Merge bitcoin-core/gui#598: Avoid recalculating the wallet balance - use model cache

4584d300a40bfd84517072f7a6eee114fb7cab08 GUI: remove now unneeded 'm_balances' field from overviewpage (furszy)
050e8b139145d6991e740b0e5f2b3364663dd348 GUI: 'getAvailableBalance', use cached balance if the user did not select UTXO manually (furszy)
96e3264a82c51b456703f500bd98e8cb98115697 GUI: use cached balance in overviewpage and sendcoinsdialog (furszy)
321335bf0292034d79afa6c44f7f072942b6cc3c GUI: add getter for WalletModel::m_cached_balances field (furszy)
e62958dc81d215a1c56318d0914dfd9a33d45973 GUI: sendCoinsDialog, remove duplicate wallet().getBalances() call (furszy)

Pull request description:

  As per the title says, we are recalculating the entire wallet balance on different situations calling to `wallet().getBalances()`, when should instead make use of the wallet model cached balance.

  This has the benefits of (1) not spending resources calculating a balance that we already have cached, and (2) avoid blocking the main thread for a long time, in case of big wallets, walking through the entire wallet's tx map more than what it's really needed.

  Changes:

  1) Fix: `SendCoinsDialog` was calling `wallet().getBalances()` twice during `setModel`.
  2) Use the cached balance if the user did not select any UTXO manually inside the wallet model `getAvailableBalance` call.

  -----------------------
  As an extra note, this work born in [#25005](https://github.com/bitcoin/bitcoin/pull/25005) but grew out of scope of it.

ACKs for top commit:
  jarolrod:
    ACK 4584d300a40bfd84517072f7a6eee114fb7cab08
  hebasto:
    re-ACK 4584d300a40bfd84517072f7a6eee114fb7cab08, only suggested changes and commit message formatting since my [recent](https://github.com/bitcoin-core/gui/pull/598#pullrequestreview-1071268192) review.

Tree-SHA512: 6633ce7f9a82a3e46e75aa7295df46c80a4cd4a9f3305427af203c9bc8670573fa8a1927f14a279260c488cc975a08d238faba2e9751588086fea1dcf8ea2b28

1 files changed, 18 insertions, 2 deletions

diff --git a/src/qt/walletmodel.cpp b/src/qt/walletmodel.cpp
index ac6c8bea46..c6f3f5b00c 100644
--- a/src/qt/walletmodel.cpp
+++ b/src/qt/walletmodel.cpp
@@ -67,6 +67,10 @@ WalletModel::~WalletModel()
 
 void WalletModel::startPollBalance()
 {
+    // Update the cached balance right away, so every view can make use of it,
+    // so them don't need to waste resources recalculating it.
+    pollBalanceChanged();
+
     // This timer will be fired repeatedly to update the balance
     // Since the QTimer::timeout is a private signal, it cannot be used
     // in the GUIUtil::ExceptionSafeConnect directly.
@@ -120,12 +124,17 @@ void WalletModel::pollBalanceChanged()
 
 void WalletModel::checkBalanceChanged(const interfaces::WalletBalances& new_balances)
 {
-    if(new_balances.balanceChanged(m_cached_balances)) {
+    if (new_balances.balanceChanged(m_cached_balances)) {
         m_cached_balances = new_balances;
         Q_EMIT balanceChanged(new_balances);
     }
 }
 
+interfaces::WalletBalances WalletModel::getCachedBalance() const
+{
+    return m_cached_balances;
+}
+
 void WalletModel::updateTransaction()
 {
     // Balance and number of transactions might have changed
@@ -194,7 +203,9 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         return DuplicateAddress;
     }
 
-    CAmount nBalance = m_wallet->getAvailableBalance(coinControl);
+    // If no coin was manually selected, use the cached balance
+    // Future: can merge this call with 'createTransaction'.
+    CAmount nBalance = getAvailableBalance(&coinControl);
 
     if(total > nBalance)
     {
@@ -602,3 +613,8 @@ uint256 WalletModel::getLastBlockProcessed() const
 {
     return m_client_model ? m_client_model->getBestBlockHash() : uint256{};
 }
+
+CAmount WalletModel::getAvailableBalance(const CCoinControl* control)
+{
+    return control && control->HasSelected() ? wallet().getAvailableBalance(*control) : getCachedBalance().balance;
+}