fix: implement correct sdp offer/answer exchange and ice handling

internal/api/preview.go

@@ -89,34 +89,103 @@ func (api *API) previewHandler(c echo.Context) error {
 	}
 	defer conn.Close()
 
+	// --- WebRTC Signal Handling ---
+
+	// on-negotiation-needed: Triggered when webrtcbin needs to create an offer or answer.
+	// In our case (server receiving an offer), this fires after the remote description is set.
 	previewPipeline.webrtcbin.Connect("on-negotiation-needed", func(self *gst.Element) {
-		log.Debug().Msg("🧠 Negotiation needed (ignored)")
+		log.Info().Msg("🧠 Negotiation needed, creating answer...")
+
+		// Create Answer
+		promise, err := self.Emit("create-answer")
+		if err != nil {
+			log.Error().Err(err).Msg("Failed to emit create-answer")
+			return
+		}
+		if promise == nil {
+			log.Error().Msg("Emit create-answer returned nil promise")
+			return
+		}
+
+		// Handle the promise result (asynchronously)
+		promise.(*gst.Promise).Interrupt() // Interrupt any previous waits
+		promise.(*gst.Promise).Wait()      // Wait for the answer to be created
+		reply := promise.(*gst.Promise).GetReply()
+		if reply == nil {
+			log.Error().Msg("Promise reply for create-answer was nil")
+			return
+		}
+
+		answerValue, ok := reply.GetValue("answer")
+		if !ok || answerValue == nil {
+			log.Error().Msg("Failed to get answer from promise reply")
+			return
+		}
+		answer, ok := answerValue.(*gst.WebRTCSessionDescription)
+		if !ok || answer == nil {
+			log.Error().Msg("Answer value is not a WebRTCSessionDescription")
+			return
+		}
+
+		log.Debug().Str("sdp", answer.GetSDP().String()).Msg("✅ Answer created")
+
+		// Set Local Description
+		promise, err = self.Emit("set-local-description", answer)
+		if err != nil {
+			log.Error().Err(err).Msg("Failed to emit set-local-description")
+			return
+		}
+		if promise == nil {
+			log.Error().Msg("Emit set-local-description returned nil promise")
+			return
+		}
+		promise.(*gst.Promise).Interrupt() // Interrupt any previous waits
+
+		log.Info().Msg("➡️ Set local description (answer)")
+
+		// Send Answer back to the client
+		log.Info().Msg("📨 Sending SDP answer back to browser")
+		response := SignalMessage{
+			Type: "answer",
+			SDP:  answer.GetSDP().String(),
+		}
+		msg, err := json.Marshal(response)
+		if err != nil {
+			log.Error().Err(err).Msg("Failed to marshal answer")
+			return
+		}
+		if err := conn.WriteMessage(websocket.TextMessage, msg); err != nil {
+			log.Error().Err(err).Msg("Failed to send answer over WebSocket")
+		}
 	})
 
-	previewPipeline.webrtcbin.Connect("on-ice-candidate", func(self *gst.Element, mlineindex int, candidate string) {
+	// on-ice-candidate: Triggered when webrtcbin generates a new ICE candidate.
+	previewPipeline.webrtcbin.Connect("on-ice-candidate", func(self *gst.Element, mlineindex uint, candidate string) {
+		log.Debug().Uint("mlineindex", mlineindex).Str("candidate", candidate).Msg("🧊 Generated ICE candidate")
+		// Note: The 'sdpMid' is often derived from the mlineindex or context,
+		// but the signal itself only provides mlineindex and candidate string directly.
+		// The JS side seems to handle this structure correctly.
 		ice := SignalMessage{
 			Type: "ice",
 			ICE: &IceCandidate{
 				Candidate:     candidate,
-				SDPMLineIndex: uint16(mlineindex),
+				SDPMLineIndex: uint16(mlineindex), // Assuming JS expects uint16
+				// sdpMid might need to be derived if required, but often index is enough
 			},
 		}
-		msg, _ := json.Marshal(ice)
+		msg, err := json.Marshal(ice)
-		conn.WriteMessage(websocket.TextMessage, msg)
+		if err != nil {
-	})
+			log.Error().Err(err).Msg("Failed to marshal ICE candidate")
-
+			return
-	previewPipeline.webrtcbin.Connect("on-sdp-answer", func(self *gst.Element, answer string) {
+		}
-		log.Info().Msg("📨 Sending SDP answer back to browser")
+		if err := conn.WriteMessage(websocket.TextMessage, msg); err != nil {
-		response := SignalMessage{
+			log.Error().Err(err).Msg("Failed to send ICE candidate over WebSocket")
-			Type: "answer",
-			SDP:  answer,
 		}
-		msg, _ := json.Marshal(response)
-		conn.WriteMessage(websocket.TextMessage, msg)
 	})
 
