wallet: skip available coins fetch if "other inputs" are disallowed

no need to waste resources calculating the wallet available coins if
they are not going to be used.

The 'm_allow_other_inputs=true` default value change is to correct
an ugly misleading behavior:

The tx creation process was having a workaround patch to automatically
fall back to select coins from the wallet if `m_allow_other_inputs=false`
(previous default value) and no manual inputs were selected.

This could be seen in master in flows like `sendtoaddress`, `sendmany`
and even the GUI, where the `m_allow_other_inputs` value isn't customized
and the wallet still selects and adds coins to the tx internally.

src/qt/sendcoinsdialog.cpp

@@ -289,7 +289,9 @@ bool SendCoinsDialog::PrepareSendText(QString& question_string, QString& informa
 
     updateCoinControlState();
 
-    prepareStatus = model->prepareTransaction(*m_current_transaction, *m_coin_control);
+    CCoinControl coin_control = *m_coin_control;
+    coin_control.m_allow_other_inputs = !coin_control.HasSelected(); // future, could introduce a checkbox to customize this value.
+    prepareStatus = model->prepareTransaction(*m_current_transaction, coin_control);
 
     // process prepareStatus and on error generate message shown to user
     processSendCoinsReturn(prepareStatus,

src/wallet/coincontrol.h

@@ -37,7 +37,7 @@ public:
     bool m_include_unsafe_inputs = false;
     //! If true, the selection process can add extra unselected inputs from the wallet
     //! while requires all selected inputs be used
-    bool m_allow_other_inputs = false;
+    bool m_allow_other_inputs = true;
     //! Includes watch only addresses which are solvable
     bool fAllowWatchOnly = false;
     //! Override automatic min/max checks on fee, m_feerate must be set if true

src/wallet/spend.cpp

@@ -579,7 +579,7 @@ std::optional<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& a
     }
 
     // coin control -> return all selected outputs (we want all selected to go into the transaction for sure)
-    if (coin_control.HasSelected() && !coin_control.m_allow_other_inputs) {
+    if (!coin_control.m_allow_other_inputs) {
         SelectionResult result(nTargetValue, SelectionAlgorithm::MANUAL);
         result.AddInput(preset_inputs);
 
@@ -893,14 +893,18 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
     const CAmount not_input_fees = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.tx_noinputs_size);
     CAmount selection_target = recipients_sum + not_input_fees;
 
-    // Get available coins
-    auto available_coins = AvailableCoins(wallet,
-                                              &coin_control,
-                                              coin_selection_params.m_effective_feerate,
-                                              1,            /*nMinimumAmount*/
-                                              MAX_MONEY,    /*nMaximumAmount*/
-                                              MAX_MONEY,    /*nMinimumSumAmount*/
-                                              0);           /*nMaximumCount*/
+    // Fetch wallet available coins if "other inputs" are
+    // allowed (coins automatically selected by the wallet)
+    CoinsResult available_coins;
+    if (coin_control.m_allow_other_inputs) {
+        available_coins = AvailableCoins(wallet,
+                                         &coin_control,
+                                         coin_selection_params.m_effective_feerate,
+                                         1,            /*nMinimumAmount*/
+                                         MAX_MONEY,    /*nMaximumAmount*/
+                                         MAX_MONEY,    /*nMinimumSumAmount*/
+                                         0);           /*nMaximumCount*/
+    }
 
     // Choose coins to use
     std::optional<SelectionResult> result = SelectCoins(wallet, available_coins, /*nTargetValue=*/selection_target, coin_control, coin_selection_params);

test/functional/rpc_fundrawtransaction.py

@@ -1095,6 +1095,8 @@ class RawTransactionsTest(BitcoinTestFramework):
         #       Expect: only preset inputs are used.
         # 5. Explicit add_inputs=true, no preset inputs (same as (1) but with an explicit set):
         #       Expect: include inputs from the wallet.
+        # 6. Explicit add_inputs=false, no preset inputs:
+        #       Expect: failure as we did not provide inputs and the process cannot automatically select coins.
 
         # Case (1), 'send' command
         # 'add_inputs' value is true unless "inputs" are specified, in such case, add_inputs=false.
@@ -1146,6 +1148,10 @@ class RawTransactionsTest(BitcoinTestFramework):
         tx = wallet.send(outputs=[{addr1: 8}], options=options)
         assert tx["complete"]
 
+        # 6. Explicit add_inputs=false, no preset inputs:
+        options = {"add_inputs": False}
+        assert_raises_rpc_error(-4, "Insufficient funds", wallet.send, outputs=[{addr1: 3}], options=options)
+
         ################################################
 
         # Case (1), 'walletcreatefundedpsbt' command
@@ -1187,6 +1193,10 @@ class RawTransactionsTest(BitcoinTestFramework):
         }
         assert "psbt" in wallet.walletcreatefundedpsbt(inputs=[], outputs=outputs, options=options)
 
+        # Case (6). Explicit add_inputs=false, no preset inputs:
+        options = {"add_inputs": False}
+        assert_raises_rpc_error(-4, "Insufficient funds", wallet.walletcreatefundedpsbt, inputs=[], outputs=outputs, options=options)
+
         self.nodes[2].unloadwallet("test_preset_inputs")
 
     def test_weight_calculation(self):