MAXIMUM_RETRIES set to 2, as 3 retries may have been slightly too many.

If a peer fails to reply with all requested blocks, we will now only apply the blocks we have received so far if at least one of them is recent. This should prevent or greatly reduce the scenario where our chain is taken from a recent to an outdated state due to only partially syncing with a peer. It is best to keep our chain "recent" if possible, as this ensures that the peer selection code always runs, and therefore avoids unnecessarily syncing to a random peer on an inferior chain.

WindowsInstaller/Qortal.aip

@@ -19,10 +19,10 @@
     <ROW Property="Manufacturer" Value="Qortal"/>
     <ROW Property="MsiLogging" MultiBuildValue="DefaultBuild:vp"/>
     <ROW Property="NTP_GOOD" Value="false"/>
-    <ROW Property="ProductCode" Value="1033:{9EA6B58D-641E-442E-8F16-5D35B92B9F9B} 1049:{B16722F6-C2FA-418D-A9DA-69707FE2034B} 2052:{459AC873-98DC-43AC-8787-B23BAF976FF5} 2057:{CD923B63-65A0-4F2A-93B6-6362AAC8608E} " Type="16"/>
+    <ROW Property="ProductCode" Value="1033:{DE0C17EF-F9D2-48A3-A0D7-FF7BB01698A2} 1049:{89129011-9CE7-44AD-97EC-152371A87D58} 2052:{C93D5260-C6BD-4341-A96D-7D878EE6D59B} 2057:{AE6EDB44-3120-4588-8C9A-558F1BE42D73} " Type="16"/>
     <ROW Property="ProductLanguage" Value="2057"/>
     <ROW Property="ProductName" Value="Qortal"/>
-    <ROW Property="ProductVersion" Value="1.4.2" Type="32"/>
+    <ROW Property="ProductVersion" Value="1.4.5" Type="32"/>
     <ROW Property="RECONFIG_NTP" Value="true"/>
     <ROW Property="REMOVE_BLOCKCHAIN" Value="YES" Type="4"/>
     <ROW Property="REPAIR_BLOCKCHAIN" Value="YES" Type="4"/>
@@ -174,7 +174,7 @@
     <ROW Component="ADDITIONAL_LICENSE_INFO_97" ComponentId="{D5544706-E2A7-424F-AEA5-3963E355AA29}" Directory_="jdk.crypto.mscapi_Dir" Attributes="0" KeyPath="ADDITIONAL_LICENSE_INFO_97" Type="0"/>
     <ROW Component="ADDITIONAL_LICENSE_INFO_98" ComponentId="{104DBCE8-A458-4B3E-9EFA-2D8613561619}" Directory_="jdk.dynalink_Dir" Attributes="0" KeyPath="ADDITIONAL_LICENSE_INFO_98" Type="0"/>
     <ROW Component="ADDITIONAL_LICENSE_INFO_99" ComponentId="{D02E3C37-E81A-48FA-9E28-B26B728AECD9}" Directory_="jdk.httpserver_Dir" Attributes="0" KeyPath="ADDITIONAL_LICENSE_INFO_99" Type="0"/>
-    <ROW Component="AI_CustomARPName" ComponentId="{F9375D31-26C0-4E23-948D-3570B43B7FA2}" Directory_="APPDIR" Attributes="260" KeyPath="DisplayName" Options="1"/>
+    <ROW Component="AI_CustomARPName" ComponentId="{CFB6FB43-13A6-4A4F-BCF9-7698463CE032}" Directory_="APPDIR" Attributes="260" KeyPath="DisplayName" Options="1"/>
     <ROW Component="AI_ExePath" ComponentId="{3644948D-AE0B-41BB-9FAF-A79E70490A08}" Directory_="APPDIR" Attributes="260" KeyPath="AI_ExePath"/>
     <ROW Component="APPDIR" ComponentId="{680DFDDE-3FB4-47A5-8FF5-934F576C6F91}" Directory_="APPDIR" Attributes="0"/>
     <ROW Component="DATA_PATH" ComponentId="{EE0B6107-E244-4CDB-B195-E9038D2F1E0E}" Directory_="DATA_PATH" Attributes="0"/>

pom.xml

@@ -3,7 +3,7 @@
 	<modelVersion>4.0.0</modelVersion>
 	<groupId>org.qortal</groupId>
 	<artifactId>qortal</artifactId>
-	<version>1.4.3</version>
+	<version>1.4.6</version>
 	<packaging>jar</packaging>
 	<properties>
 		<skipTests>true</skipTests>

src/main/java/org/qortal/api/resource/AdminResource.java

@@ -547,7 +547,7 @@ public class AdminResource {
 			blockchainLock.lockInterruptibly();
 
 			try {
-				repository.exportNodeLocalData();
+				repository.exportNodeLocalData(true);
 				return "true";
 			} finally {
 				blockchainLock.unlock();

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

@@ -135,16 +135,19 @@ public class BlockMinter extends Thread {
 				// Disregard peers that have "misbehaved" recently
 				peers.removeIf(Controller.hasMisbehaved);
 
-				// Disregard peers that don't have a recent block
-				peers.removeIf(Controller.hasNoRecentBlock);
+				// Disregard peers that don't have a recent block, but only if we're not in recovery mode.
+				// In that mode, we want to allow minting on top of older blocks, to recover stalled networks.
+				if (Controller.getInstance().getRecoveryMode() == false)
+					peers.removeIf(Controller.hasNoRecentBlock);
 
 				// Don't mint if we don't have enough up-to-date peers as where would the transactions/consensus come from?
 				if (peers.size() < Settings.getInstance().getMinBlockchainPeers())
 					continue;
 
-				// If our latest block isn't recent then we need to synchronize instead of minting.
+				// If our latest block isn't recent then we need to synchronize instead of minting, unless we're in recovery mode.
 				if (!peers.isEmpty() && lastBlockData.getTimestamp() < minLatestBlockTimestamp)
-					continue;
+					if (Controller.getInstance().getRecoveryMode() == false)
+						continue;
 
 				// There are enough peers with a recent block and our latest block is recent
 				// so go ahead and mint a block if possible.
@@ -165,6 +168,14 @@ public class BlockMinter extends Thread {
 				// Do we need to build any potential new blocks?
 				List<PrivateKeyAccount> newBlocksMintingAccounts = mintingAccountsData.stream().map(accountData -> new PrivateKeyAccount(repository, accountData.getPrivateKey())).collect(Collectors.toList());
 
+				// We might need to sit the next block out, if one of our minting accounts signed the previous one
+				final byte[] previousBlockMinter = previousBlockData.getMinterPublicKey();
+				final boolean mintedLastBlock = mintingAccountsData.stream().anyMatch(mintingAccount -> Arrays.equals(mintingAccount.getPublicKey(), previousBlockMinter));
+				if (mintedLastBlock) {
+					LOGGER.trace(String.format("One of our keys signed the last block, so we won't sign the next one"));
+					continue;
+				}
+
 				for (PrivateKeyAccount mintingAccount : newBlocksMintingAccounts) {
 					// First block does the AT heavy-lifting
 					if (newBlocks.isEmpty()) {
@@ -282,15 +293,17 @@ public class BlockMinter extends Thread {
 						RewardShareData rewardShareData = repository.getAccountRepository().getRewardShare(newBlock.getBlockData().getMinterPublicKey());
 
 						if (rewardShareData != null) {
-							LOGGER.info(String.format("Minted block %d, sig %.8s by %s on behalf of %s",
+							LOGGER.info(String.format("Minted block %d, sig %.8s, parent sig: %.8s by %s on behalf of %s",
 									newBlock.getBlockData().getHeight(),
 									Base58.encode(newBlock.getBlockData().getSignature()),
+									Base58.encode(newBlock.getParent().getSignature()),
 									rewardShareData.getMinter(),
 									rewardShareData.getRecipient()));
 						} else {
-							LOGGER.info(String.format("Minted block %d, sig %.8s by %s",
+							LOGGER.info(String.format("Minted block %d, sig %.8s, parent sig: %.8s by %s",
 									newBlock.getBlockData().getHeight(),
 									Base58.encode(newBlock.getBlockData().getSignature()),
+									Base58.encode(newBlock.getParent().getSignature()),
 									newBlock.getMinter().getAddress()));
 						}
 

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

@@ -67,8 +67,8 @@ import org.qortal.gui.SysTray;
 import org.qortal.network.Network;
 import org.qortal.network.Peer;
 import org.qortal.network.message.ArbitraryDataMessage;
-import org.qortal.network.message.BlockMessage;
 import org.qortal.network.message.BlockSummariesMessage;
+import org.qortal.network.message.CachedBlockMessage;
 import org.qortal.network.message.GetArbitraryDataMessage;
 import org.qortal.network.message.GetBlockMessage;
 import org.qortal.network.message.GetBlockSummariesMessage;
@@ -121,6 +121,7 @@ public class Controller extends Thread {
 	private static final long NTP_PRE_SYNC_CHECK_PERIOD = 5 * 1000L; // ms
 	private static final long NTP_POST_SYNC_CHECK_PERIOD = 5 * 60 * 1000L; // ms
 	private static final long DELETE_EXPIRED_INTERVAL = 5 * 60 * 1000L; // ms
+	private static final long RECOVERY_MODE_TIMEOUT = 10 * 60 * 1000L; // ms
 
 	// To do with online accounts list
 	private static final long ONLINE_ACCOUNTS_TASKS_INTERVAL = 10 * 1000L; // ms
@@ -148,9 +149,9 @@ public class Controller extends Thread {
 
