wallet: Cache SigningProviders

In order to avoid constantly re-deriving the same keys in
DescriptorScriptPubKeyMan, cache the SigningProviders generated inside
of GetSigningProvider.

src/wallet/scriptpubkeyman.cpp

@@ -2075,10 +2075,21 @@ std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvid
 std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvider(int32_t index, bool include_private) const
 {
     AssertLockHeld(cs_desc_man);
-    // Get the scripts, keys, and key origins for this script
+
     std::unique_ptr<FlatSigningProvider> out_keys = std::make_unique<FlatSigningProvider>();
-    std::vector<CScript> scripts_temp;
-    if (!m_wallet_descriptor.descriptor->ExpandFromCache(index, m_wallet_descriptor.cache, scripts_temp, *out_keys)) return nullptr;
+
+    // Fetch SigningProvider from cache to avoid re-deriving
+    auto it = m_map_signing_providers.find(index);
+    if (it != m_map_signing_providers.end()) {
+        *out_keys = Merge(*out_keys, it->second);
+    } else {
+        // Get the scripts, keys, and key origins for this script
+        std::vector<CScript> scripts_temp;
+        if (!m_wallet_descriptor.descriptor->ExpandFromCache(index, m_wallet_descriptor.cache, scripts_temp, *out_keys)) return nullptr;
+
+        // Cache SigningProvider so we don't need to re-derive if we need this SigningProvider again
+        m_map_signing_providers[index] = *out_keys;
+    }
 
     if (HavePrivateKeys() && include_private) {
         FlatSigningProvider master_provider;

src/wallet/scriptpubkeyman.h

@@ -547,6 +547,8 @@ private:
 
     KeyMap GetKeys() const EXCLUSIVE_LOCKS_REQUIRED(cs_desc_man);
 
+    // Cached FlatSigningProviders to avoid regenerating them each time they are needed.
+    mutable std::map<int32_t, FlatSigningProvider> m_map_signing_providers;
     // Fetch the SigningProvider for the given script and optionally include private keys
     std::unique_ptr<FlatSigningProvider> GetSigningProvider(const CScript& script, bool include_private = false) const;
     // Fetch the SigningProvider for the given pubkey and always include private keys. This should only be called by signing code.