Keep track of peers which are too divergent, and return an `isTooDivergent` boolean in /peers APIs.

isTooDivergent will be true or false if a definitive decision has been made, or missing from the response if not yet known. Therefore it should be safe to treat `"isTooDivergent": false` as a peer that is on the same chain.

src/main/java/org/qortal/api/model/ConnectedPeer.java

@@ -1,6 +1,7 @@
 package org.qortal.api.model;
 
 import io.swagger.v3.oas.annotations.media.Schema;
+import org.qortal.controller.Controller;
 import org.qortal.data.block.BlockSummaryData;
 import org.qortal.data.network.PeerData;
 import org.qortal.network.Handshake;
@@ -36,6 +37,7 @@ public class ConnectedPeer {
     public Long lastBlockTimestamp;
     public UUID connectionId;
     public String age;
+    public Boolean isTooDivergent;
 
     protected ConnectedPeer() {
     }
@@ -69,6 +71,11 @@ public class ConnectedPeer {
             this.lastBlockSignature = peerChainTipData.getSignature();
             this.lastBlockTimestamp = peerChainTipData.getTimestamp();
         }
+
+        // Only include isTooDivergent decision if we've had the opportunity to request block summaries this peer
+        if (peer.getLastTooDivergentTime() != null) {
+            this.isTooDivergent = Controller.wasRecentlyTooDivergent.test(peer);
+        }
     }
 
 }

src/main/java/org/qortal/controller/Controller.java

@@ -769,6 +769,16 @@ public class Controller extends Thread {
 		}
 	};
 
+	public static final Predicate<Peer> wasRecentlyTooDivergent = peer -> {
+		Long now = NTP.getTime();
+		Long peerLastTooDivergentTime = peer.getLastTooDivergentTime();
+		if (now == null || peerLastTooDivergentTime == null)
+			return false;
+
+		// Exclude any peers that were TOO_DIVERGENT in the last 5 mins
+		return (now - peerLastTooDivergentTime < 5 * 60 * 1000L);
+	};
+
 	private long getRandomRepositoryMaintenanceInterval() {
 		final long minInterval = Settings.getInstance().getRepositoryMaintenanceMinInterval();
 		final long maxInterval = Settings.getInstance().getRepositoryMaintenanceMaxInterval();

src/main/java/org/qortal/controller/Synchronizer.java

@@ -1121,6 +1121,7 @@ public class Synchronizer extends Thread {
 			// If common block is too far behind us then we're on massively different forks so give up.
 			if (!force && testHeight < ourHeight - MAXIMUM_COMMON_DELTA) {
 				LOGGER.info(String.format("Blockchain too divergent with peer %s", peer));
+				peer.setLastTooDivergentTime(NTP.getTime());
 				return SynchronizationResult.TOO_DIVERGENT;
 			}
 
@@ -1130,6 +1131,9 @@ public class Synchronizer extends Thread {
 			testHeight = Math.max(testHeight - step, 1);
 		}
 
+		// Peer not considered too divergent
+		peer.setLastTooDivergentTime(0L);
+
 		// Prepend test block's summary as first block summary, as summaries returned are *after* test block
 		BlockSummaryData testBlockSummary = new BlockSummaryData(testBlockData);
 		blockSummariesFromCommon.add(0, testBlockSummary);

src/main/java/org/qortal/network/Peer.java

@@ -155,6 +155,11 @@ public class Peer {
      */
     private CommonBlockData commonBlockData;
 
+    /**
+     * Last time we detected this peer as TOO_DIVERGENT
+     */
+    private Long lastTooDivergentTime;
+
     // Message stats
 
     private static class MessageStats {
@@ -383,6 +388,14 @@ public class Peer {
         this.commonBlockData = commonBlockData;
     }
 
+    public Long getLastTooDivergentTime() {
+        return this.lastTooDivergentTime;
+    }
+
+    public void setLastTooDivergentTime(Long lastTooDivergentTime) {
+        this.lastTooDivergentTime = lastTooDivergentTime;
+    }
+
     public boolean isSyncInProgress() {
         return this.syncInProgress;
     }