 	/** Cache of BlockMessages, indexed by block signature */
 	@SuppressWarnings("serial")
-	private final LinkedHashMap<ByteArray, BlockMessage> blockMessageCache = new LinkedHashMap<>() {
+	private final LinkedHashMap<ByteArray, CachedBlockMessage> blockMessageCache = new LinkedHashMap<>() {
 		@Override
-		protected boolean removeEldestEntry(Map.Entry<ByteArray, BlockMessage> eldest) {
+		protected boolean removeEldestEntry(Map.Entry<ByteArray, CachedBlockMessage> eldest) {
 			return this.size() > Settings.getInstance().getBlockCacheSize();
 		}
 	};
@@ -175,6 +176,11 @@ public class Controller extends Thread {
 	/** Latest block signatures from other peers that we know are on inferior chains. */
 	List<ByteArray> inferiorChainSignatures = new ArrayList<>();
 
+	/** Recovery mode, which is used to bring back a stalled network */
+	private boolean recoveryMode = false;
+	private boolean peersAvailable = true; // peersAvailable must default to true
+	private long timePeersLastAvailable = 0;
+
 	/**
 	 * Map of recent requests for ARBITRARY transaction data payloads.
 	 * <p>
@@ -358,6 +364,10 @@ public class Controller extends Thread {
 		}
 	}
 
+	public boolean getRecoveryMode() {
+		return this.recoveryMode;
+	}
+
 	// Entry point
 
 	public static void main(String[] args) {
@@ -629,6 +639,13 @@ public class Controller extends Thread {
 		// Disregard peers that don't have a recent block
 		peers.removeIf(hasNoRecentBlock);
 
+		checkRecoveryModeForPeers(peers);
+		if (recoveryMode) {
+			peers = Network.getInstance().getHandshakedPeers();
+			peers.removeIf(hasOnlyGenesisBlock);
+			peers.removeIf(hasMisbehaved);
+		}
+
 		// Check we have enough peers to potentially synchronize
 		if (peers.size() < Settings.getInstance().getMinBlockchainPeers())
 			return;
@@ -639,9 +656,31 @@ public class Controller extends Thread {
 		// Disregard peers that are on the same block as last sync attempt and we didn't like their chain
 		peers.removeIf(hasInferiorChainTip);
 
+		final int peersBeforeComparison = peers.size();
+
+		// Request recent block summaries from the remaining peers, and locate our common block with each
+		Synchronizer.getInstance().findCommonBlocksWithPeers(peers);
+
+		// Compare the peers against each other, and against our chain, which will return an updated list excluding those without common blocks
+		peers = Synchronizer.getInstance().comparePeers(peers);
+
+		// We may have added more inferior chain tips when comparing peers, so remove any peers that are currently on those chains
+		peers.removeIf(hasInferiorChainTip);
+
+		final int peersRemoved = peersBeforeComparison - peers.size();
+		if (peersRemoved > 0)
+			LOGGER.info(String.format("Ignoring %d peers on inferior chains. Peers remaining: %d", peersRemoved, peers.size()));
+
 		if (peers.isEmpty())
 			return;
 
+		if (peers.size() > 1) {
+			StringBuilder finalPeersString = new StringBuilder();
+			for (Peer peer : peers)
+				finalPeersString = finalPeersString.length() > 0 ? finalPeersString.append(", ").append(peer) : finalPeersString.append(peer);
+			LOGGER.info(String.format("Choosing random peer from: [%s]", finalPeersString.toString()));
+		}
+
 		// Pick random peer to sync with
 		int index = new SecureRandom().nextInt(peers.size());
 		Peer peer = peers.get(index);
@@ -744,6 +783,46 @@ public class Controller extends Thread {
 		}
 	}
 
+	private boolean checkRecoveryModeForPeers(List<Peer> qualifiedPeers) {
+		List<Peer> handshakedPeers = Network.getInstance().getHandshakedPeers();
+
+		if (handshakedPeers.size() > 0) {
+			// There is at least one handshaked peer
+			if (qualifiedPeers.isEmpty()) {
+				// There are no 'qualified' peers - i.e. peers that have a recent block we can sync to
+				boolean werePeersAvailable = peersAvailable;
+				peersAvailable = false;
+
+				// If peers only just became unavailable, update our record of the time they were last available
+				if (werePeersAvailable)
+					timePeersLastAvailable = NTP.getTime();
+
+				// If enough time has passed, enter recovery mode, which lifts some restrictions on who we can sync with and when we can mint
+				if (NTP.getTime() - timePeersLastAvailable > RECOVERY_MODE_TIMEOUT) {
+					if (recoveryMode == false) {
+						LOGGER.info(String.format("Peers have been unavailable for %d minutes. Entering recovery mode...", RECOVERY_MODE_TIMEOUT/60/1000));
+						recoveryMode = true;
+					}
+				}
+			} else {
+				// We now have at least one peer with a recent block, so we can exit recovery mode and sync normally
+				peersAvailable = true;
+				if (recoveryMode) {
+					LOGGER.info("Peers have become available again. Exiting recovery mode...");
+					recoveryMode = false;
+				}
+			}
+		}
+		return recoveryMode;
+	}
+
+	public void addInferiorChainSignature(byte[] inferiorSignature) {
+		// Update our list of inferior chain tips
+		ByteArray inferiorChainSignature = new ByteArray(inferiorSignature);
+		if (!inferiorChainSignatures.contains(inferiorChainSignature))
+			inferiorChainSignatures.add(inferiorChainSignature);
+	}
+
 	public static class StatusChangeEvent implements Event {
 		public StatusChangeEvent() {
 		}
@@ -775,7 +854,7 @@ public class Controller extends Thread {
 				actionText = Translator.INSTANCE.translate("SysTray", "MINTING_DISABLED");
 		}
 
-		String tooltip = String.format("%s - %d %s - %s %d", actionText, numberOfPeers, connectionsText, heightText, height);
+		String tooltip = String.format("%s - %d %s - %s %d", actionText, numberOfPeers, connectionsText, heightText, height) + "\n" + String.format("Build version: %s", this.buildVersion);
 		SysTray.getInstance().setToolTipText(tooltip);
 
 		this.callbackExecutor.execute(() -> {
@@ -1151,7 +1230,7 @@ public class Controller extends Thread {
 
 		ByteArray signatureAsByteArray = new ByteArray(signature);
 
-		BlockMessage cachedBlockMessage = this.blockMessageCache.get(signatureAsByteArray);
+		CachedBlockMessage cachedBlockMessage = this.blockMessageCache.get(signatureAsByteArray);
 		int blockCacheSize = Settings.getInstance().getBlockCacheSize();
 
 		// Check cached latest block message
@@ -1159,7 +1238,7 @@ public class Controller extends Thread {
 			this.stats.getBlockMessageStats.cacheHits.incrementAndGet();
 
 			// We need to duplicate it to prevent multiple threads setting ID on the same message
-			BlockMessage clonedBlockMessage = cachedBlockMessage.cloneWithNewId(message.getId());
+			CachedBlockMessage clonedBlockMessage = cachedBlockMessage.cloneWithNewId(message.getId());
 
 			if (!peer.sendMessage(clonedBlockMessage))
 				peer.disconnect("failed to send block");
@@ -1187,12 +1266,15 @@ public class Controller extends Thread {
 
 			Block block = new Block(repository, blockData);
 
-			BlockMessage blockMessage = new BlockMessage(block);
+			CachedBlockMessage blockMessage = new CachedBlockMessage(block);
 			blockMessage.setId(message.getId());
 
 			// This call also causes the other needed data to be pulled in from repository
-			if (!peer.sendMessage(blockMessage))
+			if (!peer.sendMessage(blockMessage)) {
 				peer.disconnect("failed to send block");
+				// Don't fall-through to caching because failure to send might be from failure to build message
+				return;
+			}
 
 			// If request is for a recent block, cache it
 			if (getChainHeight() - blockData.getHeight() <= blockCacheSize) {
@@ -1209,6 +1291,18 @@ public class Controller extends Thread {
 		TransactionMessage transactionMessage = (TransactionMessage) message;
 		TransactionData transactionData = transactionMessage.getTransactionData();
 
+		/*
+		 *  If we can't obtain blockchain lock immediately,
+		 *  e.g. Synchronizer is active, or another transaction is taking a while to validate,
+		 *  then we're using up a network thread for ages and clogging things up
+		 *  so bail out early
+		 */
+		ReentrantLock blockchainLock = Controller.getInstance().getBlockchainLock();
+		if (!blockchainLock.tryLock()) {
+			LOGGER.trace(() -> String.format("Too busy to import %s transaction %s from peer %s", transactionData.getType().name(), Base58.encode(transactionData.getSignature()), peer));
+			return;
+		}
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
 			Transaction transaction = Transaction.fromData(repository, transactionData);
 
@@ -1238,6 +1332,8 @@ public class Controller extends Thread {
 			LOGGER.debug(() -> String.format("Imported %s transaction %s from peer %s", transactionData.getType().name(), Base58.encode(transactionData.getSignature()), peer));
 		} catch (DataException e) {
 			LOGGER.error(String.format("Repository issue while processing transaction %s from peer %s", Base58.encode(transactionData.getSignature()), peer), e);
+		} finally {
+			blockchainLock.unlock();
 		}
 	}
 

