RealtimeSession: uplink pump + downlink audio.delta forwarding

backend/Realtime/RealtimeSession.cs

@@ -26,6 +26,16 @@ public class RealtimeSession
     private EndReason _endReason = EndReason.Unknown;
     private bool _endRequested;
 
+    private readonly System.Threading.Channels.Channel<byte[]> _uplink =
+        System.Threading.Channels.Channel.CreateBounded<byte[]>(
+            new System.Threading.Channels.BoundedChannelOptions(64)
+            {
+                FullMode = System.Threading.Channels.BoundedChannelFullMode.DropOldest,
+            });
+
+    public ValueTask WriteUplinkAsync(byte[] frame, CancellationToken ct)
+        => _uplink.Writer.WriteAsync(frame, ct);
+
     public RealtimeSession(
         Guid deviceId,
         IRealtimeUpstream upstream,
@@ -69,7 +79,10 @@ public class RealtimeSession
                 ["conversation_id"] = _conversation.Id.ToString(),
             }, ct);
 
+            var uplinkTask = Task.Run(() => PumpUplinkAsync(ct), ct);
             await PumpAsync(ct);
+            _uplink.Writer.TryComplete();
+            await uplinkTask;
         }
         catch (OperationCanceledException)
         {
@@ -102,9 +115,37 @@ public class RealtimeSession
         {
             var evt = await _upstream.ReceiveJsonAsync(ct);
             if (evt is null) { _endReason = EndReason.Error; return; }
+            var type = (string?)evt["type"];
+            switch (type)
+            {
+                case "response.audio.delta":
+                    await ForwardAudioDeltaAsync(evt, ct);
+                    break;
+            }
         }
     }
 
+    private async Task ForwardAudioDeltaAsync(JsonObject evt, CancellationToken ct)
+    {
+        var b64 = (string?)evt["delta"];
+        if (string.IsNullOrEmpty(b64)) return;
+        var bytes = Convert.FromBase64String(b64);
+        await _output.WriteBinaryAsync(bytes, ct);
+    }
+
+    private async Task PumpUplinkAsync(CancellationToken ct)
+    {
+        try
+        {
+            await foreach (var frame in _uplink.Reader.ReadAllAsync(ct))
+            {
+                await _upstream.SendJsonAsync(RealtimeEvents.InputAudioBufferAppend(frame), ct);
+            }
+        }
+        catch (OperationCanceledException) { }
+        catch (Exception ex) { _logger.LogWarning(ex, "uplink pump errored"); }
+    }
+
     private async Task<JsonObject?> WaitForUpstreamTypeAsync(string type, CancellationToken ct)
     {
         while (true)