Merge bitcoin/bitcoin#25748: refactor: Avoid copies in FlatSigningProvider Merge

fa3f15f2dd refactor: Avoid copies in FlatSigningProvider Merge (MacroFake)

Pull request description:

  `Merge` will create several copies unconditionally:
  * To initialize the args `a`, and `b`
  * `ret`, which is the merge of the two args

  So change the code to let the caller decide how many copies they need/want:
  * `a`, and `b` must be explicitly moved or copied by the caller
  * `ret` is no longer needed, as `a` can be used for it in place "for free"

ACKs for top commit:
  achow101:
    ACK fa3f15f2dd
  furszy:
    looks good, ACK fa3f15f2
  ryanofsky:
    Code review ACK fa3f15f2dd. Confirmed that all the places `std::move` was added the argument actually did seem safe to move from. Compiler enforces that temporary copies are explicitly created in non-move cases.

Tree-SHA512: 7c027ccdea1549cd9f37403344ecbb76e008adf545f6ce52996bf95e89eb7dc89af6cb31435a9289d6f2eea1c416961b2fb96348bc8a211d550728f1d99ac49c

src/script/descriptor.cpp

@@ -644,7 +644,7 @@ public:
             assert(outscripts.size() == 1);
             subscripts.emplace_back(std::move(outscripts[0]));
         }
-        out = Merge(std::move(out), std::move(subprovider));
+        out.Merge(std::move(subprovider));
 
         std::vector<CPubKey> pubkeys;
         pubkeys.reserve(entries.size());

src/script/signingprovider.cpp

@@ -77,20 +77,14 @@ bool FlatSigningProvider::GetTaprootBuilder(const XOnlyPubKey& output_key, Tapro
     return LookupHelper(tr_trees, output_key, builder);
 }
 
-FlatSigningProvider Merge(const FlatSigningProvider& a, const FlatSigningProvider& b)
+FlatSigningProvider& FlatSigningProvider::Merge(FlatSigningProvider&& b)
 {
-    FlatSigningProvider ret;
-    ret.scripts = a.scripts;
-    ret.scripts.insert(b.scripts.begin(), b.scripts.end());
-    ret.pubkeys = a.pubkeys;
-    ret.pubkeys.insert(b.pubkeys.begin(), b.pubkeys.end());
-    ret.keys = a.keys;
-    ret.keys.insert(b.keys.begin(), b.keys.end());
-    ret.origins = a.origins;
-    ret.origins.insert(b.origins.begin(), b.origins.end());
-    ret.tr_trees = a.tr_trees;
-    ret.tr_trees.insert(b.tr_trees.begin(), b.tr_trees.end());
-    return ret;
+    scripts.merge(b.scripts);
+    pubkeys.merge(b.pubkeys);
+    keys.merge(b.keys);
+    origins.merge(b.origins);
+    tr_trees.merge(b.tr_trees);
+    return *this;
 }
 
 void FillableSigningProvider::ImplicitlyLearnRelatedKeyScripts(const CPubKey& pubkey)

src/script/signingprovider.h

@@ -6,6 +6,7 @@
 #ifndef BITCOIN_SCRIPT_SIGNINGPROVIDER_H
 #define BITCOIN_SCRIPT_SIGNINGPROVIDER_H
 
+#include <attributes.h>
 #include <key.h>
 #include <pubkey.h>
 #include <script/keyorigin.h>
@@ -85,9 +86,9 @@ struct FlatSigningProvider final : public SigningProvider
     bool GetKey(const CKeyID& keyid, CKey& key) const override;
     bool GetTaprootSpendData(const XOnlyPubKey& output_key, TaprootSpendData& spenddata) const override;
     bool GetTaprootBuilder(const XOnlyPubKey& output_key, TaprootBuilder& builder) const override;
-};
 
-FlatSigningProvider Merge(const FlatSigningProvider& a, const FlatSigningProvider& b);
+    FlatSigningProvider& Merge(FlatSigningProvider&& b) LIFETIMEBOUND;
+};
 
 /** Fillable signing provider that keeps keys in an address->secret map */
 class FillableSigningProvider : public SigningProvider

src/test/descriptor_tests.cpp

@@ -312,7 +312,7 @@ void DoCheck(const std::string& prv, const std::string& pub, const std::string&
                     txdata.Init(spend, std::move(utxos), /*force=*/true);
                     MutableTransactionSignatureCreator creator{spend, 0, CAmount{0}, &txdata, SIGHASH_DEFAULT};
                     SignatureData sigdata;
-                    BOOST_CHECK_MESSAGE(ProduceSignature(Merge(keys_priv, script_provider), creator, spks[n], sigdata), prv);
+                    BOOST_CHECK_MESSAGE(ProduceSignature(FlatSigningProvider{keys_priv}.Merge(FlatSigningProvider{script_provider}), creator, spks[n], sigdata), prv);
                 }
 
                 /* Infer a descriptor from the generated script, and verify its solvability and that it roundtrips. */

src/wallet/rpc/spend.cpp

@@ -644,7 +644,7 @@ void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out,
                     throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Unable to parse descriptor '%s': %s", desc_str, error));
                 }
                 desc->Expand(0, desc_out, scripts_temp, desc_out);
-                coinControl.m_external_provider = Merge(coinControl.m_external_provider, desc_out);
+                coinControl.m_external_provider.Merge(std::move(desc_out));
             }
         }
     }

src/wallet/scriptpubkeyman.cpp

@@ -2085,7 +2085,7 @@ std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvid
     // 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);
+        out_keys->Merge(FlatSigningProvider{it->second});
     } else {
         // Get the scripts, keys, and key origins for this script
         std::vector<CScript> scripts_temp;
@@ -2122,7 +2122,7 @@ bool DescriptorScriptPubKeyMan::SignTransaction(CMutableTransaction& tx, const s
         if (!coin_keys) {
             continue;
         }
-        *keys = Merge(*keys, *coin_keys);
+        keys->Merge(std::move(*coin_keys));
     }
 
     return ::SignTransaction(tx, keys.get(), coins, sighash, input_errors);
@@ -2183,7 +2183,7 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
         std::unique_ptr<FlatSigningProvider> keys = std::make_unique<FlatSigningProvider>();
         std::unique_ptr<FlatSigningProvider> script_keys = GetSigningProvider(script, sign);
         if (script_keys) {
-            *keys = Merge(*keys, *script_keys);
+            keys->Merge(std::move(*script_keys));
         } else {
             // Maybe there are pubkeys listed that we can sign for
             script_keys = std::make_unique<FlatSigningProvider>();
@@ -2191,7 +2191,7 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
                 const CPubKey& pubkey = pk_pair.first;
                 std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(pubkey);
                 if (pk_keys) {
-                    *keys = Merge(*keys, *pk_keys);
+                    keys->Merge(std::move(*pk_keys));
                 }
             }
             for (const auto& pk_pair : input.m_tap_bip32_paths) {
@@ -2203,7 +2203,7 @@ TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction&
                     fullpubkey.Set(b, b + 33);
                     std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(fullpubkey);
                     if (pk_keys) {
-                        *keys = Merge(*keys, *pk_keys);
+                        keys->Merge(std::move(*pk_keys));
                     }
                 }
             }