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

@@ -8,6 +8,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
+import java.util.Iterator;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -17,6 +18,7 @@ import org.qortal.block.Block;
 import org.qortal.block.Block.ValidationResult;
 import org.qortal.data.block.BlockData;
 import org.qortal.data.block.BlockSummaryData;
+import org.qortal.data.block.CommonBlockData;
 import org.qortal.data.network.PeerChainTipData;
 import org.qortal.data.transaction.RewardShareTransactionData;
 import org.qortal.data.transaction.TransactionData;
@@ -39,10 +41,23 @@ public class Synchronizer {
 
 	private static final Logger LOGGER = LogManager.getLogger(Synchronizer.class);
 
+	/** Max number of new blocks we aim to add to chain tip in each sync round */
+	private static final int SYNC_BATCH_SIZE = 200; // XXX move to Settings?
+
+	/** Initial jump back of block height when searching for common block with peer */
 	private static final int INITIAL_BLOCK_STEP = 8;
-	private static final int MAXIMUM_BLOCK_STEP = 500;
+	/** Maximum jump back of block height when searching for common block with peer */
+	private static final int MAXIMUM_BLOCK_STEP = 128;
+
+	/** Maximum difference in block height between tip and peer's common block before peer is considered TOO DIVERGENT */
 	private static final int MAXIMUM_COMMON_DELTA = 240; // XXX move to Settings?
-	private static final int SYNC_BATCH_SIZE = 200;
+
+	/** Maximum number of block signatures we ask from peer in one go */
+	private static final int MAXIMUM_REQUEST_SIZE = 200; // XXX move to Settings?
+
+	/** Number of retry attempts if a peer fails to respond with the requested data */
+	private static final int MAXIMUM_RETRIES = 2; // XXX move to Settings?
+
 
 	private static Synchronizer instance;
 
