Latency-aware fetch pack target selection

src/ripple/app/ledger/LedgerMaster.cpp

@@ -596,16 +596,23 @@ public:
             return;
         }
 
+        // Select target Peer based on highest score.
+        // The score is randomized but biased in favor of Peers with low latency.
         Peer::ptr target;
-        int count = 0;
-
-        Overlay::PeerSequence peerList = getApp().overlay ().getActivePeers ();
-        for (auto const& peer : peerList)
         {
-            if (peer->hasRange (missingIndex, missingIndex + 1))
+            int maxScore = 0;
+            Overlay::PeerSequence peerList = getApp().overlay ().getActivePeers ();
+            for (auto const& peer : peerList)
             {
-                if ((count++ == 0) || ((rand() % count) == 0))
-                    target = peer;
+                if (peer->hasRange (missingIndex, missingIndex + 1))
+                {
+                    int score = peer->getScore (true);
+                    if (! target || (score > maxScore))
+                    {
+                        target = peer;
+                        maxScore = score;
+                    }
+                }
             }
         }
 

src/ripple/overlay/Peer.h

@@ -81,6 +81,10 @@ public:
     bool
     isHighLatency() const = 0;
 
+    virtual
+    int
+    getScore (bool) const = 0;
+
     virtual
     RippleAddress const&
     getNodePublic() const = 0;

src/ripple/overlay/impl/PeerImp.cpp

@@ -2171,7 +2171,7 @@ PeerImp::peerTXData (Job&, uint256 const& hash,
 }
 
 int
-PeerImp::getScore (bool haveItem)
+PeerImp::getScore (bool haveItem) const
 {
    // Random component of score, used to break ties and avoid
    // overloading the "best" peer

src/ripple/overlay/impl/PeerImp.h

@@ -305,7 +305,7 @@ public:
 
     // Called to determine our priority for querying
     int
-    getScore (bool haveItem);
+    getScore (bool haveItem) const override;
 
     bool
     isHighLatency() const override;