BugFix: fix not updating GUI balance race condition.

The problem arises when we trigger the balance update worker task and the chain tip changes (due a reorg) before one of the threadpool's threads start executing the update task. As the tip in this situation has the same height that the one cached inside the model, the `processBalanceChangeInternal` returns true without setting the processing flag to false again. Blocking every future balance update.

Github-Pull: #2379
Rebased-From: fbda4d2

src/qt/walletmodel.cpp

@@ -205,24 +205,26 @@ static bool processBalanceChangeInternal(WalletModel* walletModel)
     int chainHeight = walletModel->getLastBlockProcessedNum();
     const uint256& blockHash = walletModel->getLastBlockProcessed();
 
-    if (walletModel->hasForceCheckBalance() || chainHeight != walletModel->getCacheNumBLocks()) {
-        // Try to get lock only if needed
-        TRY_LOCK(pwalletMain->cs_wallet, lockWallet);
-        if (!lockWallet)
-            return false;
-
-        walletModel->setfForceCheckBalanceChanged(false);
-
-        // Balance and number of transactions might have changed
-        walletModel->setCacheNumBlocks(chainHeight);
-        walletModel->setCacheBlockHash(blockHash);
-        walletModel->checkBalanceChanged(walletModel->getBalances());
-        QMetaObject::invokeMethod(walletModel, "updateTxModelData", Qt::QueuedConnection);
-        QMetaObject::invokeMethod(walletModel, "pollFinished", Qt::QueuedConnection);
-
-        // Address in receive tab may have been used
-        Q_EMIT walletModel->notifyReceiveAddressChanged();
-    }
+    // Avoid recomputing wallet balances unless a tx changed or BlockTip notification was received.
+    // Extra note: This needs to be done before and after the update task trigger and execution because, as it runs concurrently,
+    // there is no guarantee that the threadpool will execute the task right away.
+    if (!walletModel->hasForceCheckBalance() && walletModel->getCacheBlockHash() == blockHash) return false;
+
+    // Try to get lock only if needed
+    TRY_LOCK(pwalletMain->cs_wallet, lockWallet);
+    if (!lockWallet) return false;
+
+    walletModel->setfForceCheckBalanceChanged(false);
+
+    // Balance and number of transactions might have changed
+    walletModel->setCacheNumBlocks(chainHeight);
+    walletModel->setCacheBlockHash(blockHash);
+    walletModel->checkBalanceChanged(walletModel->getBalances());
+    QMetaObject::invokeMethod(walletModel, "updateTxModelData", Qt::QueuedConnection);
+    QMetaObject::invokeMethod(walletModel, "pollFinished", Qt::QueuedConnection);
+
+    // Address in receive tab may have been used
+    Q_EMIT walletModel->notifyReceiveAddressChanged();
     return true;
 }
 

src/qt/walletmodel.h

@@ -340,6 +340,7 @@ class WalletModel : public QObject
     void setCacheNumBlocks(int _cachedNumBlocks) { cachedNumBlocks = _cachedNumBlocks; }
     int getCacheNumBLocks() { return cachedNumBlocks; }
     void setCacheBlockHash(const uint256& _blockHash) { m_cached_best_block_hash = _blockHash; }
+    uint256 getCacheBlockHash() { return m_cached_best_block_hash; }
     void setfForceCheckBalanceChanged(bool _fForceCheckBalanceChanged) { fForceCheckBalanceChanged = _fForceCheckBalanceChanged; }
     Q_INVOKABLE void checkBalanceChanged(const interfaces::WalletBalances& new_balances);