@@ -62,6 +77,368 @@ public class Synchronizer {
 		return instance;
 	}
 
+
+	/**
+	 * Iterate through a list of supplied peers, and attempt to find our common block with each.
+	 * If a common block is found, its summary will be retained in the peer's commonBlockSummary property, for processing later.
+	 * <p>
+	 * Will return <tt>SynchronizationResult.OK</tt> on success.
+	 * <p>
+	 * @param peers
+	 * @return SynchronizationResult.OK if the process completed successfully, or a different SynchronizationResult if something went wrong.
+	 * @throws InterruptedException
+	 */
+	public SynchronizationResult findCommonBlocksWithPeers(List<Peer> peers) throws InterruptedException {
+		try (final Repository repository = RepositoryManager.getRepository()) {
+			try {
+
+				if (peers.size() == 0)
+					return SynchronizationResult.NOTHING_TO_DO;
+
+				// If our latest block is very old, it's best that we don't try and determine the best peers to sync to.
+				// This is because it can involve very large chain comparisons, which is too intensive.
+				// In reality, most forking problems occur near the chain tips, so we will reserve this functionality for those situations.
+				final Long minLatestBlockTimestamp = Controller.getMinimumLatestBlockTimestamp();
+				if (minLatestBlockTimestamp == null)
+					return SynchronizationResult.REPOSITORY_ISSUE;
+
+				final BlockData ourLatestBlockData = repository.getBlockRepository().getLastBlock();
+				if (ourLatestBlockData.getTimestamp() < minLatestBlockTimestamp) {
+					LOGGER.debug(String.format("Our latest block is very old, so we won't collect common block info from peers"));
+					return SynchronizationResult.NOTHING_TO_DO;
+				}
+
+				LOGGER.debug(String.format("Searching for common blocks with %d peers...", peers.size()));
+				final long startTime = System.currentTimeMillis();
+				int commonBlocksFound = 0;
+
+				for (Peer peer : peers) {
+					// Are we shutting down?
+					if (Controller.isStopping())
+						return SynchronizationResult.SHUTTING_DOWN;
+
+					// Check if we can use the cached common block data, by comparing the peer's current chain tip against the peer's chain tip when we last found our common block
+					if (peer.canUseCachedCommonBlockData()) {
+						LOGGER.debug(String.format("Skipping peer %s because we already have the latest common block data in our cache. Cached common block sig is %.08s", peer, Base58.encode(peer.getCommonBlockData().getCommonBlockSummary().getSignature())));
+						continue;
+					}
+
+					// Cached data is stale, so clear it and repopulate
+					peer.setCommonBlockData(null);
+
+					// Search for the common block
+					Synchronizer.getInstance().findCommonBlockWithPeer(peer, repository);
+					if (peer.getCommonBlockData() != null)
+						commonBlocksFound++;
+				}
+
+				final long totalTimeTaken = System.currentTimeMillis() - startTime;
+				LOGGER.info(String.format("Finished searching for common blocks with %d peer%s. Found: %d. Total time taken: %d ms", peers.size(), (peers.size() != 1 ? "s" : ""), commonBlocksFound, totalTimeTaken));
+
+				return SynchronizationResult.OK;
+			} finally {
+				repository.discardChanges(); // Free repository locks, if any, also in case anything went wrong
+			}
+		} catch (DataException e) {
+			LOGGER.error("Repository issue during synchronization with peer", e);
+			return SynchronizationResult.REPOSITORY_ISSUE;
+		}
+	}
+
+	/**
+	 * Attempt to find the find our common block with supplied peer.
+	 * If a common block is found, its summary will be retained in the peer's commonBlockSummary property, for processing later.
+	 * <p>
+	 * Will return <tt>SynchronizationResult.OK</tt> on success.
+	 * <p>
+	 * @param peer
+	 * @param repository
+	 * @return SynchronizationResult.OK if the process completed successfully, or a different SynchronizationResult if something went wrong.
+	 * @throws InterruptedException
+	 */
+	public SynchronizationResult findCommonBlockWithPeer(Peer peer, Repository repository) throws InterruptedException {
+		try {
+			final BlockData ourLatestBlockData = repository.getBlockRepository().getLastBlock();
+			final int ourInitialHeight = ourLatestBlockData.getHeight();
+
+			PeerChainTipData peerChainTipData = peer.getChainTipData();
+			int peerHeight = peerChainTipData.getLastHeight();
+			byte[] peersLastBlockSignature = peerChainTipData.getLastBlockSignature();
+
+			byte[] ourLastBlockSignature = ourLatestBlockData.getSignature();
+			LOGGER.debug(String.format("Fetching summaries from peer %s at height %d, sig %.8s, ts %d; our height %d, sig %.8s, ts %d", peer,
+					peerHeight, Base58.encode(peersLastBlockSignature), peer.getChainTipData().getLastBlockTimestamp(),
+					ourInitialHeight, Base58.encode(ourLastBlockSignature), ourLatestBlockData.getTimestamp()));
+
+			List<BlockSummaryData> peerBlockSummaries = new ArrayList<>();
+			SynchronizationResult findCommonBlockResult = fetchSummariesFromCommonBlock(repository, peer, ourInitialHeight, false, peerBlockSummaries);
+			if (findCommonBlockResult != SynchronizationResult.OK) {
+				// Logging performed by fetchSummariesFromCommonBlock() above
+				peer.setCommonBlockData(null);
+				return findCommonBlockResult;
+			}
+
+			// First summary is common block
+			final BlockData commonBlockData = repository.getBlockRepository().fromSignature(peerBlockSummaries.get(0).getSignature());
+			final BlockSummaryData commonBlockSummary = new BlockSummaryData(commonBlockData);
+			final int commonBlockHeight = commonBlockData.getHeight();
+			final byte[] commonBlockSig = commonBlockData.getSignature();
+			final String commonBlockSig58 = Base58.encode(commonBlockSig);
+			LOGGER.debug(String.format("Common block with peer %s is at height %d, sig %.8s, ts %d", peer,
+					commonBlockHeight, commonBlockSig58, commonBlockData.getTimestamp()));
+			peerBlockSummaries.remove(0);
+
+			// Store the common block summary against the peer, and the current chain tip (for caching)
+			peer.setCommonBlockData(new CommonBlockData(commonBlockSummary, peerChainTipData));
+
+			return SynchronizationResult.OK;
+		} catch (DataException e) {
+			LOGGER.error("Repository issue during synchronization with peer", e);
+			return SynchronizationResult.REPOSITORY_ISSUE;
+		}
+	}
+
+
+	/**
+	 * Compare a list of peers to determine the best peer(s) to sync to next.
+	 * <p>
+	 * Will return a filtered list of peers on success, or an identical list of peers on failure.
+	 * This allows us to fall back to legacy behaviour (random selection from the entire list of peers), if we are unable to make the comparison.
+	 * <p>
+	 * @param peers
+	 * @return a list of peers, possibly filtered.
+	 * @throws InterruptedException
+	 */
+	public List<Peer> comparePeers(List<Peer> peers) throws InterruptedException {
+		try (final Repository repository = RepositoryManager.getRepository()) {
+			try {
+
+				// If our latest block is very old, it's best that we don't try and determine the best peers to sync to.
+				// This is because it can involve very large chain comparisons, which is too intensive.
+				// In reality, most forking problems occur near the chain tips, so we will reserve this functionality for those situations.
+				final Long minLatestBlockTimestamp = Controller.getMinimumLatestBlockTimestamp();
+				if (minLatestBlockTimestamp == null)
+					return peers;
+
+				final BlockData ourLatestBlockData = repository.getBlockRepository().getLastBlock();
+				if (ourLatestBlockData.getTimestamp() < minLatestBlockTimestamp) {
+					LOGGER.debug(String.format("Our latest block is very old, so we won't filter the peers list"));
+					return peers;
+				}
+
+				// Retrieve a list of unique common blocks from this list of peers
+				List<BlockSummaryData> commonBlocks = this.uniqueCommonBlocks(peers);
+
+				// Order common blocks by height, in ascending order
+				// This is essential for the logic below to make the correct decisions when discarding chains - do not remove
+				commonBlocks.sort((b1, b2) -> Integer.valueOf(b1.getHeight()).compareTo(Integer.valueOf(b2.getHeight())));
+
+				// Get our latest height
+				final int ourHeight = ourLatestBlockData.getHeight();
+
+				// Create a placeholder to track of common blocks that we can discard due to being inferior chains
+				int dropPeersAfterCommonBlockHeight = 0;
+
+				// Remove peers with no common block data
+				Iterator iterator = peers.iterator();
+				while (iterator.hasNext()) {
+					Peer peer = (Peer) iterator.next();
+					if (peer.getCommonBlockData() == null) {
+						LOGGER.debug(String.format("Removed peer %s because it has no common block data", peer));
+						iterator.remove();
+					}
+				}
+
+				// Loop through each group of common blocks
+				for (BlockSummaryData commonBlockSummary : commonBlocks) {
+					List<Peer> peersSharingCommonBlock = peers.stream().filter(peer -> peer.getCommonBlockData().getCommonBlockSummary().equals(commonBlockSummary)).collect(Collectors.toList());
+
+					// Check if we need to discard this group of peers
+					if (dropPeersAfterCommonBlockHeight > 0) {
+						if (commonBlockSummary.getHeight() > dropPeersAfterCommonBlockHeight) {
+							// We have already determined that the correct chain diverged from a lower height. We are safe to skip these peers.
+							for (Peer peer : peersSharingCommonBlock) {
+								LOGGER.debug(String.format("Peer %s has common block at height %d but the superior chain is at height %d. Removing it from this round.", peer, commonBlockSummary.getHeight(), dropPeersAfterCommonBlockHeight));
+								Controller.getInstance().addInferiorChainSignature(peer.getChainTipData().getLastBlockSignature());
+							}
+							continue;
+						}
+					}
+
+					// Calculate the length of the shortest peer chain sharing this common block, including our chain
+					final int ourAdditionalBlocksAfterCommonBlock = ourHeight - commonBlockSummary.getHeight();
+					int minChainLength = this.calculateMinChainLength(commonBlockSummary, ourAdditionalBlocksAfterCommonBlock, peersSharingCommonBlock);
+
+					// Fetch block summaries from each peer
+					for (Peer peer : peersSharingCommonBlock) {
+
+						// If we're shutting down, just return the latest peer list
+						if (Controller.isStopping())
+							return peers;
+
+						// Count the number of blocks this peer has beyond our common block
+						final int peerHeight = peer.getChainTipData().getLastHeight();
+						final int peerAdditionalBlocksAfterCommonBlock = peerHeight - commonBlockSummary.getHeight();
+						// Limit the number of blocks we are comparing. FUTURE: we could request more in batches, but there may not be a case when this is needed
+						int summariesRequired = Math.min(peerAdditionalBlocksAfterCommonBlock, MAXIMUM_REQUEST_SIZE);
+
+						// Check if we can use the cached common block summaries, by comparing the peer's current chain tip against the peer's chain tip when we last found our common block
+						boolean useCachedSummaries = false;
+						if (peer.canUseCachedCommonBlockData()) {
+							if (peer.getCommonBlockData().getBlockSummariesAfterCommonBlock() != null) {
+								if (peer.getCommonBlockData().getBlockSummariesAfterCommonBlock().size() == summariesRequired) {
+									LOGGER.debug(String.format("Using cached block summaries for peer %s", peer));
+									useCachedSummaries = true;
+								}
+							}
+						}
+
+						if (useCachedSummaries == false) {
+							if (summariesRequired > 0) {
+								LOGGER.trace(String.format("Requesting %d block summar%s from peer %s after common block %.8s. Peer height: %d", summariesRequired, (summariesRequired != 1 ? "ies" : "y"), peer, Base58.encode(commonBlockSummary.getSignature()), peerHeight));
+
+								List<BlockSummaryData> blockSummaries = this.getBlockSummaries(peer, commonBlockSummary.getSignature(), summariesRequired);
+								peer.getCommonBlockData().setBlockSummariesAfterCommonBlock(blockSummaries);
+
+								if (blockSummaries != null) {
+									LOGGER.trace(String.format("Peer %s returned %d block summar%s", peer, blockSummaries.size(), (blockSummaries.size() != 1 ? "ies" : "y")));
+
+									// We need to adjust minChainLength if peers fail to return all expected block summaries
+									if (blockSummaries.size() < summariesRequired) {
+										// This could mean that the peer has re-orged. But we still have the same common block, so it's safe to proceed with this set of signatures instead.
+										LOGGER.debug(String.format("Peer %s returned %d block summar%s instead of expected %d", peer, blockSummaries.size(), (blockSummaries.size() != 1 ? "ies" : "y"), summariesRequired));
+
+										// Update minChainLength if we have at least 1 block for this peer. If we don't have any blocks, this peer will be excluded from chain weight comparisons later in the process, so we shouldn't update minChainLength
+										if (blockSummaries.size() > 0)
+											minChainLength = blockSummaries.size();
+									}
+								}
+							} else {
+								// There are no block summaries after this common block
+								peer.getCommonBlockData().setBlockSummariesAfterCommonBlock(null);
+							}
+						}
+					}
+
+					// Fetch our corresponding block summaries. Limit to MAXIMUM_REQUEST_SIZE, in order to make the comparison fairer, as peers have been limited too
+					final int ourSummariesRequired = Math.min(ourAdditionalBlocksAfterCommonBlock, MAXIMUM_REQUEST_SIZE);
+					LOGGER.trace(String.format("About to fetch our block summaries from %d to %d. Our height: %d", commonBlockSummary.getHeight() + 1, commonBlockSummary.getHeight() + ourSummariesRequired, ourHeight));
+					List<BlockSummaryData> ourBlockSummaries = repository.getBlockRepository().getBlockSummaries(commonBlockSummary.getHeight() + 1, commonBlockSummary.getHeight() + ourSummariesRequired);
+					if (ourBlockSummaries.isEmpty())
+						LOGGER.debug(String.format("We don't have any block summaries so can't compare our chain against peers with this common block. We can still compare them against each other."));
+					else
+						populateBlockSummariesMinterLevels(repository, ourBlockSummaries);
+
+					// Create array to hold peers for comparison
+					List<Peer> superiorPeersForComparison = new ArrayList<>();
+
+					// Calculate our chain weight
+					BigInteger ourChainWeight = BigInteger.valueOf(0);
+					if (ourBlockSummaries.size() > 0)
+						ourChainWeight = Block.calcChainWeight(commonBlockSummary.getHeight(), commonBlockSummary.getSignature(), ourBlockSummaries, minChainLength);
+
+					NumberFormat formatter = new DecimalFormat("0.###E0");
+					NumberFormat accurateFormatter = new DecimalFormat("0.################E0");
+					LOGGER.debug(String.format("Our chain weight based on %d blocks is %s", ourBlockSummaries.size(), formatter.format(ourChainWeight)));
+
+					LOGGER.debug(String.format("Listing peers with common block %.8s...", Base58.encode(commonBlockSummary.getSignature())));
+					for (Peer peer : peersSharingCommonBlock) {
+						final int peerHeight = peer.getChainTipData().getLastHeight();
+						final int peerAdditionalBlocksAfterCommonBlock = peerHeight - commonBlockSummary.getHeight();
+						final CommonBlockData peerCommonBlockData = peer.getCommonBlockData();
+
+						if (peerCommonBlockData == null || peerCommonBlockData.getBlockSummariesAfterCommonBlock() == null || peerCommonBlockData.getBlockSummariesAfterCommonBlock().isEmpty()) {
+							// No response - remove this peer for now
+							LOGGER.debug(String.format("Peer %s doesn't have any block summaries - removing it from this round", peer));
+							peers.remove(peer);
+							continue;
+						}
+
+						final List<BlockSummaryData> peerBlockSummariesAfterCommonBlock = peerCommonBlockData.getBlockSummariesAfterCommonBlock();
+						populateBlockSummariesMinterLevels(repository, peerBlockSummariesAfterCommonBlock);
+
+						// Calculate cumulative chain weight of this blockchain subset, from common block to highest mutual block held by all peers in this group.
+						LOGGER.debug(String.format("About to calculate chain weight based on %d blocks for peer %s with common block %.8s (peer has %d blocks after common block)", peerBlockSummariesAfterCommonBlock.size(), peer, Base58.encode(commonBlockSummary.getSignature()), peerAdditionalBlocksAfterCommonBlock));
+						BigInteger peerChainWeight = Block.calcChainWeight(commonBlockSummary.getHeight(), commonBlockSummary.getSignature(), peerBlockSummariesAfterCommonBlock, minChainLength);
+						peer.getCommonBlockData().setChainWeight(peerChainWeight);
+						LOGGER.debug(String.format("Chain weight of peer %s based on %d blocks (%d - %d) is %s", peer, peerBlockSummariesAfterCommonBlock.size(), peerBlockSummariesAfterCommonBlock.get(0).getHeight(), peerBlockSummariesAfterCommonBlock.get(peerBlockSummariesAfterCommonBlock.size()-1).getHeight(), formatter.format(peerChainWeight)));
+
+						// Compare against our chain - if our blockchain has greater weight then don't synchronize with peer (or any others in this group)
+						if (ourChainWeight.compareTo(peerChainWeight) > 0) {
+							// This peer is on an inferior chain - remove it
+							LOGGER.debug(String.format("Peer %s is on an inferior chain to us - removing it from this round", peer));
+							peers.remove(peer);
+						}
+						else {
+							// Our chain is inferior
+							LOGGER.debug(String.format("Peer %s is on a better chain to us. We will compare the other peers sharing this common block against each other, and drop all peers sharing higher common blocks.", peer));
+							dropPeersAfterCommonBlockHeight = commonBlockSummary.getHeight();
+							superiorPeersForComparison.add(peer);
+						}
+					}
+
+					// Now that we have selected the best peers, compare them against each other and remove any with lower weights
+					if (superiorPeersForComparison.size() > 0) {
+						BigInteger bestChainWeight = null;
+						for (Peer peer : superiorPeersForComparison) {
+							// Increase bestChainWeight if needed
+							if (bestChainWeight == null || peer.getCommonBlockData().getChainWeight().compareTo(bestChainWeight) >= 0)
+								bestChainWeight = peer.getCommonBlockData().getChainWeight();
+						}
+						for (Peer peer : superiorPeersForComparison) {
+							// Check if we should discard an inferior peer
+							if (peer.getCommonBlockData().getChainWeight().compareTo(bestChainWeight) < 0) {
+								BigInteger difference = bestChainWeight.subtract(peer.getCommonBlockData().getChainWeight());
+								LOGGER.debug(String.format("Peer %s has a lower chain weight (difference: %s) than other peer(s) in this group - removing it from this round.", peer, accurateFormatter.format(difference)));
+								peers.remove(peer);
+							}
+						}
+					}
+				}
+
+				return peers;
+			} finally {
+				repository.discardChanges(); // Free repository locks, if any, also in case anything went wrong
+			}
+		} catch (DataException e) {
+			LOGGER.error("Repository issue during peer comparison", e);
+			return peers;
+		}
+	}
+
+	private List<BlockSummaryData> uniqueCommonBlocks(List<Peer> peers) {
+		List<BlockSummaryData> commonBlocks = new ArrayList<>();
+
+		for (Peer peer : peers) {
+			if (peer.getCommonBlockData() != null && peer.getCommonBlockData().getCommonBlockSummary() != null) {
+				LOGGER.debug(String.format("Peer %s has common block %.8s", peer, Base58.encode(peer.getCommonBlockData().getCommonBlockSummary().getSignature())));
+
+				BlockSummaryData commonBlockSummary = peer.getCommonBlockData().getCommonBlockSummary();
+				if (!commonBlocks.contains(commonBlockSummary))
+					commonBlocks.add(commonBlockSummary);
+			}
+			else {
+				LOGGER.debug(String.format("Peer %s has no common block data. Skipping...", peer));
+			}
+		}
+
+		return commonBlocks;
+	}
+
+	private int calculateMinChainLength(BlockSummaryData commonBlockSummary, int ourAdditionalBlocksAfterCommonBlock, List<Peer> peersSharingCommonBlock) {
+		// Calculate the length of the shortest peer chain sharing this common block, including our chain
+		int minChainLength = ourAdditionalBlocksAfterCommonBlock;
+		for (Peer peer : peersSharingCommonBlock) {
+			final int peerHeight = peer.getChainTipData().getLastHeight();
+			final int peerAdditionalBlocksAfterCommonBlock = peerHeight - commonBlockSummary.getHeight();
+
+			if (peerAdditionalBlocksAfterCommonBlock < minChainLength)
+				minChainLength = peerAdditionalBlocksAfterCommonBlock;
+		}
+		return minChainLength;
+	}
+
+
 	/**
 	 * Attempt to synchronize blockchain with peer.
 	 * <p>
@@ -97,9 +474,12 @@ public class Synchronizer {
 
