wallet: GetAvailableBalance, remove double walk-through every available coin

Filtering `AvailableCoins` by spendable outputs only and using the retrieved total_amount.

Author: furszy

src/wallet/spend.cpp

@@ -96,7 +96,6 @@ CoinsResult AvailableCoins(const CWallet& wallet,
     AssertLockHeld(wallet.cs_wallet);
 
     CoinsResult result;
-    CAmount nTotal = 0;
     // Either the WALLET_FLAG_AVOID_REUSE flag is not set (in which case we always allow), or we default to avoiding, and only in the case where
     // a coin control object is provided, and has the avoid address reuse flag set to false, do we allow already used addresses
     bool allow_used_addresses = !wallet.IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE) || (coinControl && !coinControl->m_avoid_address_reuse);
@@ -206,12 +205,11 @@ CoinsResult AvailableCoins(const CWallet& wallet,
 
             int input_bytes = GetTxSpendSize(wallet, wtx, i, (coinControl && coinControl->fAllowWatchOnly));
             result.coins.emplace_back(outpoint, output, nDepth, input_bytes, spendable, solvable, safeTx, wtx.GetTxTime(), tx_from_me, feerate);
+            result.total_amount += output.nValue;
 
             // Checks the sum amount of all UTXO's.
             if (nMinimumSumAmount != MAX_MONEY) {
-                nTotal += output.nValue;
-
-                if (nTotal >= nMinimumSumAmount) {
+                if (result.total_amount >= nMinimumSumAmount) {
                     return result;
                 }
             }
@@ -234,14 +232,14 @@ CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl*
 CAmount GetAvailableBalance(const CWallet& wallet, const CCoinControl* coinControl)
 {
     LOCK(wallet.cs_wallet);
-
-    CAmount balance = 0;
-    for (const COutput& out : AvailableCoinsListUnspent(wallet, coinControl).coins) {
-        if (out.spendable) {
-            balance += out.txout.nValue;
-        }
-    }
-    return balance;
+    return AvailableCoins(wallet,
+                          coinControl,
+                          std::nullopt, /*feerate=*/
+                          1,            /*nMinimumAmount*/
+                          MAX_MONEY,    /*nMaximumAmount*/
+                          MAX_MONEY,    /*nMinimumSumAmount*/
+                          0             /*nMaximumCount*/
+                          ).total_amount;
 }
 
 const CTxOut& FindNonChangeParentOutput(const CWallet& wallet, const CTransaction& tx, int output)

src/wallet/spend.h

@@ -36,6 +36,8 @@ TxSize CalculateMaximumSignedTxSize(const CTransaction& tx, const CWallet* walle
 
 struct CoinsResult {
     std::vector<COutput> coins;
+    // Sum of all the coins amounts
+    CAmount total_amount{0};
 };
 /**
  * Return vector of available COutputs.