Merge bitcoin/bitcoin#26668: wallet: if only have one output type, don't perform "mixed" coin selection

89c1491d35389eac0c1fecc59333cdfae3b1bd2c wallet: if only have one output type, don't perform "mixed" coin selection (furszy)

Pull request description:

  For wallets that only have one output type, we are currently performing the same
  selection process over the same coins twice.

  The "mixed coin selection" doesn't add any value to the result
  (there is nothing to mix if the available coins struct has only one type).

ACKs for top commit:
  achow101:
    ACK 89c1491d35389eac0c1fecc59333cdfae3b1bd2c
  john-moffett:
    ACK 89c1491d35389eac0c1fecc59333cdfae3b1bd2c
  kristapsk:
    cr utACK 89c1491d35389eac0c1fecc59333cdfae3b1bd2c

Tree-SHA512: 672eaeed3ba911d13fa61a46f719c8fe1ebe4d2dc7d723040e71937c693659411bc99cdbd9f0014e836b70eebeff1b8ca861f4d81d39e6f79f437364a526edbe

2 files changed, 5 insertions, 2 deletions

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index dd3f0e99d1..33289c1d2f 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -525,8 +525,9 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
     if (results.size() > 0) return *std::min_element(results.begin(), results.end());
 
     // If we can't fund the transaction from any individual OutputType, run coin selection one last time
-    // over all available coins, which would allow mixing
-    if (allow_mixed_output_types) {
+    // 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;
         }
diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index 2b861c2361..46144d4c92 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -46,6 +46,8 @@ struct CoinsResult {
     /** The following methods are provided so that CoinsResult can mimic a vector,
      * i.e., methods can work with individual OutputType vectors or on the entire object */
     size_t Size() const;
+    /** Return how many different output types this struct stores */
+    size_t TypesCount() const { return coins.size(); }
     void Clear();
     void Erase(const std::unordered_set<COutPoint, SaltedOutpointHasher>& coins_to_remove);
     void Shuffle(FastRandomContext& rng_fast);