From 74d54de1f6ec58c76e3a97df11df592be0a4f616 Mon Sep 17 00:00:00 2001
From: Andreas Schildbach <andreas.schildbach@bloq.com>
Date: Mon, 21 Nov 2022 15:39:24 +0100
Subject: [PATCH] AddressEventListener: new event produced by Peer if an `addr`
 or `addrv2` message is received

---
 .../src/main/java/org/bitcoinj/core/Peer.java | 24 +++++++++++--
 .../core/listeners/AddressEventListener.java  | 34 +++++++++++++++++++
 2 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 core/src/main/java/org/bitcoinj/core/listeners/AddressEventListener.java

diff --git a/core/src/main/java/org/bitcoinj/core/Peer.java b/core/src/main/java/org/bitcoinj/core/Peer.java
index 77d52f9f1..42b275a18 100644
--- a/core/src/main/java/org/bitcoinj/core/Peer.java
+++ b/core/src/main/java/org/bitcoinj/core/Peer.java
@@ -23,6 +23,7 @@ import com.google.common.base.Throwables;
 import net.jcip.annotations.GuardedBy;
 import org.bitcoinj.base.Coin;
 import org.bitcoinj.base.Sha256Hash;
+import org.bitcoinj.core.listeners.AddressEventListener;
 import org.bitcoinj.core.listeners.BlocksDownloadedEventListener;
 import org.bitcoinj.core.listeners.ChainDownloadStartedEventListener;
 import org.bitcoinj.core.listeners.GetDataEventListener;
@@ -101,6 +102,8 @@ public class Peer extends PeerSocketHandler {
         = new CopyOnWriteArrayList<>();
     private final CopyOnWriteArrayList<ListenerRegistration<OnTransactionBroadcastListener>> onTransactionEventListeners
         = new CopyOnWriteArrayList<>();
+    private final CopyOnWriteArrayList<ListenerRegistration<AddressEventListener>> addressEventListeners
+        = new CopyOnWriteArrayList<>();
     // Whether to try and download blocks and transactions from this peer. Set to false by PeerGroup if not the
     // primary peer. This is to avoid redundant work and concurrency problems with downloading the same chain
     // in parallel.
@@ -331,6 +334,16 @@ public class Peer extends PeerSocketHandler {
         preMessageReceivedEventListeners.add(new ListenerRegistration<>(listener, executor));
     }
 
+    /** Registers a listener that is called when addr or addrv2 messages are received. */
+    public void addAddressEventListener(AddressEventListener listener) {
+        addAddressEventListener(Threading.USER_THREAD, listener);
+    }
+
+    /** Registers a listener that is called when addr or addrv2 messages are received. */
+    public void addAddressEventListener(Executor executor, AddressEventListener listener) {
+        addressEventListeners.add(new ListenerRegistration<>(listener, executor));
+    }
+
     public boolean removeBlocksDownloadedEventListener(BlocksDownloadedEventListener listener) {
         return ListenerRegistration.removeFromList(listener, blocksDownloadedEventListeners);
     }
@@ -359,6 +372,10 @@ public class Peer extends PeerSocketHandler {
         return ListenerRegistration.removeFromList(listener, preMessageReceivedEventListeners);
     }
 
+    public boolean removeAddressEventListener(AddressEventListener listener) {
+        return ListenerRegistration.removeFromList(listener, addressEventListeners);
+    }
+
     @Override
     public String toString() {
         final MoreObjects.ToStringHelper helper = MoreObjects.toStringHelper(this).omitNullValues();
@@ -485,14 +502,17 @@ public class Peer extends PeerSocketHandler {
         }
     }
 
-    private void processAddressMessage(AddressMessage m) {
+    private void processAddressMessage(AddressMessage message) {
+        for (final ListenerRegistration<AddressEventListener> registration : addressEventListeners) {
+            registration.executor.execute(() -> registration.listener.onAddr(Peer.this, message));
+        }
         CompletableFuture<AddressMessage> future;
         synchronized (getAddrFutures) {
             future = getAddrFutures.poll();
             if (future == null)  // Not an addr message we are waiting for.
                 return;
         }
-        future.complete(m);
+        future.complete(message);
     }
 
     private void processVersionMessage(VersionMessage peerVersionMessage) throws ProtocolException {
diff --git a/core/src/main/java/org/bitcoinj/core/listeners/AddressEventListener.java b/core/src/main/java/org/bitcoinj/core/listeners/AddressEventListener.java
new file mode 100644
index 000000000..7aa94094f
--- /dev/null
+++ b/core/src/main/java/org/bitcoinj/core/listeners/AddressEventListener.java
@@ -0,0 +1,34 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.bitcoinj.core.listeners;
+
+import org.bitcoinj.core.AddressMessage;
+import org.bitcoinj.core.Peer;
+
+/**
+ * <p>Implementors can listen to addresses being received from remote peers.</p>
+ */
+public interface AddressEventListener {
+
+    /**
+     * <p>Called when a peer receives an addr or addrv2 message, usually in response to a getaddr message.</p>
+     *
+     * @param peer    the peer that received the addr or addrv2 message
+     * @param message the addr or addrv2 message that was received
+     */
+    void onAddr(Peer peer, AddressMessage message);
+}