From 8d983649d3f339799a70a2d67bf69b37597c51d4 Mon Sep 17 00:00:00 2001
From: Andreas Schildbach <andreas.schildbach@bloq.com>
Date: Thu, 27 Feb 2020 20:21:21 +0100
Subject: [PATCH] PeerGroup: Don't immediately select a download peer in
 startBlockChainDownload(). Wait for enough peers to be connected.

---
 core/src/main/java/org/bitcoinj/core/PeerGroup.java   | 11 +++--------
 .../test/java/org/bitcoinj/core/PeerGroupTest.java    |  5 +++--
 2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/core/src/main/java/org/bitcoinj/core/PeerGroup.java b/core/src/main/java/org/bitcoinj/core/PeerGroup.java
index c62a91c3c..fc83f60a1 100644
--- a/core/src/main/java/org/bitcoinj/core/PeerGroup.java
+++ b/core/src/main/java/org/bitcoinj/core/PeerGroup.java
@@ -1394,7 +1394,7 @@ public class PeerGroup implements TransactionBroadcaster {
     }
 
     /**
-     * <p>Start downloading the blockchain from the first available peer.</p>
+     * <p>Start downloading the blockchain.</p>
      *
      * <p>If no peers are currently connected, the download will be started once a peer starts.  If the peer dies,
      * the download will resume with another peer.</p>
@@ -1413,12 +1413,6 @@ public class PeerGroup implements TransactionBroadcaster {
                 }
             }
             this.downloadListener = listener;
-            // TODO: be more nuanced about which peer to download from.  We can also try
-            // downloading from multiple peers and handle the case when a new peer comes along
-            // with a longer chain after we thought we were done.
-            if (!peers.isEmpty()) {
-                startBlockChainDownloadFromPeer(peers.iterator().next()); // Will add the new download listener
-            }
         } finally {
             lock.unlock();
         }
@@ -1859,7 +1853,8 @@ public class PeerGroup implements TransactionBroadcaster {
     }
     @Nullable private ChainDownloadSpeedCalculator chainDownloadSpeedCalculator;
 
-    private void startBlockChainDownloadFromPeer(Peer peer) {
+    @VisibleForTesting
+    void startBlockChainDownloadFromPeer(Peer peer) {
         lock.lock();
         try {
             setDownloadPeer(peer);
diff --git a/core/src/test/java/org/bitcoinj/core/PeerGroupTest.java b/core/src/test/java/org/bitcoinj/core/PeerGroupTest.java
index 9b429cf82..a0e4d7b4c 100644
--- a/core/src/test/java/org/bitcoinj/core/PeerGroupTest.java
+++ b/core/src/test/java/org/bitcoinj/core/PeerGroupTest.java
@@ -326,8 +326,8 @@ public class PeerGroupTest extends TestWithPeerGroup {
         Block b3 = FakeTxBuilder.makeSolvedTestBlock(b2);
 
         // Expect a zero hash getblocks on p1. This is how the process starts.
-        peerGroup.startBlockChainDownload(new AbstractPeerDataEventListener() {
-        });
+        peerGroup.startBlockChainDownload(new AbstractPeerDataEventListener() {});
+        peerGroup.startBlockChainDownloadFromPeer(peerGroup.getConnectedPeers().iterator().next());
         GetBlocksMessage getblocks = (GetBlocksMessage) outbound(p1);
         assertEquals(Sha256Hash.ZERO_HASH, getblocks.getStopHash());
         // We give back an inv with some blocks in it.
@@ -804,6 +804,7 @@ public class PeerGroupTest extends TestWithPeerGroup {
         assertTrue(p1.lastReceivedFilter.contains(keys.get(5).getPubKeyHash()));
         assertFalse(p1.lastReceivedFilter.contains(keys.get(keys.size() - 1).getPubKey()));
         peerGroup.startBlockChainDownload(null);
+        peerGroup.startBlockChainDownloadFromPeer(peerGroup.getConnectedPeers().iterator().next());
         assertNextMessageIs(p1, GetBlocksMessage.class);
 
         // Make some transactions and blocks that send money to the wallet thus using up all the keys.