qt: get required locks upfront in polling functions

This avoids the GUI from getting stuck on
periodical polls if the core is holding the locks for a longer time -
for example, during a wallet rescan.

src/qt/clientmodel.cpp

@@ -92,6 +92,12 @@ double ClientModel::getVerificationProgress() const
 
 void ClientModel::updateTimer()
 {
+    // Get required lock upfront. This avoids the GUI from getting stuck on
+    // periodical polls if the core is holding the locks for a longer time -
+    // for example, during a wallet rescan.
+    TRY_LOCK(cs_main, lockMain);
+    if(!lockMain)
+        return;
     // Some quantities (such as number of blocks) change so fast that we don't want to be notified for each change.
     // Periodically check and update with a timer.
     int newNumBlocks = getNumBlocks();

src/qt/transactiontablemodel.cpp

@@ -24,7 +24,6 @@
 #include <QDebug>
 #include <QIcon>
 #include <QList>
-#include <QTimer>
 
 // Amount column is right-aligned it contains numbers
 static int column_alignments[] = {
@@ -187,11 +186,18 @@ public:
         {
             TransactionRecord *rec = &cachedWallet[idx];
 
+            // Get required locks upfront. This avoids the GUI from getting
+            // stuck if the core is holding the locks for a longer time - for
+            // example, during a wallet rescan.
+            //
             // If a status update is needed (blocks came in since last check),
             //  update the status of this transaction from the wallet. Otherwise,
             // simply re-use the cached status.
-            LOCK2(cs_main, wallet->cs_wallet);
+            TRY_LOCK(cs_main, lockMain);
-            if(rec->statusUpdateNeeded())
+            if(lockMain)
+            {
+                TRY_LOCK(wallet->cs_wallet, lockWallet);
+                if(lockWallet && rec->statusUpdateNeeded())
                 {
                     std::map<uint256, CWalletTx>::iterator mi = wallet->mapWallet.find(rec->hash);
 
@@ -200,6 +206,7 @@ public:
                         rec->updateStatus(mi->second);
                     }
                 }
+            }
             return rec;
         }
         else

src/qt/walletmodel.cpp

@@ -98,18 +98,21 @@ void WalletModel::updateStatus()
 
 void WalletModel::pollBalanceChanged()
 {
-    bool heightChanged = false;
+    // Get required locks upfront. This avoids the GUI from getting stuck on
-    {
+    // periodical polls if the core is holding the locks for a longer time -
-        LOCK(cs_main);
+    // for example, during a wallet rescan.
+    TRY_LOCK(cs_main, lockMain);
+    if(!lockMain)
+        return;
+    TRY_LOCK(wallet->cs_wallet, lockWallet);
+    if(!lockWallet)
+        return;
+
     if(chainActive.Height() != cachedNumBlocks)
     {
         // Balance and number of transactions might have changed
         cachedNumBlocks = chainActive.Height();
-            heightChanged = true;
+
-        }
-    }
-    if(heightChanged)
-    {
         checkBalanceChanged();
         if(transactionTableModel)
             transactionTableModel->updateConfirmations();