+	// --- WebSocket Message Loop ---
 	for {
-		_, message, err := conn.ReadMessage()
+		messageType, message, err := conn.ReadMessage()
 		if err != nil {
 			log.Error().Err(err).Msg("WebSocket read error")
 			break
@@ -124,22 +193,57 @@ func (api *API) previewHandler(c echo.Context) error {
 		var signal SignalMessage
 		if err := json.Unmarshal(message, &signal); err != nil {
 			log.Warn().Err(err).Msg("Failed to unmarshal signal message")
+			log.Warn().Err(err).Str("message", string(message)).Msg("Failed to unmarshal signal message")
 			continue
 		}
 
-		if signal.Type == "offer" {
+		switch signal.Type {
+		case "offer":
 			log.Info().Msg("📥 Received SDP offer from browser")
-			// Pass the SDP offer to webrtcbin
+			log.Debug().Str("sdp", signal.SDP).Msg("Offer SDP")
-			previewPipeline.webrtcbin.Emit("set-remote-description", "offer", signal.SDP)
 
-			// Create an answer
+			// Create WebRTCSessionDescription for the offer
-			previewPipeline.webrtcbin.Emit("create-answer")
+			offerDesc, err := gst.NewWebRTCSessionDescription(gst.WebRTCSDPTypeOffer, gst.NewSDPMessageFromString(signal.SDP))
-			log.Debug().Msg("➡️ Sent SDP offer into webrtcbin and requested answer")
+			if err != nil {
-		}
+				log.Error().Err(err).Msg("Failed to create offer description from SDP")
+				continue
+			}
 
-		if signal.Type == "ice" && signal.ICE != nil {
+			// Set Remote Description
-			log.Debug().Str("candidate", signal.ICE.Candidate).Msg("➕ Adding ICE candidate")
+			// This will trigger on-negotiation-needed if successful and state allows
-			previewPipeline.webrtcbin.Emit("add-ice-candidate", signal.ICE.SDPMLineIndex, signal.ICE.Candidate)
+			promise, err := previewPipeline.webrtcbin.Emit("set-remote-description", offerDesc)
+			if err != nil {
+				log.Error().Err(err).Msg("Failed to emit set-remote-description")
+				continue
+			}
+			if promise == nil {
+				log.Error().Msg("Emit set-remote-description returned nil promise")
+				continue
+			}
+			promise.(*gst.Promise).Interrupt() // Interrupt previous waits if any
+
+			log.Info().Msg("➡️ Set remote description (offer)")
+			// Answer creation is now handled in on-negotiation-needed
+
+		case "ice":
+			if signal.ICE == nil {
+				log.Warn().Msg("Received ICE signal with nil ICE field")
+				continue
+			}
+			log.Debug().Str("candidate", signal.ICE.Candidate).Uint16("mlineindex", signal.ICE.SDPMLineIndex).Str("sdpMid", signal.ICE.SDPMid).Msg("➕ Received ICE candidate from browser")
+
+			// Add ICE Candidate
+			// Note: The signal takes mlineindex (uint) and candidate (string).
+			// sdpMid is usually associated but not directly passed here.
+			_, err := previewPipeline.webrtcbin.Emit("add-ice-candidate", uint(signal.ICE.SDPMLineIndex), signal.ICE.Candidate)
+			if err != nil {
+				log.Error().Err(err).Msg("Failed to emit add-ice-candidate")
+			} else {
+				log.Debug().Msg("Added ICE candidate")
+			}
+
+		default:
+			log.Warn().Str("type", signal.Type).Msg("Received unknown signal type")
 		}
 	}