Reuse the work buffer when verifying online accounts from the OnlineAccountsManager import queue.

This is a hopeful fix for extra memory usage since mempow activated, due to adding a lot of load to the garbage collector. It only applies to accounts verified from the import queue; the optimization hasn't been applied to block processing. But verifying online accounts when processing blocks is rare and generally would only last a short amount of time.

src/main/java/org/qortal/block/Block.java

@@ -1072,7 +1072,7 @@ public class Block {
 
 		// Validate the rest
 		for (OnlineAccountData onlineAccount : onlineAccounts)
-			if (!OnlineAccountsManager.getInstance().verifyMemoryPoW(onlineAccount))
+			if (!OnlineAccountsManager.getInstance().verifyMemoryPoW(onlineAccount, null))
 				return ValidationResult.ONLINE_ACCOUNT_NONCE_INCORRECT;
 
 		// Cache the valid online accounts as they will likely be needed for the next block

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

@@ -72,6 +72,11 @@ public class OnlineAccountsManager {
     public static final int POW_BUFFER_SIZE_TESTNET = 1 * 1024 * 1024; // bytes
     public static final int POW_DIFFICULTY_TESTNET = 5; // leading zero bits
 
+    // IMPORTANT: if we ever need to dynamically modify the buffer size using a feature trigger, the
+    // pre-allocated buffer below will NOT work, and we should instead use a dynamically allocated
+    // one for the transition period.
+    private static long[] POW_VERIFY_WORK_BUFFER = new long[getPoWBufferSize() / 8];
+
     private final ScheduledExecutorService executor = Executors.newScheduledThreadPool(4, new NamedThreadFactory("OnlineAccounts"));
     private volatile boolean isStopping = false;
 
@@ -339,7 +344,7 @@ public class OnlineAccountsManager {
         }
 
         // Validate mempow
-        if (!getInstance().verifyMemoryPoW(onlineAccountData)) {
+        if (!getInstance().verifyMemoryPoW(onlineAccountData, POW_VERIFY_WORK_BUFFER)) {
             LOGGER.trace(() -> String.format("Rejecting online reward-share for account %s due to invalid PoW nonce", mintingAccount.getAddress()));
             return false;
         }
@@ -582,7 +587,7 @@ public class OnlineAccountsManager {
             OnlineAccountData ourOnlineAccountData = new OnlineAccountData(onlineAccountsTimestamp, signature, publicKey, nonce);
 
             // Make sure to verify before adding
-            if (verifyMemoryPoW(ourOnlineAccountData)) {
+            if (verifyMemoryPoW(ourOnlineAccountData, null)) {
                 ourOnlineAccounts.add(ourOnlineAccountData);
             }
         }
@@ -637,7 +642,7 @@ public class OnlineAccountsManager {
         return nonce;
     }
 
-    public boolean verifyMemoryPoW(OnlineAccountData onlineAccountData) {
+    public boolean verifyMemoryPoW(OnlineAccountData onlineAccountData, long[] workBuffer) {
         // Require a valid nonce value
         if (onlineAccountData.getNonce() == null || onlineAccountData.getNonce() < 0) {
             return false;
@@ -653,7 +658,7 @@ public class OnlineAccountsManager {
         }
 
         // Verify the nonce
-        return MemoryPoW.verify2(mempowBytes, getPoWBufferSize(), getPoWDifficulty(), nonce);
+        return MemoryPoW.verify2(mempowBytes, workBuffer, getPoWBufferSize(), getPoWDifficulty(), nonce);
     }
 
 

src/main/java/org/qortal/crypto/MemoryPoW.java

@@ -99,6 +99,10 @@ public class MemoryPoW {
 	}
 
 	public static boolean verify2(byte[] data, int workBufferLength, long difficulty, int nonce) {
+		return verify2(data, null, workBufferLength, difficulty, nonce);
+	}
+
+	public static boolean verify2(byte[] data, long[] workBuffer, int workBufferLength, long difficulty, int nonce) {
 		// Hash data with SHA256
 		byte[] hash = Crypto.digest(data);
 
@@ -111,7 +115,10 @@ public class MemoryPoW {
 		byteBuffer = null;
 
 		int longBufferLength = workBufferLength / 8;
-		long[] workBuffer = new long[longBufferLength];
+
+		if (workBuffer == null)
+			workBuffer = new long[longBufferLength];
+
 		long[] state = new long[4];
 
 		long seed = 8682522807148012L;