fix(whip,whep): make do-retransmission configurable to work around GStreamer RTX+hdrext crash

backend/src/blocks/builtin/whep.rs

@@ -167,6 +167,17 @@ fn explicit_track_count(properties: &HashMap<String, PropertyValue>, name: &str)
     })
 }
 
+/// Parse do_retransmission from properties (default: true).
+fn parse_do_retransmission(properties: &HashMap<String, PropertyValue>) -> bool {
+    properties
+        .get("do_retransmission")
+        .and_then(|v| match v {
+            PropertyValue::Bool(b) => Some(*b),
+            _ => None,
+        })
+        .unwrap_or(true)
+}
+
 /// Migrate a legacy `mode` property on a WHEP Output block to explicit
 /// `num_audio_tracks` / `num_video_tracks` counts, and drop `mode`.
 ///
@@ -951,6 +962,8 @@ fn build_whepserversink(
         num_audio_tracks, num_video_tracks
     );
 
+    let do_retransmission = parse_do_retransmission(properties);
+
     // Timestamp offset in milliseconds. A negative value shifts playout earlier,
     // reducing end-to-end latency for this output while maintaining A/V sync.
     // Applied as ts-offset on clocksync and appsink inside whepserversink.
@@ -1021,16 +1034,19 @@ fn build_whepserversink(
         whepserversink.set_property("turn-servers", turn_servers);
     }
 
-    // Disable FEC but keep RTX (retransmission) enabled.
+    // Disable FEC but keep RTX (retransmission) configurable (default: on).
     // - FEC adds proactive redundancy packets on every stream (~50% constant
     //   overhead, near-double bandwidth for pre-encoded high-bitrate video),
     //   so it stays off.
     // - RTX is reactive: it costs nothing while no packets are lost and only
     //   resends the exact packets the client NACKs. Without it, every loss
     //   escalates to PLI -> forced keyframe, which is far more expensive and
-    //   leaves the picture broken until the keyframe arrives.
+    //   leaves the picture broken until the keyframe arrives. It can be
+    //   disabled as a workaround for a GStreamer bug where RTX combined with
+    //   RTP header-extension aggregation can crash the process (assertion in
+    //   gst_rtp_base_depayload_handle_buffer on priv->hdrext_buffers).
     whepserversink.set_property("do-fec", false);
-    whepserversink.set_property("do-retransmission", true);
+    whepserversink.set_property("do-retransmission", do_retransmission);
 
     // Access the signaller child and set its properties
     // Bind to localhost only - axum will proxy external requests
@@ -2341,6 +2357,20 @@ fn whep_output_definition() -> BlockDefinition {
                 live: false,
                 persist: None,
             },
+            ExposedProperty {
+                name: "do_retransmission".to_string(),
+                label: "Retransmission (RTX)".to_string(),
+                description: "Resend lost packets to viewers on request (NACK-based). Disable only for diagnostics — a known GStreamer bug can crash the process when RTX is combined with RTP header-extension aggregation; without it, packet loss forces a full keyframe request instead of a cheap resend.".to_string(),
+                property_type: PropertyType::Bool,
+                default_value: Some(PropertyValue::Bool(true)),
+                mapping: PropertyMapping {
+                    element_id: "_block".to_string(),
+                    property_name: "do_retransmission".to_string(),
+                    transform: None,
+                },
+                live: false,
+                persist: None,
+            },
         ],
         // Note: external_pads here are the static defaults (1 video + 1 audio).
         // The actual pads are determined dynamically by WHEPOutputBuilder::get_external_pads()
@@ -2416,6 +2446,36 @@ mod tests {
             .collect()
     }
 
+    /// Build a property map from explicit key/value pairs, for tests of pure
+    /// parsing helpers that don't need the pad-count `props` helper above.
+    fn raw_props(entries: &[(&str, PropertyValue)]) -> HashMap<String, PropertyValue> {
+        entries
+            .iter()
+            .map(|(k, v)| (k.to_string(), v.clone()))
+            .collect()
+    }
+
+    #[test]
+    fn do_retransmission_defaults_to_true() {
+        assert!(parse_do_retransmission(&raw_props(&[])));
+    }
+
+    #[test]
+    fn do_retransmission_respects_explicit_true() {
+        assert!(parse_do_retransmission(&raw_props(&[(
+            "do_retransmission",
+            PropertyValue::Bool(true)
+        )])));
+    }
+
+    #[test]
+    fn do_retransmission_respects_explicit_false() {
+        assert!(!parse_do_retransmission(&raw_props(&[(
+            "do_retransmission",
+            PropertyValue::Bool(false)
+        )])));
+    }
+
     #[test]
     fn external_pads_default_is_one_video_and_one_audio() {
         let pads = WHEPOutputBuilder

backend/src/blocks/builtin/whip.rs

@@ -139,6 +139,17 @@ impl BlockBuilder for WHIPInputBuilder {
 // WHIP Input (whipserversrc - hosts WHIP server)
 // ============================================================================
 