 					List<BlockSummaryData> peerBlockSummaries = new ArrayList<>();
 					SynchronizationResult findCommonBlockResult = fetchSummariesFromCommonBlock(repository, peer, ourInitialHeight, force, peerBlockSummaries);
-					if (findCommonBlockResult != SynchronizationResult.OK)
+					if (findCommonBlockResult != SynchronizationResult.OK) {
 						// Logging performed by fetchSummariesFromCommonBlock() above
+						// Clear our common block cache for this peer
+						peer.setCommonBlockData(null);
 						return findCommonBlockResult;
+					}
 
 					// First summary is common block
 					final BlockData commonBlockData = repository.getBlockRepository().fromSignature(peerBlockSummaries.get(0).getSignature());
@@ -244,9 +624,13 @@ public class Synchronizer {
 		// Currently we work forward from common block until we hit a block we don't have
 		// TODO: rewrite as modified binary search!
 		int i;
-		for (i = 1; i < blockSummariesFromCommon.size(); ++i)
+		for (i = 1; i < blockSummariesFromCommon.size(); ++i) {
+			if (Controller.isStopping())
+				return SynchronizationResult.SHUTTING_DOWN;
+
 			if (!repository.getBlockRepository().exists(blockSummariesFromCommon.get(i).getSignature()))
 				break;
+		}
 
 		// Note: index i - 1 isn't cleared: List.subList is fromIndex inclusive to toIndex exclusive
 		blockSummariesFromCommon.subList(0, i - 1).clear();
@@ -295,6 +679,9 @@ public class Synchronizer {
 
 				// Check peer sent valid heights
 				for (int i = 0; i < moreBlockSummaries.size(); ++i) {
+					if (Controller.isStopping())
+						return SynchronizationResult.SHUTTING_DOWN;
+
 					++lastSummaryHeight;
 
 					BlockSummaryData blockSummary = moreBlockSummaries.get(i);
@@ -341,52 +728,118 @@ public class Synchronizer {
 		final byte[] commonBlockSig = commonBlockData.getSignature();
 		String commonBlockSig58 = Base58.encode(commonBlockSig);
 
+		byte[] latestPeerSignature = commonBlockSig;
+		int height = commonBlockHeight;
+
 		LOGGER.debug(() -> String.format("Fetching peer %s chain from height %d, sig %.8s", peer, commonBlockHeight, commonBlockSig58));
 
-		int ourHeight = ourInitialHeight;
 
 		// Overall plan: fetch peer's blocks first, then orphan, then apply
 
 		// Convert any leftover (post-common) block summaries into signatures to request from peer
 		List<byte[]> peerBlockSignatures = peerBlockSummaries.stream().map(BlockSummaryData::getSignature).collect(Collectors.toList());
 
-		// Fetch remaining block signatures, if needed
-		int numberSignaturesRequired = peerBlockSignatures.size() - (peerHeight - commonBlockHeight);
-		if (numberSignaturesRequired > 0) {
-			byte[] latestPeerSignature = peerBlockSignatures.isEmpty() ? commonBlockSig : peerBlockSignatures.get(peerBlockSignatures.size() - 1);
-
-			LOGGER.trace(String.format("Requesting %d signature%s after height %d, sig %.8s",
-					numberSignaturesRequired, (numberSignaturesRequired != 1 ? "s": ""), ourHeight, Base58.encode(latestPeerSignature)));
-
-			List<byte[]> moreBlockSignatures = this.getBlockSignatures(peer, latestPeerSignature, numberSignaturesRequired);
-
-			if (moreBlockSignatures == null || moreBlockSignatures.isEmpty()) {
-				LOGGER.info(String.format("Peer %s failed to respond with more block signatures after height %d, sig %.8s", peer,
-						ourHeight, Base58.encode(latestPeerSignature)));
-				return SynchronizationResult.NO_REPLY;
-			}
-
-			LOGGER.trace(String.format("Received %s signature%s", peerBlockSignatures.size(), (peerBlockSignatures.size() != 1 ? "s" : "")));
-
-			peerBlockSignatures.addAll(moreBlockSignatures);
-		}
-
-		// Fetch blocks using signatures
-		LOGGER.debug(String.format("Fetching new blocks from peer %s", peer));
+		// Keep a list of blocks received so far
 		List<Block> peerBlocks = new ArrayList<>();
 
