fix: Do not relay a transport the bridge did not restart

jicofo-common/src/main/kotlin/org/jitsi/jicofo/metrics/IceRestartMetrics.kt

@@ -42,6 +42,17 @@ class IceRestartMetrics {
             "Number of in-place ICE restarts for which the bridge's rotated transport was signaled to the participant."
         )
 
+        /**
+         * The bridge answered with the transport it already had, because it did not need to restart (its own
+         * transport has not connected yet). Nothing was relayed and the participant keeps the session it has, so
+         * this is neither a completed restart nor a failed one.
+         */
+        @JvmField
+        val notNeeded = metricsContainer.registerCounter(
+            "ice_restarts_not_needed",
+            "Number of in-place ICE restarts for which the bridge kept its existing transport."
+        )
+
         /**
          * An ICE restart did not complete: the request was rejected (disabled, rate-limited, stale bridge-session ID)
          * or the bridge's response could not be relayed (no transport in the response, stale generation, participant

jicofo-selector/src/main/kotlin/org/jitsi/jicofo/bridge/colibri/ColibriV2SessionManager.kt

@@ -793,6 +793,11 @@ class ColibriV2SessionManager @JvmOverloads constructor(
      * if the participant restarts twice in quick succession the two responses can arrive in either order. The
      * `ice-generation` the bridge echoes tells us which round each belongs to, and anything older than what we
      * have already relayed is discarded.
+     *
+     * A transport with no `ice-generation` at all means something different: the bridge did not restart and
+     * answered with the credentials of the Agent it already has. It does this when its transport has not
+     * connected yet, where there is nothing to preserve and nothing for the participant to do. There is nothing
+     * to relay in that case, and no restart to record.
      */
     internal fun endpointIceRestarted(endpointId: String, transport: IceUdpTransportPacketExtension) {
         val generation = transport.iceGeneration
@@ -802,8 +807,12 @@ class ColibriV2SessionManager @JvmOverloads constructor(
                 IceRestartMetrics.failed.inc()
                 return
             }
-            if (generation != IceUdpTransportPacketExtension.GENERATION_UNSPECIFIED &&
-                participantInfo.lastRelayedIceGeneration != IceUdpTransportPacketExtension.GENERATION_UNSPECIFIED &&
+            if (generation == IceUdpTransportPacketExtension.GENERATION_UNSPECIFIED) {
+                logger.info("ICE restart: the bridge kept its existing transport for $endpointId, not relaying.")
+                IceRestartMetrics.notNeeded.inc()
+                return
+            }
+            if (participantInfo.lastRelayedIceGeneration != IceUdpTransportPacketExtension.GENERATION_UNSPECIFIED &&
                 generation <= participantInfo.lastRelayedIceGeneration
             ) {
                 logger.info(

jicofo-selector/src/test/kotlin/org/jitsi/jicofo/bridge/colibri/ColibriV2SessionManagerTest.kt

@@ -45,7 +45,6 @@ import org.jitsi.utils.time.FakeClock
 import org.jitsi.xmpp.extensions.colibri2.ConferenceModifyIQ
 import org.jitsi.xmpp.extensions.jingle.DtlsFingerprintPacketExtension
 import org.jitsi.xmpp.extensions.jingle.IceUdpTransportPacketExtension
-import org.jitsi.xmpp.extensions.jingle.IceUdpTransportPacketExtension.GENERATION_UNSPECIFIED
 import org.jivesoftware.smack.packet.IQ
 import org.jxmpp.jid.Jid
 import org.jxmpp.jid.impl.JidCreate
@@ -359,15 +358,18 @@ class ColibriV2SessionManagerTest : ShouldSpec() {
                 }
             }
 
-            context("With a bridge that does not tag generations") {
-                sessionManager.endpointIceRestarted("p1", transportWithGeneration(null)).also { drain() }
+            context("With a transport that carries no generation") {
+                // The bridge kept the Agent it already had instead of restarting, so there is nothing to relay.
+                sessionManager.endpointIceRestarted("p1", transportWithGeneration(2)).also { drain() }
                 sessionManager.endpointIceRestarted("p1", transportWithGeneration(null)).also { drain() }
 
-                should("relay everything (the guard can not order untagged transports)") {
-                    iceRestartedTransports.size shouldBe 2
-                    iceRestartedTransports.map {
-                        it.second.iceGeneration
-                    } shouldBe listOf(GENERATION_UNSPECIFIED, GENERATION_UNSPECIFIED)
+                should("not relay it") {
+                    iceRestartedTransports.map { it.second.iceGeneration } shouldBe listOf(2)
+                }
+                should("not lose the generation of the last transport that was relayed") {
+                    sessionManager.endpointIceRestarted("p1", transportWithGeneration(1)).also { drain() }
+
+                    iceRestartedTransports.map { it.second.iceGeneration } shouldBe listOf(2)
                 }
             }
         }