wallet: return accurate error messages from Coin Selection

and not the general "Insufficient funds" when the wallet
actually have funds.

Two new error messages:

1) If the selection result exceeds the maximum transaction weight,
   we now will return: "The inputs size exceeds the maximum weight".

2) If the user preselected inputs and disallowed the automatic coin
   selection process (no other inputs are allowed), we now will
   return: "The preselected coins total amount does not cover the
   transaction target".

2 files changed, 40 insertions, 15 deletions

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index 95fec6e96c..6bd9563eae 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -510,15 +510,20 @@ std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<C
     return groups_out;
 }
 
+// Returns true if the result contains an error and the message is not empty
+static bool HasErrorMsg(const util::Result<SelectionResult>& res) { return !util::ErrorString(res).empty(); }
+
 util::Result<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const CoinsResult& available_coins,
                                const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types)
 {
     // Run coin selection on each OutputType and compute the Waste Metric
     std::vector<SelectionResult> results;
     for (const auto& it : available_coins.coins) {
-        if (auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, it.second, coin_selection_params)}) {
-            results.push_back(*result);
-        }
+        auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, it.second, coin_selection_params)};
+        // If any specific error message appears here, then something particularly wrong happened.
+        if (HasErrorMsg(result)) return result; // So let's return the specific error.
+        // Append the favorable result.
+        if (result) results.push_back(*result);
     }
     // If we have at least one solution for funding the transaction without mixing, choose the minimum one according to waste metric
     // and return the result
@@ -528,9 +533,7 @@ util::Result<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmo
     // over all available coins, which would allow mixing.
     // If TypesCount() <= 1, there is nothing to mix.
     if (allow_mixed_output_types && available_coins.TypesCount() > 1) {
-        if (auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, available_coins.All(), coin_selection_params)}) {
-            return result;
-        }
+        return ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, available_coins.All(), coin_selection_params);
     }
     // Either mixing is not allowed and we couldn't find a solution from any single OutputType, or mixing was allowed and we still couldn't
     // find a solution using all available coins
@@ -571,7 +574,8 @@ util::Result<SelectionResult> ChooseSelectionResult(const CWallet& wallet, const
     });
 
     if (eligible_results.empty()) {
-        return util::Error();
+        return util::Error{_("The inputs size exceeds the maximum weight. "
+                             "Please try sending a smaller amount or manually consolidating your wallet's UTXOs")};
     }
 
     // Choose the result with the least waste
@@ -588,7 +592,10 @@ util::Result<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& av
     CAmount selection_target = nTargetValue - pre_set_inputs.total_amount;
 
     // Return if automatic coin selection is disabled, and we don't cover the selection target
-    if (!coin_control.m_allow_other_inputs && selection_target > 0) return util::Error();
+    if (!coin_control.m_allow_other_inputs && selection_target > 0) {
+        return util::Error{_("The preselected coins total amount does not cover the transaction target. "
+                             "Please allow other inputs to be automatically selected or include more coins manually")};
+    }
 
     // Return if we can cover the target only with the preset inputs
     if (selection_target <= 0) {
@@ -681,13 +688,23 @@ util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, Coin
             }
         }
 
-        // Walk-through the filters until the solution gets found
+        // Walk-through the filters until the solution gets found.
+        // If no solution is found, return the first detailed error (if any).
+        // future: add "error level" so the worst one can be picked instead.
+        std::vector<util::Result<SelectionResult>> res_detailed_errors;
         for (const auto& select_filter : ordered_filters) {
             if (auto res{AttemptSelection(wallet, value_to_select, select_filter.filter, available_coins,
-                                          coin_selection_params, select_filter.allow_mixed_output_types)}) return res;
+                                          coin_selection_params, select_filter.allow_mixed_output_types)}) {
+                return res; // result found
+            } else {
+                // If any specific error message appears here, then something particularly wrong might have happened.
+                // Save the error and continue the selection process. So if no solutions gets found, we can return
+                // the detailed error to the upper layers.
+                if (HasErrorMsg(res)) res_detailed_errors.emplace_back(res);
+            }
         }
         // Coin Selection failed.
-        return util::Result<SelectionResult>(util::Error());
+        return res_detailed_errors.empty() ? util::Result<SelectionResult>(util::Error()) : res_detailed_errors.front();
     }();
 
     return res;
@@ -916,7 +933,9 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
     // Choose coins to use
     auto select_coins_res = SelectCoins(wallet, available_coins, preset_inputs, /*nTargetValue=*/selection_target, coin_control, coin_selection_params);
     if (!select_coins_res) {
-        return util::Error{_("Insufficient funds")};
+        // 'SelectCoins' either returns a specific error message or, if empty, means a general "Insufficient funds".
+        const bilingual_str& err = util::ErrorString(select_coins_res);
+        return util::Error{err.empty() ?_("Insufficient funds") : err};
     }
     const SelectionResult& result = *select_coins_res;
     TRACE5(coin_selection, selected_coins, wallet.GetName().c_str(), GetAlgorithmName(result.GetAlgo()).c_str(), result.GetTarget(), result.GetWaste(), result.GetSelectedValue());
diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index 68a277f85c..7afab27acf 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -119,7 +119,9 @@ std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<C
  * param@[in]  coin_selection_params     Parameters for the coin selection
  * param@[in]  allow_mixed_output_types  Relax restriction that SelectionResults must be of the same OutputType
  * returns                               If successful, a SelectionResult containing the input set
- *                                       If failed, a nullopt
+ *                                       If failed, returns (1) an empty error message if the target was not reached (general "Insufficient funds")
+ *                                                  or (2) an specific error message if there was something particularly wrong (e.g. a selection
+ *                                                  result that surpassed the tx max weight size).
  */
 util::Result<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const CoinsResult& available_coins,
                         const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types);
@@ -135,7 +137,9 @@ util::Result<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmo
  * param@[in]  available_coins           The struct of coins, organized by OutputType, available for selection prior to filtering
  * param@[in]  coin_selection_params     Parameters for the coin selection
  * returns                               If successful, a SelectionResult containing the input set
- *                                       If failed, a nullopt
+ *                                       If failed, returns (1) an empty error message if the target was not reached (general "Insufficient funds")
+ *                                                  or (2) an specific error message if there was something particularly wrong (e.g. a selection
+ *                                                  result that surpassed the tx max weight size).
  */
 util::Result<SelectionResult> ChooseSelectionResult(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const std::vector<COutput>& available_coins,
                         const CoinSelectionParams& coin_selection_params);
@@ -175,7 +179,9 @@ util::Result<PreSelectedInputs> FetchSelectedInputs(const CWallet& wallet, const
  * param@[in]   coin_selection_params  Parameters for this coin selection such as feerates, whether to avoid partial spends,
  *                                     and whether to subtract the fee from the outputs.
  * returns                             If successful, a SelectionResult containing the selected coins
- *                                     If failed, a nullopt.
+ *                                     If failed, returns (1) an empty error message if the target was not reached (general "Insufficient funds")
+ *                                                or (2) an specific error message if there was something particularly wrong (e.g. a selection
+ *                                                result that surpassed the tx max weight size).
  */
 util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, CoinsResult& available_coins, const CAmount& nTargetValue, const CCoinControl& coin_control,
                  const CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);