-		for (byte[] blockSignature : peerBlockSignatures) {
-			Block newBlock = this.fetchBlock(repository, peer, blockSignature);
+		// Calculate the total number of additional blocks this peer has beyond the common block
+		int additionalPeerBlocksAfterCommonBlock = peerHeight - commonBlockHeight;
+		// Subtract the number of signatures that we already have, as we don't need to request them again
+		int numberSignaturesRequired = additionalPeerBlocksAfterCommonBlock - peerBlockSignatures.size();
+
+		int retryCount = 0;
+		while (height < peerHeight) {
+			if (Controller.isStopping())
+				return SynchronizationResult.SHUTTING_DOWN;
+
+            // Ensure we don't request more than MAXIMUM_REQUEST_SIZE
+            int numberRequested = Math.min(numberSignaturesRequired, MAXIMUM_REQUEST_SIZE);
+
+            // Do we need more signatures?
+			if (peerBlockSignatures.isEmpty() && numberRequested > 0) {
+                LOGGER.trace(String.format("Requesting %d signature%s after height %d, sig %.8s",
+                        numberRequested, (numberRequested != 1 ? "s" : ""), height, Base58.encode(latestPeerSignature)));
+
+                peerBlockSignatures = this.getBlockSignatures(peer, latestPeerSignature, numberRequested);
+
+                if (peerBlockSignatures == null || peerBlockSignatures.isEmpty()) {
+                    LOGGER.info(String.format("Peer %s failed to respond with more block signatures after height %d, sig %.8s", peer,
+                            height, Base58.encode(latestPeerSignature)));
+
+					// Clear our cache of common block summaries for this peer, as they are likely to be invalid
+					CommonBlockData cachedCommonBlockData = peer.getCommonBlockData();
+					if (cachedCommonBlockData != null)
+						cachedCommonBlockData.setBlockSummariesAfterCommonBlock(null);
+
+                    // If we have already received RECENT blocks from this peer, go ahead and apply them
+                    if (peerBlocks.size() > 0) {
+                    	final Block peerLatestBlock = peerBlocks.get(peerBlocks.size() - 1);
+						final Long minLatestBlockTimestamp = Controller.getMinimumLatestBlockTimestamp();
+                    	if (peerLatestBlock != null && minLatestBlockTimestamp != null
+								&& peerLatestBlock.getBlockData().getTimestamp() > minLatestBlockTimestamp) {
+							LOGGER.debug("Newly received blocks are recent, so we will apply them");
+							break;
+						}
+                    }
+                    // Otherwise, give up and move on to the next peer, to avoid putting our chain into an outdated state
+                    return SynchronizationResult.NO_REPLY;
+                }
+
+                numberSignaturesRequired = peerHeight - height - peerBlockSignatures.size();
+                LOGGER.trace(String.format("Received %s signature%s", peerBlockSignatures.size(), (peerBlockSignatures.size() != 1 ? "s" : "")));
+			}
+
+			if (peerBlockSignatures.isEmpty()) {
+				LOGGER.trace(String.format("No more signatures or blocks to request from peer %s", peer));
+				break;
+			}
+
+			byte[] nextPeerSignature = peerBlockSignatures.get(0);
+			int nextHeight = height + 1;
+
+			LOGGER.trace(String.format("Fetching block %d, sig %.8s from %s", nextHeight, Base58.encode(nextPeerSignature), peer));
+			Block newBlock = this.fetchBlock(repository, peer, nextPeerSignature);
 
 			if (newBlock == null) {
 				LOGGER.info(String.format("Peer %s failed to respond with block for height %d, sig %.8s", peer,
-						ourHeight, Base58.encode(blockSignature)));
-				return SynchronizationResult.NO_REPLY;
+						nextHeight, Base58.encode(nextPeerSignature)));
+
+				if (retryCount >= MAXIMUM_RETRIES) {
+
+					// If we have already received RECENT blocks from this peer, go ahead and apply them
+					if (peerBlocks.size() > 0) {
+						final Block peerLatestBlock = peerBlocks.get(peerBlocks.size() - 1);
+						final Long minLatestBlockTimestamp = Controller.getMinimumLatestBlockTimestamp();
+						if (peerLatestBlock != null && minLatestBlockTimestamp != null
+								&& peerLatestBlock.getBlockData().getTimestamp() > minLatestBlockTimestamp) {
+							LOGGER.debug("Newly received blocks are recent, so we will apply them");
+							break;
+						}
+					}
+					// Otherwise, give up and move on to the next peer, to avoid putting our chain into an outdated state
+					return SynchronizationResult.NO_REPLY;
+
+				} else {
+					// Re-fetch signatures, in case the peer is now on a different fork
+					peerBlockSignatures.clear();
+					numberSignaturesRequired = peerHeight - height;
+
+					// Retry until retryCount reaches MAXIMUM_RETRIES
+					retryCount++;
+					int triesRemaining = MAXIMUM_RETRIES - retryCount;
+					LOGGER.info(String.format("Re-issuing request to peer %s (%d attempt%s remaining)", peer, triesRemaining, (triesRemaining != 1 ? "s" : "")));
+					continue;
+				}
 			}
 
+			// Reset retryCount because the last request succeeded
+			retryCount = 0;
+
+			LOGGER.trace(String.format("Fetched block %d, sig %.8s from %s", nextHeight, Base58.encode(latestPeerSignature), peer));
+
 			if (!newBlock.isSignatureValid()) {
 				LOGGER.info(String.format("Peer %s sent block with invalid signature for height %d, sig %.8s", peer,
-						ourHeight, Base58.encode(blockSignature)));
+						nextHeight, Base58.encode(latestPeerSignature)));
 				return SynchronizationResult.INVALID_DATA;
 			}
 
@@ -395,12 +848,18 @@ public class Synchronizer {
 				transaction.setInitialApprovalStatus();
 
 			peerBlocks.add(newBlock);
+
+			// Now that we've received this block, we can increase our height and move on to the next one
+			latestPeerSignature = nextPeerSignature;
+			peerBlockSignatures.remove(0);
+			++height;
 		}
 
 		// Unwind to common block (unless common block is our latest block)
-		LOGGER.debug(String.format("Orphaning blocks back to common block height %d, sig %.8s", commonBlockHeight, commonBlockSig58));
+		int ourHeight = ourInitialHeight;
+		LOGGER.debug(String.format("Orphaning blocks back to common block height %d, sig %.8s. Our height: %d", commonBlockHeight, commonBlockSig58, ourHeight));
 
-		BlockData orphanBlockData = repository.getBlockRepository().fromHeight(ourHeight);
+		BlockData orphanBlockData = repository.getBlockRepository().fromHeight(ourInitialHeight);
 		while (ourHeight > commonBlockHeight) {
 			if (Controller.isStopping())
 				return SynchronizationResult.SHUTTING_DOWN;
@@ -422,10 +881,13 @@ public class Synchronizer {
 		LOGGER.debug(String.format("Orphaned blocks back to height %d, sig %.8s - applying new blocks from peer %s", commonBlockHeight, commonBlockSig58, peer));
 
 		for (Block newBlock : peerBlocks) {
+			if (Controller.isStopping())
+				return SynchronizationResult.SHUTTING_DOWN;
+
 			ValidationResult blockResult = newBlock.isValid();
 			if (blockResult != ValidationResult.OK) {
 				LOGGER.info(String.format("Peer %s sent invalid block for height %d, sig %.8s: %s", peer,
-						ourHeight, Base58.encode(newBlock.getSignature()), blockResult.name()));
+						newBlock.getBlockData().getHeight(), Base58.encode(newBlock.getSignature()), blockResult.name()));
 				return SynchronizationResult.INVALID_DATA;
 			}
 
@@ -469,7 +931,8 @@ public class Synchronizer {
 
 			// Do we need more signatures?
 			if (peerBlockSignatures.isEmpty()) {
-				int numberRequested = maxBatchHeight - ourHeight;
+				int numberRequested = Math.min(maxBatchHeight - ourHeight, MAXIMUM_REQUEST_SIZE);
+
 				LOGGER.trace(String.format("Requesting %d signature%s after height %d, sig %.8s",
 						numberRequested, (numberRequested != 1 ? "s": ""), ourHeight, Base58.encode(latestPeerSignature)));
 
@@ -573,6 +1036,9 @@ public class Synchronizer {
 		final int firstBlockHeight = blockSummaries.get(0).getHeight();
 
 		for (int i = 0; i < blockSummaries.size(); ++i) {
+			if (Controller.isStopping())
+				return;
+
 			BlockSummaryData blockSummary = blockSummaries.get(i);
 
 			// Qortal: minter is always a reward-share, so find actual minter and get their effective minting level

src/main/java/org/qortal/controller/tradebot/BitcoinACCTv1TradeBot.java

@@ -386,7 +386,7 @@ public class BitcoinACCTv1TradeBot implements AcctTradeBot {
 
 				// If it has been over 24 hours since we last updated this trade-bot entry then assume AT is never coming back
 				// and so wipe the trade-bot entry
-				if (tradeBotData.getTimestamp() + MAX_AT_CONFIRMATION_PERIOD > NTP.getTime()) {
+				if (tradeBotData.getTimestamp() + MAX_AT_CONFIRMATION_PERIOD < NTP.getTime()) {
 					LOGGER.info(() -> String.format("AT %s has been gone for too long - deleting trade-bot entry", tradeBotData.getAtAddress()));
 					repository.getCrossChainRepository().delete(tradeBotData.getTradePrivateKey());
 					repository.saveChanges();

src/main/java/org/qortal/controller/tradebot/LitecoinACCTv1TradeBot.java

@@ -211,6 +211,9 @@ public class LitecoinACCTv1TradeBot implements AcctTradeBot {
 
 		TradeBot.updateTradeBotState(repository, tradeBotData, () -> String.format("Built AT %s. Waiting for deployment", atAddress));
 
+		// Attempt to backup the trade bot data
+		TradeBot.backupTradeBotData(repository);
+
 		// Return to user for signing and broadcast as we don't have their Qortal private key
 		try {
 			return DeployAtTransactionTransformer.toBytes(deployAtTransactionData);
@@ -283,6 +286,9 @@ public class LitecoinACCTv1TradeBot implements AcctTradeBot {
 				tradeForeignPublicKey, tradeForeignPublicKeyHash,
 				crossChainTradeData.expectedForeignAmount, xprv58, null, lockTimeA, receivingPublicKeyHash);
 
+		// Attempt to backup the trade bot data
+		TradeBot.backupTradeBotData(repository);
+
 		// Check we have enough funds via xprv58 to fund P2SH to cover expectedForeignAmount
 		long p2shFee;
 		try {
@@ -384,7 +390,7 @@ public class LitecoinACCTv1TradeBot implements AcctTradeBot {
 
