wallet: encapsulate pre-selected-inputs lookup into its own function

First step towards decoupling the pre-selected-inputs fetching functionality
from `SelectCoins`. Which, will let us not waste resources calculating the
available coins if one of the pre-set inputs has an error.

(right now, if one of the pre-set inputs is invalid, we first walk through
the entire wallet txes map just to end up failing right after it finish)

4 files changed, 82 insertions, 51 deletions

diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
index b568e90998..a8be6cd83a 100644
--- a/src/wallet/coinselection.cpp
+++ b/src/wallet/coinselection.cpp
@@ -444,6 +444,12 @@ void SelectionResult::AddInput(const OutputGroup& group)
     m_use_effective = !group.m_subtract_fee_outputs;
 }
 
+void SelectionResult::AddInputs(const std::set<COutput>& inputs, bool subtract_fee_outputs)
+{
+    util::insert(m_selected_inputs, inputs);
+    m_use_effective = !subtract_fee_outputs;
+}
+
 void SelectionResult::Merge(const SelectionResult& other)
 {
     m_target += other.m_target;
diff --git a/src/wallet/coinselection.h b/src/wallet/coinselection.h
index 761c2be0b3..b23dd10867 100644
--- a/src/wallet/coinselection.h
+++ b/src/wallet/coinselection.h
@@ -308,6 +308,7 @@ public:
     void Clear();
 
     void AddInput(const OutputGroup& group);
+    void AddInputs(const std::set<COutput>& inputs, bool subtract_fee_outputs);
 
     /** Calculates and stores the waste for this selection via GetSelectionWaste */
     void ComputeAndSetWaste(const CAmount min_viable_change, const CAmount change_cost, const CAmount change_fee);
diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index cd0c7c4a09..0b7cb7faa4 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -143,6 +143,51 @@ static OutputType GetOutputType(TxoutType type, bool is_from_p2sh)
     }
 }
 
+// Fetch and validate the coin control selected inputs.
+// Coins could be internal (from the wallet) or external.
+util::Result<PreSelectedInputs> FetchSelectedInputs(const CWallet& wallet, const CCoinControl& coin_control,
+                                            const CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
+{
+    PreSelectedInputs result;
+    std::vector<COutPoint> vPresetInputs;
+    coin_control.ListSelected(vPresetInputs);
+    for (const COutPoint& outpoint : vPresetInputs) {
+        int input_bytes = -1;
+        CTxOut txout;
+        if (auto ptr_wtx = wallet.GetWalletTx(outpoint.hash)) {
+            // Clearly invalid input, fail
+            if (ptr_wtx->tx->vout.size() <= outpoint.n) {
+                return util::Error{strprintf(_("Invalid pre-selected input %s"), outpoint.ToString())};
+            }
+            txout = ptr_wtx->tx->vout.at(outpoint.n);
+            input_bytes = CalculateMaximumSignedInputSize(txout, &wallet, &coin_control);
+        } else {
+            // The input is external. We did not find the tx in mapWallet.
+            if (!coin_control.GetExternalOutput(outpoint, txout)) {
+                return util::Error{strprintf(_("Not found pre-selected input %s"), outpoint.ToString())};
+            }
+        }
+
+        if (input_bytes == -1) {
+            input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, &coin_control);
+        }
+
+        // If available, override calculated size with coin control specified size
+        if (coin_control.HasInputWeight(outpoint)) {
+            input_bytes = GetVirtualTransactionSize(coin_control.GetInputWeight(outpoint), 0, 0);
+        }
+
+        if (input_bytes == -1) {
+            return util::Error{strprintf(_("Not solvable pre-selected input %s"), outpoint.ToString())}; // Not solvable, can't estimate size for fee
+        }
+
+        /* Set some defaults for depth, spendable, solvable, safe, time, and from_me as these don't matter for preset inputs since no selection is being done. */
+        COutput output(outpoint, txout, /*depth=*/ 0, input_bytes, /*spendable=*/ true, /*solvable=*/ true, /*safe=*/ true, /*time=*/ 0, /*from_me=*/ false, coin_selection_params.m_effective_feerate);
+        result.Insert(output, coin_selection_params.m_subtract_fee_outputs);
+    }
+    return result;
+}
+
 CoinsResult AvailableCoins(const CWallet& wallet,
                            const CCoinControl* coinControl,
                            std::optional<CFeeRate> feerate,
@@ -527,58 +572,14 @@ std::optional<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& a
 {
     CAmount value_to_select = nTargetValue;
 
-    OutputGroup preset_inputs(coin_selection_params);
+    util::Result<PreSelectedInputs> pre_selected_inputs = FetchSelectedInputs(wallet, coin_control, coin_selection_params);
+    if (!pre_selected_inputs) return std::nullopt;
+    PreSelectedInputs inputs = *pre_selected_inputs;
 
