check for empty in disconnect reason bytes (#6938)

Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>

ethereum/p2p/src/main/java/org/hyperledger/besu/ethereum/p2p/network/PeerDenylistManager.java

@@ -52,7 +52,7 @@ public class PeerDenylistManager implements DisconnectCallback {
       final boolean initiatedByPeer) {
     // we have a number of reasons that use the same code, but with different message strings
     // so here we use the code of the reason param to ensure we get the no-message version
-    if (shouldBlock(DisconnectReason.forCode(reason.getValue().get(0)), initiatedByPeer)) {
+    if (shouldBlock(DisconnectReason.forCode(reason.getValue()), initiatedByPeer)) {
       if (maintainedPeers.contains(connection.getPeer())) {
         LOG.debug(
             "Skip adding maintained peer {} to peer denylist for reason {}",

ethereum/p2p/src/main/java/org/hyperledger/besu/ethereum/p2p/rlpx/wire/messages/DisconnectMessage.java

@@ -174,6 +174,14 @@ public final class DisconnectMessage extends AbstractMessageData {
       return BY_ID[code];
     }
 
+    public static DisconnectReason forCode(final Bytes codeBytes) {
+      if (codeBytes == null || codeBytes.isEmpty()) {
+        return UNKNOWN;
+      } else {
+        return forCode(codeBytes.get(0));
+      }
+    }
+
     DisconnectReason(final Byte code) {
       this.code = Optional.ofNullable(code);
       this.message = Optional.empty();

ethereum/p2p/src/test/java/org/hyperledger/besu/ethereum/p2p/network/PeerDenylistManagerTest.java

@@ -111,6 +111,15 @@ public class PeerDenylistManagerTest {
     checkPermissions(denylist, peer.getPeer(), false);
   }
 
+  @Test
+  public void disconnectReasonWithEmptyValue_doesNotAddToDenylist() {
+    final PeerConnection peer = generatePeerConnection();
+
+    checkPermissions(denylist, peer.getPeer(), true);
+    peerDenylistManager.onDisconnect(peer, DisconnectReason.UNKNOWN, false);
+    checkPermissions(denylist, peer.getPeer(), true);
+  }
+
   private void checkPermissions(
       final PeerPermissionsDenylist denylist, final Peer remotePeer, final boolean expectedResult) {
     for (PeerPermissions.Action action : PeerPermissions.Action.values()) {