 				// If it has been over 24 hours since we last updated this trade-bot entry then assume AT is never coming back
 				// and so wipe the trade-bot entry
-				if (tradeBotData.getTimestamp() + MAX_AT_CONFIRMATION_PERIOD > NTP.getTime()) {
+				if (tradeBotData.getTimestamp() + MAX_AT_CONFIRMATION_PERIOD < NTP.getTime()) {
 					LOGGER.info(() -> String.format("AT %s has been gone for too long - deleting trade-bot entry", tradeBotData.getAtAddress()));
 					repository.getCrossChainRepository().delete(tradeBotData.getTradePrivateKey());
 					repository.saveChanges();

src/main/java/org/qortal/controller/tradebot/TradeBot.java

@@ -7,6 +7,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Random;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -267,6 +268,22 @@ public class TradeBot implements Listener {
 		return secret;
 	}
 
+	/*package*/ static void backupTradeBotData(Repository repository) {
+		// Attempt to backup the trade bot data. This an optional step and doesn't impact trading, so don't throw an exception on failure
+		try {
+			LOGGER.info("About to backup trade bot data...");
+			ReentrantLock blockchainLock = Controller.getInstance().getBlockchainLock();
+			blockchainLock.lockInterruptibly();
+			try {
+				repository.exportNodeLocalData(true);
+			} finally {
+				blockchainLock.unlock();
+			}
+		} catch (InterruptedException | DataException e) {
+			LOGGER.info(String.format("Failed to obtain blockchain lock when exporting trade bot data: %s", e.getMessage()));
+		}
+	}
+
 	/** Updates trade-bot entry to new state, with current timestamp, logs message and notifies state-change listeners. */
 	/*package*/ static void updateTradeBotState(Repository repository, TradeBotData tradeBotData,
 			String newState, int newStateValue, Supplier<String> logMessageSupplier) throws DataException {

src/main/java/org/qortal/data/block/BlockSummaryData.java

@@ -2,6 +2,7 @@ package org.qortal.data.block;
 
 import javax.xml.bind.annotation.XmlAccessType;
 import javax.xml.bind.annotation.XmlAccessorType;
+import java.util.Arrays;
 
 @XmlAccessorType(XmlAccessType.FIELD)
 public class BlockSummaryData {
@@ -84,4 +85,21 @@ public class BlockSummaryData {
 		this.minterLevel = minterLevel;
 	}
 
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		BlockSummaryData otherBlockSummary = (BlockSummaryData) o;
+		if (this.getSignature() == null || otherBlockSummary.getSignature() == null)
+			return false;
+
+		// Treat two block summaries as equal if they have matching signatures
+		return Arrays.equals(this.getSignature(), otherBlockSummary.getSignature());
+	}
+
 }

src/main/java/org/qortal/data/block/CommonBlockData.java

@@ -0,0 +1,56 @@
+package org.qortal.data.block;
+
+import org.qortal.data.network.PeerChainTipData;
+
+import javax.xml.bind.annotation.XmlAccessType;
+import javax.xml.bind.annotation.XmlAccessorType;
+import java.math.BigInteger;
+import java.util.List;
+
+@XmlAccessorType(XmlAccessType.FIELD)
+public class CommonBlockData {
+
+	// Properties
+	private BlockSummaryData commonBlockSummary = null;
+	private List<BlockSummaryData> blockSummariesAfterCommonBlock = null;
+	private BigInteger chainWeight = null;
+	private PeerChainTipData chainTipData = null;
+
+	// Constructors
+
+	protected CommonBlockData() {
+	}
+
+	public CommonBlockData(BlockSummaryData commonBlockSummary, PeerChainTipData chainTipData) {
+		this.commonBlockSummary = commonBlockSummary;
+		this.chainTipData = chainTipData;
+	}
+
+
+	// Getters / setters
+
+	public BlockSummaryData getCommonBlockSummary() {
+		return this.commonBlockSummary;
+	}
+
+	public List<BlockSummaryData> getBlockSummariesAfterCommonBlock() {
+		return this.blockSummariesAfterCommonBlock;
+	}
+
+	public void setBlockSummariesAfterCommonBlock(List<BlockSummaryData> blockSummariesAfterCommonBlock) {
+		this.blockSummariesAfterCommonBlock = blockSummariesAfterCommonBlock;
+	}
+
+	public BigInteger getChainWeight() {
+		return this.chainWeight;
+	}
+
+	public void setChainWeight(BigInteger chainWeight) {
+		this.chainWeight = chainWeight;
+	}
+
+	public PeerChainTipData getChainTipData() {
+		return this.chainTipData;
+	}
+
+}

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

@@ -15,6 +15,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Random;
+import java.util.Arrays;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
@@ -22,6 +23,7 @@ import java.util.concurrent.TimeUnit;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.qortal.data.block.CommonBlockData;
 import org.qortal.data.network.PeerChainTipData;
 import org.qortal.data.network.PeerData;
 import org.qortal.network.message.ChallengeMessage;
@@ -46,7 +48,7 @@ public class Peer {
 	private static final int CONNECT_TIMEOUT = 2000; // ms
 
 	/** Maximum time to wait for a message reply to arrive from peer. (ms) */
-	private static final int RESPONSE_TIMEOUT = 2000; // ms
+	private static final int RESPONSE_TIMEOUT = 3000; // ms
 
 	/**
 	 * Interval between PING messages to a peer. (ms)
@@ -106,6 +108,9 @@ public class Peer {
 	/** Latest block info as reported by peer. */
 	private PeerChainTipData peersChainTipData;
 
+	/** Our common block with this peer */
+	private CommonBlockData commonBlockData;
+
 	// Constructors
 
 	/** Construct unconnected, outbound Peer using socket address in peer data */
@@ -272,6 +277,18 @@ public class Peer {
 		}
 	}
 
+	public CommonBlockData getCommonBlockData() {
+		synchronized (this.peerInfoLock) {
+			return this.commonBlockData;
+		}
+	}
+
+	public void setCommonBlockData(CommonBlockData commonBlockData) {
+		synchronized (this.peerInfoLock) {
+			this.commonBlockData = commonBlockData;
+		}
+	}
+
 	/*package*/ void queueMessage(Message message) {
 		if (!this.pendingMessages.offer(message))
 			LOGGER.info(() -> String.format("No room to queue message from peer %s - discarding", this));
@@ -507,6 +524,7 @@ public class Peer {
 			}
 		} catch (MessageException e) {
 			LOGGER.warn(String.format("Failed to send %s message with ID %d to peer %s: %s", message.getType().name(), message.getId(), this, e.getMessage()));
+			return false;
 		} catch (IOException e) {
 			// Send failure
 			return false;
@@ -615,6 +633,25 @@ public class Peer {
 		}
 	}
 
+
+	// Common block data
+
+	public boolean canUseCachedCommonBlockData() {
+		PeerChainTipData peerChainTipData = this.getChainTipData();
+		CommonBlockData commonBlockData = this.getCommonBlockData();
+
+		if (peerChainTipData != null && commonBlockData != null) {
+			PeerChainTipData commonBlockChainTipData = commonBlockData.getChainTipData();
+			if (peerChainTipData.getLastBlockSignature() != null && commonBlockChainTipData != null && commonBlockChainTipData.getLastBlockSignature() != null) {
+				if (Arrays.equals(peerChainTipData.getLastBlockSignature(), commonBlockChainTipData.getLastBlockSignature())) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
+
 	// Utility methods
 
 	/** Returns true if ports and addresses (or hostnames) match */

src/main/java/org/qortal/network/message/CachedBlockMessage.java

@@ -0,0 +1,70 @@
+package org.qortal.network.message;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.UnsupportedEncodingException;
+import java.nio.ByteBuffer;
+
+import org.qortal.block.Block;
+import org.qortal.transform.TransformationException;
+import org.qortal.transform.block.BlockTransformer;
+
+import com.google.common.primitives.Ints;
+
+// This is an OUTGOING-only Message which more readily lends itself to being cached
+public class CachedBlockMessage extends Message {
+
+	private Block block = null;
+	private byte[] cachedBytes = null;
+
+	public CachedBlockMessage(Block block) {
+		super(MessageType.BLOCK);
+
+		this.block = block;
+	}
+
+	private CachedBlockMessage(byte[] cachedBytes) {
+		super(MessageType.BLOCK);
+
+		this.block = null;
+		this.cachedBytes = cachedBytes;
+	}
+	
+	public static Message fromByteBuffer(int id, ByteBuffer byteBuffer) throws UnsupportedEncodingException {
+		throw new UnsupportedOperationException("CachedBlockMessage is for outgoing messages only");
+	}
+
+	@Override
+	protected byte[] toData() {
+		// Already serialized?
+		if (this.cachedBytes != null)
+			return cachedBytes;
+
+		if (this.block == null)
+			return null;
+
+		try {
+			ByteArrayOutputStream bytes = new ByteArrayOutputStream();
+
+			bytes.write(Ints.toByteArray(this.block.getBlockData().getHeight()));
+
+			bytes.write(BlockTransformer.toBytes(this.block));
+
+			this.cachedBytes = bytes.toByteArray();
+			// We no longer need source Block
+			// and Block contains repository handle which is highly likely to be invalid after this call
+			this.block = null;
+
+			return this.cachedBytes;
+		} catch (TransformationException | IOException e) {
+			return null;
+		}
+	}
+
+	public CachedBlockMessage cloneWithNewId(int newId) {
+		CachedBlockMessage clone = new CachedBlockMessage(this.cachedBytes);
+		clone.setId(newId);
+		return clone;
+	}
+
+}

src/main/java/org/qortal/repository/Repository.java

@@ -49,7 +49,7 @@ public interface Repository extends AutoCloseable {
 
 	public void performPeriodicMaintenance() throws DataException;
 