-    std::vector<COutPoint> vPresetInputs;
-    coin_control.ListSelected(vPresetInputs);
-    for (const COutPoint& outpoint : vPresetInputs) {
-        int input_bytes = -1;
-        CTxOut txout;
-        auto ptr_wtx = wallet.GetWalletTx(outpoint.hash);
-        if (ptr_wtx) {
-            // Clearly invalid input, fail
-            if (ptr_wtx->tx->vout.size() <= outpoint.n) {
-                return std::nullopt;
-            }
-            txout = ptr_wtx->tx->vout.at(outpoint.n);
-            input_bytes = CalculateMaximumSignedInputSize(txout, &wallet, &coin_control);
-        } else {
-            // The input is external. We did not find the tx in mapWallet.
-            if (!coin_control.GetExternalOutput(outpoint, txout)) {
-                return std::nullopt;
-            }
-        }
-
-        if (input_bytes == -1) {
-            input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, &coin_control);
-        }
-
-        // If available, override calculated size with coin control specified size
-        if (coin_control.HasInputWeight(outpoint)) {
-            input_bytes = GetVirtualTransactionSize(coin_control.GetInputWeight(outpoint), 0, 0);
-        }
-
-        if (input_bytes == -1) {
-            return std::nullopt; // Not solvable, can't estimate size for fee
-        }
-
-        /* Set some defaults for depth, spendable, solvable, safe, time, and from_me as these don't matter for preset inputs since no selection is being done. */
-        COutput output(outpoint, txout, /*depth=*/ 0, input_bytes, /*spendable=*/ true, /*solvable=*/ true, /*safe=*/ true, /*time=*/ 0, /*from_me=*/ false, coin_selection_params.m_effective_feerate);
-        if (coin_selection_params.m_subtract_fee_outputs) {
-            value_to_select -= output.txout.nValue;
-        } else {
-            value_to_select -= output.GetEffectiveValue();
-        }
-        /* Set ancestors and descendants to 0 as they don't matter for preset inputs since no actual selection is being done.
-         * positive_only is set to false because we want to include all preset inputs, even if they are dust.
-         */
-        preset_inputs.Insert(output, /*ancestors=*/ 0, /*descendants=*/ 0, /*positive_only=*/ false);
-    }
-
-    // coin control -> return all selected outputs (we want all selected to go into the transaction for sure)
+    // If automatic coin selection was disabled, we just want to return the preset inputs result
     if (!coin_control.m_allow_other_inputs) {
         SelectionResult result(nTargetValue, SelectionAlgorithm::MANUAL);
-        result.AddInput(preset_inputs);
+        result.AddInputs(inputs.coins, coin_selection_params.m_subtract_fee_outputs);
 
         if (!coin_selection_params.m_subtract_fee_outputs && result.GetSelectedEffectiveValue() < nTargetValue) {
             return std::nullopt;
@@ -590,6 +591,9 @@ std::optional<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& a
         return result;
     }
 
+    // Decrease the already selected amount
+    value_to_select -= inputs.total_amount;
+
     unsigned int limit_ancestor_count = 0;
     unsigned int limit_descendant_count = 0;
     wallet.chain().getPackageLimits(limit_ancestor_count, limit_descendant_count);
@@ -606,8 +610,8 @@ std::optional<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& a
         available_coins.Shuffle(coin_selection_params.rng_fast);
     }
 
-    SelectionResult preselected(preset_inputs.GetSelectionAmount(), SelectionAlgorithm::MANUAL);
-    preselected.AddInput(preset_inputs);
+    SelectionResult preselected(inputs.total_amount, SelectionAlgorithm::MANUAL);
+    preselected.AddInputs(inputs.coins, coin_selection_params.m_subtract_fee_outputs);
 
     // Coin Selection attempts to select inputs from a pool of eligible UTXOs to fund the
     // transaction at a target feerate. If an attempt fails, more attempts may be made using a more
diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index c29e5be5c7..39832f363f 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -121,6 +121,26 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
 std::optional<SelectionResult> ChooseSelectionResult(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const std::vector<COutput>& available_coins,
                         const CoinSelectionParams& coin_selection_params);
 
+// User manually selected inputs that must be part of the transaction
+struct PreSelectedInputs
+{
+    std::set<COutput> coins;
+    // If subtract fee from outputs is disabled, the 'total_amount'
+    // will be the sum of each output effective value
+    // instead of the sum of the outputs amount
+    CAmount total_amount{0};
+
+    void Insert(const COutput& output, bool subtract_fee_outputs)
+    {
+        if (subtract_fee_outputs) {
+            total_amount += output.txout.nValue;
+        } else {
+            total_amount += output.GetEffectiveValue();
+        }
+        coins.insert(output);
+    }
+};
+
 /**
  * Select a set of coins such that nTargetValue is met and at least
  * all coins from coin_control are selected; never select unconfirmed coins if they are not ours