Stabilize iOS reconnect and screen audio sessions

clients/apple/VoiceCat.iOS/IosAudioEngine.cs

@@ -88,6 +88,7 @@ internal sealed class IosAudioEngine
         // right hardware. Rebuilding it there would release an HFP headset and pay a Bluetooth
         // profile renegotiation for a transport blip that changed no audio configuration.
         if (engine?.Running == true) { playbackRing.Resynchronize(); return; }
+        if (ResumeStoppedGraph()) return;
         Volatile.Write(ref microphone, null); Rebuild();
     }
 
@@ -134,6 +135,7 @@ internal sealed class IosAudioEngine
         int channels = IosAudioRouter.Shared.CaptureChannels;
         if (!force && engine?.Running == true && tapInstalled && builtCaptureChannels == channels
             && voiceProcessing == IosAudioRouter.Shared.UsesVoiceProcessing && !HardwareChanged()) return;
+        if (!force && ResumeStoppedGraph()) return;
         if (current is not null)
         {
             // Stop the old tap before publishing a route with a different sample width. Otherwise
@@ -155,9 +157,30 @@ internal sealed class IosAudioEngine
         // Still starting: report it as running rather than replacing it, since that is what it is
         // about to be, and a rebuild here would start the cycle over.
         if (Settling) return true;
+        if (ResumeStoppedGraph()) return true;
         Rebuild(); return engine?.Running == true;
     }
 
+    private bool ResumeStoppedGraph()
+    {
+        if (engine is not { } paused || !tapInstalled ||
+            builtCaptureChannels != IosAudioRouter.Shared.CaptureChannels ||
+            voiceProcessing != IosAudioRouter.Shared.UsesVoiceProcessing || HardwareChanged()) return false;
+        IosAudioRouter.Shared.Apply(true);
+        bool started = paused.StartAndReturnError(out NSError? error);
+        if (started)
+        {
+            playbackRing.Resynchronize();
+            // Voice-processing IO may still be starting when Start returns. Give the existing
+            // graph its settling window before a watchdog judges it, just as a fresh graph gets.
+            Volatile.Write(ref lastRebuildAt, Environment.TickCount64);
+            Console.Error.WriteLine($"VC_RESUME running={paused.Running}");
+            return true;
+        }
+        Console.Error.WriteLine($"VC_RESUME running={paused.Running} error={error?.LocalizedDescription ?? "none"}");
+        return false;
+    }
+
     private void Rebuild([System.Runtime.CompilerServices.CallerMemberName] string cause = "")
     {
         Console.Error.WriteLine($"VC_REBUILD cause={cause} running={engine?.Running == true} callbacks={RenderCallbacks}");
@@ -182,7 +205,7 @@ internal sealed class IosAudioEngine
             // formats are queried or nodes are connected so the graph is built from the final IO.
             input = next.InputNode;
             if (!input.SetVoiceProcessingEnabled(IosAudioRouter.Shared.UsesVoiceProcessing, out NSError? processingError))
-                throw new InvalidOperationException(processingError.LocalizedDescription);
+                throw new InvalidOperationException(processingError?.LocalizedDescription ?? "Could not configure voice processing.");
             if (IosAudioRouter.Shared.UsesVoiceProcessing) input.VoiceProcessingAgcEnabled = IosAudioRouter.Shared.AutomaticGainControl;
         }
         outputFormat = new(AVAudioCommonFormat.PCMFloat32, 48_000, 2, false);
@@ -195,6 +218,8 @@ internal sealed class IosAudioEngine
         if (captures)
         {
             AVAudioFormat inputFormat = input!.GetBusOutputFormat(0);
+            if (inputFormat.SampleRate <= 0 || inputFormat.ChannelCount == 0)
+                throw new InvalidOperationException("No usable microphone input is available on the current audio route.");
             Console.Error.WriteLine($"VC_GRAPH vpio={IosAudioRouter.Shared.UsesVoiceProcessing} requestedCh={captureChannels} " +
                 $"inputCh={inputFormat.ChannelCount} inputRate={inputFormat.SampleRate}");
             builtInputRate = inputFormat.SampleRate; builtInputChannels = inputFormat.ChannelCount;
@@ -231,15 +256,15 @@ internal sealed class IosAudioEngine
             captureSink.OutputVolume = 0f;
         }
         next.Prepare();
-        if (!next.StartAndReturnError(out NSError? error)) { next.Dispose(); throw new InvalidOperationException(error.LocalizedDescription); }
+        if (!next.StartAndReturnError(out NSError? error)) { next.Dispose(); throw new InvalidOperationException(error?.LocalizedDescription ?? "The iOS audio graph could not start."); }
         engine = next;
         {
             AVAudioSession live = AVAudioSession.SharedInstance();
             Console.Error.WriteLine($"VC_START running={next.Running} builtCh={builtInputChannels} sessionRate={live.SampleRate} " +
                 $"sessionInCh={live.InputNumberOfChannels} sessionOutCh={live.OutputNumberOfChannels} inputAvailable={live.InputAvailable} " +
                 $"other={live.OtherAudioPlaying} mode={live.Mode} options={live.CategoryOptions} io={live.IOBufferDuration:F4} " +
-                $"out={string.Join(',', live.CurrentRoute.Outputs.Select(value => value.PortType.ToString()))} " +
-                $"in={string.Join(',', live.CurrentRoute.Inputs.Select(value => value.PortType.ToString()))}");
+                $"out={string.Join(',', live.CurrentRoute?.Outputs?.Select(value => value.PortType.ToString()) ?? [])} " +
+                $"in={string.Join(',', live.CurrentRoute?.Inputs?.Select(value => value.PortType.ToString()) ?? [])}");
             // A graph that starts and then stops on its own is the failure that matters, and it is
             // invisible at start: check again once the IO has had time to come up.
             AVAudioEngine started = next;
@@ -380,7 +405,7 @@ internal sealed class IosAudioEngine
     {
         IsConnected = false; Volatile.Write(ref microphone, null);
         if (client is { } owner) owner.Audio.MixedPcm -= ReceiveMixedPcm;
-        DestroyGraph(); client = null; IosAudioRouter.Shared.Deactivate();
+        engine?.Pause(); client = null; IosAudioRouter.Shared.Deactivate();
     }
 
     private void DestroyGraph()