-	public void exportNodeLocalData() throws DataException;
+	public void exportNodeLocalData(boolean keepArchivedCopy) throws DataException;
 
 	public void importDataFromFile(String filename) throws DataException;
 

src/main/java/org/qortal/repository/hsqldb/HSQLDBRepository.java

@@ -52,6 +52,7 @@ import org.qortal.repository.TransactionRepository;
 import org.qortal.repository.VotingRepository;
 import org.qortal.repository.hsqldb.transaction.HSQLDBTransactionRepository;
 import org.qortal.settings.Settings;
+import org.qortal.utils.NTP;
 
 public class HSQLDBRepository implements Repository {
 
@@ -459,10 +460,44 @@ public class HSQLDBRepository implements Repository {
 	}
 
 	@Override
-	public void exportNodeLocalData() throws DataException {
+	public void exportNodeLocalData(boolean keepArchivedCopy) throws DataException {
+
+		// Create the qortal-backup folder if it doesn't exist
+		Path backupPath = Paths.get("qortal-backup");
+		try {
+			Files.createDirectories(backupPath);
+		} catch (IOException e) {
+			LOGGER.info("Unable to create backup folder");
+			throw new DataException("Unable to create backup folder");
+		}
+
+		// We need to rename or delete an existing TradeBotStates backup before creating a new one
+		File tradeBotStatesBackupFile = new File("qortal-backup/TradeBotStates.script");
+		if (tradeBotStatesBackupFile.exists()) {
+			if (keepArchivedCopy) {
+				// Rename existing TradeBotStates backup, to make sure that we're not overwriting any keys
+				File archivedBackupFile = new File(String.format("qortal-backup/TradeBotStates-archive-%d.script", NTP.getTime()));
+				if (tradeBotStatesBackupFile.renameTo(archivedBackupFile))
+					LOGGER.info(String.format("Moved existing TradeBotStates backup file to %s", archivedBackupFile.getPath()));
+				else
+					throw new DataException("Unable to rename existing TradeBotStates backup");
+			} else {
+				// Delete existing copy
+				LOGGER.info("Deleting existing TradeBotStates backup because it is being replaced with a new one");
+				tradeBotStatesBackupFile.delete();
+			}
+		}
+
+		// There's currently no need to take an archived copy of the MintingAccounts data - just delete the old one if it exists
+		File mintingAccountsBackupFile = new File("qortal-backup/MintingAccounts.script");
+		if (mintingAccountsBackupFile.exists()) {
+			LOGGER.info("Deleting existing MintingAccounts backup because it is being replaced with a new one");
+			mintingAccountsBackupFile.delete();
+		}
+
 		try (Statement stmt = this.connection.createStatement()) {
-			stmt.execute("PERFORM EXPORT SCRIPT FOR TABLE MintingAccounts DATA TO 'MintingAccounts.script'");
-			stmt.execute("PERFORM EXPORT SCRIPT FOR TABLE TradeBotStates DATA TO 'TradeBotStates.script'");
+			stmt.execute("PERFORM EXPORT SCRIPT FOR TABLE MintingAccounts DATA TO 'qortal-backup/MintingAccounts.script'");
+			stmt.execute("PERFORM EXPORT SCRIPT FOR TABLE TradeBotStates DATA TO 'qortal-backup/TradeBotStates.script'");
 			LOGGER.info("Exported sensitive/node-local data: minting keys and trade bot states");
 		} catch (SQLException e) {
 			throw new DataException("Unable to export sensitive/node-local data from repository");
@@ -475,12 +510,12 @@ public class HSQLDBRepository implements Repository {
 			LOGGER.info(() -> String.format("Importing data into repository from %s", filename));
 
 			String escapedFilename = stmt.enquoteLiteral(filename);
-			stmt.execute("PERFORM IMPORT SCRIPT DATA FROM " + escapedFilename + " STOP ON ERROR");
+			stmt.execute("PERFORM IMPORT SCRIPT DATA FROM " + escapedFilename + " CONTINUE ON ERROR");
 
 			LOGGER.info(() -> String.format("Imported data into repository from %s", filename));
 		} catch (SQLException e) {
 			LOGGER.info(() -> String.format("Failed to import data into repository from %s: %s", filename, e.getMessage()));
-			throw new DataException("Unable to export sensitive/node-local data from repository: " + e.getMessage());
+			throw new DataException("Unable to import sensitive/node-local data to repository: " + e.getMessage());
 		}
 	}
 
@@ -681,7 +716,7 @@ public class HSQLDBRepository implements Repository {
 	/**
 	 * Execute PreparedStatement and return changed row count.
 	 * 
-	 * @param preparedStatement
+	 * @param sql
 	 * @param objects
 	 * @return number of changed rows
 	 * @throws SQLException
@@ -693,8 +728,8 @@ public class HSQLDBRepository implements Repository {
 	/**
 	 * Execute batched PreparedStatement
 	 * 
-	 * @param preparedStatement
-	 * @param objects
+	 * @param sql
+	 * @param batchedObjects
 	 * @return number of changed rows
 	 * @throws SQLException
 	 */
@@ -818,7 +853,7 @@ public class HSQLDBRepository implements Repository {
 	 * 
 	 * @param tableName
 	 * @param whereClause
-	 * @param objects
+	 * @param batchedObjects
 	 * @throws SQLException
 	 */
 	public int deleteBatch(String tableName, String whereClause, List<Object[]> batchedObjects) throws SQLException {
@@ -931,6 +966,8 @@ public class HSQLDBRepository implements Repository {
 
 	/** Logs other HSQLDB sessions then returns passed exception */
 	public SQLException examineException(SQLException e) {
+		// TODO: could log at DEBUG for deadlocks by checking RepositoryManager.isDeadlockRelated(e)?
+
 		LOGGER.error(() -> String.format("[Session %d] HSQLDB error: %s", this.sessionId, e.getMessage()), e);
 
 		logStatements();

src/main/java/org/qortal/repository/hsqldb/HSQLDBRepositoryFactory.java

@@ -14,11 +14,11 @@ import org.hsqldb.jdbc.HSQLDBPool;
 import org.qortal.repository.DataException;
 import org.qortal.repository.Repository;
 import org.qortal.repository.RepositoryFactory;
+import org.qortal.settings.Settings;
 
 public class HSQLDBRepositoryFactory implements RepositoryFactory {
 
 	private static final Logger LOGGER = LogManager.getLogger(HSQLDBRepositoryFactory.class);
-	private static final int POOL_SIZE = 100;
 
 	/** Log getConnection() calls that take longer than this. (ms) */
 	private static final long SLOW_CONNECTION_THRESHOLD = 1000L;
@@ -57,7 +57,7 @@ public class HSQLDBRepositoryFactory implements RepositoryFactory {
 			HSQLDBRepository.attemptRecovery(connectionUrl);
 		}
 
-		this.connectionPool = new HSQLDBPool(POOL_SIZE);
+		this.connectionPool = new HSQLDBPool(Settings.getInstance().getRepositoryConnectionPoolSize());
 		this.connectionPool.setUrl(this.connectionUrl);
 
 		Properties properties = new Properties();

src/main/java/org/qortal/settings/Settings.java

@@ -52,7 +52,7 @@ public class Settings {
 	// UI servers
 	private int uiPort = 12388;
 	private String[] uiLocalServers = new String[] {
-		"localhost", "127.0.0.1", "172.24.1.1", "qor.tal"
+		"localhost", "127.0.0.1"
 	};
 	private String[] uiRemoteServers = new String[] {
 		"node1.qortal.org", "node2.qortal.org", "node3.qortal.org", "node4.qortal.org", "node5.qortal.org",
@@ -136,6 +136,8 @@ public class Settings {
 	private Long slowQueryThreshold = null;
 	/** Repository storage path. */
 	private String repositoryPath = "db";
+	/** Repository connection pool size. Needs to be a bit bigger than maxNetworkThreadPoolSize */
+	private int repositoryConnectionPoolSize = 100;
 
 	// Auto-update sources
 	private String[] autoUpdateRepos = new String[] {
@@ -430,6 +432,10 @@ public class Settings {
 		return this.repositoryPath;
 	}
 
+	public int getRepositoryConnectionPoolSize() {
+		return this.repositoryConnectionPoolSize;
+	}
+
 	public boolean isAutoUpdateEnabled() {
 		return this.autoUpdateEnabled;
 	}

tools/publish-auto-update.pl

@@ -57,9 +57,11 @@ $timestamp *= 1000; # Convert to milliseconds
 
 # locate sha256 utility
 my $SHA256 = `which sha256sum || which sha256`;
+chomp $SHA256;
+die("Can't find sha256sum or sha256\n") unless length($SHA256) > 0;
 
 # SHA256 of actual update file
-my $sha256 = `git show auto-update-${commit_hash}:${project}.update | ${SHA256}`;
+my $sha256 = `git show auto-update-${commit_hash}:${project}.update | ${SHA256} | head -c 64`;
 die("Can't calculate SHA256 of ${project}.update\n") unless $sha256 =~ m/(\S{64})/;
 chomp $sha256;