ICE client: always reply to binding requests; upgrade route if better pair succeeds

Previously, binding requests from non-selected pairs were silently
discarded once a candidate pair was nominated.  There is no good reason
to ignore them -- the response is cheap and the RFC expects it -- so
remove that guard entirely.

Also allow the controlling side to upgrade to a higher-priority pair
after initial nomination.  Under packet loss the relay path can win the
nomination race over a direct path (relay responses bypass the lossy
adapter), leaving the connection on a suboptimal route.  Now, if a
higher-priority pair succeeds after selection, it is nominated and
SetSelectedCandidatePair is called again to switch to it.

On the controlled side, the USE-CANDIDATE and nomination guards are
relaxed in the same way, so they honor an upgrade request from the
controlling peer.

The response callback now always transitions pair state (Succeeded or
Failed) before the priority guard.  Previously the guard returned early
after clearing m_pPeerRequest but before updating m_nState, leaving
pairs stuck in InProgress with no active request and breaking the
invariant assumed by the binding request handler.

Author: zpostfacto

src/steamnetworkingsockets/clientlib/steamnetworkingsockets_ice_client.cpp

@@ -1705,12 +1705,6 @@ void CSteamNetworkingICESession::OnPacketReceived( const RecvPktInfo_t &info, IC
         }
     }
 
-    // Stale request on a pair we're not using? Ignore.
-    if ( m_pSelectedCandidatePair != nullptr && m_pSelectedCandidatePair != pThisPair )
-    {
-        return;
-    }
-
     //
     // Didn't find a pair?  Then maybe we can create one
     //
@@ -1770,12 +1764,16 @@ void CSteamNetworkingICESession::OnPacketReceived( const RecvPktInfo_t &info, IC
     {
         if (
             pThisPair->m_nState == kICECandidatePairState_Succeeded
-            && ( m_pSelectedCandidatePair == nullptr || m_pSelectedCandidatePair == pThisPair )
+            && ( m_pSelectedCandidatePair == nullptr
+                 || m_pSelectedCandidatePair == pThisPair
+                 || pThisPair->m_nPriority > m_pSelectedCandidatePair->m_nPriority )
         ) {
             SetSelectedCandidatePair( pThisPair );
         }
-        else if ( m_pSelectedCandidatePair == nullptr )
-        {
+        else if (
+            m_pSelectedCandidatePair == nullptr
+            || pThisPair->m_nPriority > m_pSelectedCandidatePair->m_nPriority
+        ) {
             bool bAlreadyHaveANomination = false;
             for ( ICECandidatePair *pOtherPair : m_vecCandidatePairs )
             {
@@ -2334,38 +2332,52 @@ void CSteamNetworkingICESession::STUNRequestCallback_PeerConnectivityCheck( cons
     const SteamNetworkingMicroseconds usPing = Max( SteamNetworkingMicroseconds( 1 ), info.m_usecNow - info.m_pRequest->m_usecLastSentTime );
     pPair->m_nLastRecordedPing = Max( 1, (int)( usPing / 1000 ) );
 
-    // Stale request on a pair we're not using? Ignore.
-    if ( m_pSelectedCandidatePair != nullptr && m_pSelectedCandidatePair != pPair )
+    // Always update the pair state so it is never left stuck in InProgress with no
+    // active request (which would break the invariant checked in OnPacketReceived).
+    if ( info.m_pHeader == nullptr )
     {
+        pPair->m_nState = kICECandidatePairState_Failed;
         return;
     }
+    pPair->m_nState = kICECandidatePairState_Succeeded;
 
-    if ( info.m_pHeader == nullptr )
+    // Don't nominate or upgrade to a pair with lower-or-equal priority than the
+    // current selection — it can't improve our route.
+    if ( m_pSelectedCandidatePair != nullptr && m_pSelectedCandidatePair != pPair
+         && pPair->m_nPriority <= m_pSelectedCandidatePair->m_nPriority )
     {
-        pPair->m_nState = kICECandidatePairState_Failed;
         return;
     }
-    pPair->m_nState = kICECandidatePairState_Succeeded;
+
     if ( pPair->m_bNominated )
     {
         SetSelectedCandidatePair( pPair );
     }
 	else if ( m_role == k_EICERole_Controlling )
     {
-		// Once we have a selected pair, or any nominated pair (including ones queued
-		// but not yet sent), don't nominate more.
-		bool bAlreadyHaveANomination = ( m_pSelectedCandidatePair != nullptr );
-        for ( ICECandidatePair *pOtherPair : m_vecCandidatePairs )
-        {
-            if ( pOtherPair->m_bNominated )
-                bAlreadyHaveANomination = true;
-        }
-		if ( !bAlreadyHaveANomination )
+		if ( m_pSelectedCandidatePair != nullptr
+             && pPair->m_nPriority > m_pSelectedCandidatePair->m_nPriority )
 		{
+			// Better path than current selection — nominate it to trigger an upgrade.
 			pPair->m_bNominated = true;
 			m_vecTriggeredCheckQueue.push_back( pPair );
 		}
-
+		else
+		{
+			// Once we have a selected pair, or any nominated pair (including ones queued
+			// but not yet sent), don't nominate more.
+			bool bAlreadyHaveANomination = ( m_pSelectedCandidatePair != nullptr );
+			for ( ICECandidatePair *pOtherPair : m_vecCandidatePairs )
+			{
+				if ( pOtherPair->m_bNominated )
+					bAlreadyHaveANomination = true;
+			}
+			if ( !bAlreadyHaveANomination )
+			{
+				pPair->m_bNominated = true;
+				m_vecTriggeredCheckQueue.push_back( pPair );
+			}
+		}
     }
 }