+/// Parse do_retransmission from properties (default: true).
+fn parse_do_retransmission(properties: &HashMap<String, PropertyValue>) -> bool {
+    properties
+        .get("do_retransmission")
+        .and_then(|v| match v {
+            PropertyValue::Bool(b) => Some(*b),
+            _ => None,
+        })
+        .unwrap_or(true)
+}
+
 /// Build WHIP Input per-slot output chains.
 ///
 /// At build time, per-slot chains are created in the main pipeline:
@@ -181,6 +192,8 @@ fn build_whipserversrc(
         })
         .unwrap_or(true);
 
+    let do_retransmission = parse_do_retransmission(properties);
+
     let max_video_bitrate_kbps = properties
         .get("max_video_bitrate")
         .and_then(|v| match v {
@@ -415,8 +428,8 @@ fn build_whipserversrc(
     let turn_server = ctx.turn_server();
 
     info!(
-        "WHIP Input configured: endpoint_id='{}', stun={:?}, turn={:?}, mode={:?}, decode={}, max_sessions={} (whipserversrc created per-session)",
-        endpoint_id, stun_server, turn_server, mode, decode, max_sessions
+        "WHIP Input configured: endpoint_id='{}', stun={:?}, turn={:?}, mode={:?}, decode={}, do_retransmission={}, max_sessions={} (whipserversrc created per-session)",
+        endpoint_id, stun_server, turn_server, mode, decode, do_retransmission, max_sessions
     );
 
     // Register WHIP endpoint with the build context (port=0 placeholder, sessions get their own ports)
@@ -436,6 +449,7 @@ fn build_whipserversrc(
             ice_transport_policy: ctx.ice_transport_policy().to_string(),
             pipeline_weak: gst::glib::WeakRef::new(),
             decode,
+            do_retransmission,
             dynamic_webrtcbin_store: ctx.dynamic_webrtcbin_store(),
             max_video_bitrate_kbps,
             max_sessions,
@@ -511,6 +525,8 @@ pub fn create_whipserversrc_for_session(
     let signaller = whipserversrc.property::<gst::glib::Object>("signaller");
     signaller.set_property("host-addr", &host_addr);
 
+    whipserversrc.set_property("do-retransmission", config.do_retransmission);
+
     // Configure codec negotiation based on mode
     if config.mode.has_audio() {
         let audio_codecs = gst::Array::new(["OPUS"]);
@@ -1444,6 +1460,20 @@ fn whip_input_definition() -> BlockDefinition {
                 live: false,
                 persist: None,
             },
+            ExposedProperty {
+                name: "do_retransmission".to_string(),
+                label: "Retransmission (RTX)".to_string(),
+                description: "Request retransmission of lost packets from the publisher (NACK-based). Disable only for diagnostics; without it, any packet loss forces a full keyframe request instead of a cheap resend.".to_string(),
+                property_type: PropertyType::Bool,
+                default_value: Some(PropertyValue::Bool(true)),
+                mapping: PropertyMapping {
+                    element_id: "_block".to_string(),
+                    property_name: "do_retransmission".to_string(),
+                    transform: None,
+                },
+                live: false,
+                persist: None,
+            },
             ExposedProperty {
                 name: "max_video_bitrate".to_string(),
                 label: "Max Video Bitrate (kbps)".to_string(),
@@ -1625,3 +1655,36 @@ fn setup_incoming_rtp_handler(
 
     info!("WHIP: Incoming RTP handler installed for {}", instance_id);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn props(entries: &[(&str, PropertyValue)]) -> HashMap<String, PropertyValue> {
+        entries
+            .iter()
+            .map(|(k, v)| (k.to_string(), v.clone()))
+            .collect()
+    }
+
+    #[test]
+    fn do_retransmission_defaults_to_true() {
+        assert!(parse_do_retransmission(&props(&[])));
+    }
+
+    #[test]
+    fn do_retransmission_respects_explicit_true() {
+        assert!(parse_do_retransmission(&props(&[(
+            "do_retransmission",
+            PropertyValue::Bool(true)
+        )])));
+    }
+
+    #[test]
+    fn do_retransmission_respects_explicit_false() {
+        assert!(!parse_do_retransmission(&props(&[(
+            "do_retransmission",
+            PropertyValue::Bool(false)
+        )])));
+    }
+}

backend/src/whip_session_manager.rs

@@ -32,6 +32,9 @@ pub struct WhipEndpointConfig {
     pub pipeline_weak: gst::glib::WeakRef<gst::Pipeline>,
     /// Whether to decode RTP to raw media (true) or pass through RTP (false)
     pub decode: bool,
+    /// Whether whipserversrc should request retransmission (NACK) of lost
+    /// packets from the publisher. Matches upstream default (true).
+    pub do_retransmission: bool,
     /// Shared dynamic webrtcbin store for ICE policy tracking
     pub dynamic_webrtcbin_store: DynamicWebrtcbinStore,
     /// Maximum video bitrate hint for Chrome (kbps). Injected into the SDP