From ea3fca970c850cecd311c6d7336b52e2f19acb52 Mon Sep 17 00:00:00 2001
From: Mike Hearn <hearn@google.com>
Date: Tue, 25 Dec 2012 00:28:49 +0000
Subject: [PATCH] Incorporate protocol version into download peer selection
 logic.

---
 .../java/com/google/bitcoin/core/PeerGroup.java   | 15 +++++++++++++--
 .../com/google/bitcoin/core/PeerGroupTest.java    |  8 +++++++-
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/core/src/main/java/com/google/bitcoin/core/PeerGroup.java b/core/src/main/java/com/google/bitcoin/core/PeerGroup.java
index b4a471c5..ec9e11b6 100644
--- a/core/src/main/java/com/google/bitcoin/core/PeerGroup.java
+++ b/core/src/main/java/com/google/bitcoin/core/PeerGroup.java
@@ -1110,10 +1110,21 @@ public class PeerGroup extends AbstractIdleService {
             return null;
         // Make sure we don't select a peer that is behind/synchronizing itself.
         int mostCommonChainHeight = getMostCommonChainHeight();
+        List<Peer> candidates = new ArrayList<Peer>();
         for (Peer peer : peers) {
-            if (peer.getBestHeight() == mostCommonChainHeight) return peer;
+            if (peer.getBestHeight() == mostCommonChainHeight) candidates.add(peer);
         }
-        throw new IllegalStateException("Unreachable");
+        // Of the candidates, find the highest protocol version.
+        int highestVersion = 0;
+        Peer leadingCandidate = null;
+        for (Peer peer : candidates) {
+            int v = peer.getPeerVersionMessage().clientVersion;
+            if (v > highestVersion) {
+                highestVersion = v;
+                leadingCandidate = peer;
+            }
+        }
+        return leadingCandidate;
     }
 
     private static class PeerGroupThreadFactory implements ThreadFactory {
diff --git a/core/src/test/java/com/google/bitcoin/core/PeerGroupTest.java b/core/src/test/java/com/google/bitcoin/core/PeerGroupTest.java
index 0d171c1c..342af25f 100644
--- a/core/src/test/java/com/google/bitcoin/core/PeerGroupTest.java
+++ b/core/src/test/java/com/google/bitcoin/core/PeerGroupTest.java
@@ -419,8 +419,14 @@ public class PeerGroupTest extends TestWithNetworkConnections {
         Peer c = PeerGroup.peerFromChannel(connectPeer(3, versionMessage3));
         assertEquals(2, peerGroup.getMostCommonChainHeight());
         assertEquals(a, peerGroup.getDownloadPeer());  // No change yet.
-        Peer d = PeerGroup.peerFromChannel(connectPeer(4, versionMessage3));
+        PeerGroup.peerFromChannel(connectPeer(4, versionMessage3));
         assertEquals(3, peerGroup.getMostCommonChainHeight());
         assertEquals(c, peerGroup.getDownloadPeer());  // Switch to first peer advertising new height.
+
+        // New peer with a higher protocol version but same chain height.
+        VersionMessage versionMessage4 = new VersionMessage(params, 3);
+        versionMessage4.clientVersion = 100000;
+        Peer d = PeerGroup.peerFromChannel(connectPeer(5, versionMessage4));
+        assertEquals(d, peerGroup.getDownloadPeer());
     }
 }