clients", "apple", "VoiceCat.iOS", "IosAudioRouter.cs"));
+
+        // The render callback is the only clock for the device-clocked pipeline, so a graph that
+        // stops while backgrounded froze capture, mix and send until the app was foregrounded.
+        Assert.Contains("internal long RenderCallbacks", microphone);
+        Assert.Contains("internal bool IsRunning", microphone);
+        Assert.Contains("private void TickWatchdog()", router);
+        Assert.Contains("IosAudioEngine.Shared.RenderCallbacks", router);
+        Assert.Contains("IosAudioEngine.Shared.IsRunning", router);
+        Assert.Contains("IosAudioEngine.Shared.Reconfigure()", router);
+
+        // SetActive fails for as long as an interruption is in force, so Began suppresses
+        // recovery and the watchdog retries the rebuild that Ended asks for.
+        Assert.Contains("AVAudioSessionInterruptionType.Began", router);
+        Assert.Contains("interrupted = true", router);
+        Assert.Contains("internal void ResumeForeground()", router);
+
+        // A stalled render callback resynchronizes its own rings, and a refill is bounded so it
+        // cannot overrun the callback deadline that it is recovering from.
+        Assert.Contains("owner.Audio.ResynchronizeInputs()", microphone);
+        Assert.Contains("playbackRing.Resynchronize()", microphone);
+        Assert.Contains("Ring.Resynchronize()", microphone);
+        Assert.Contains("Math.Min(frames + playbackRing.TargetFrames - playbackRing.CountFrames, frames + 960)", microphone);
+    }
+
     [Fact]
     public async Task IosAudioCommitsInputRouteBeforeChannelsAndVoiceProcessingBeforeConnections()
     {