wallet: remove dead code in legacy wallet migration

A discussion on a previous PR 32481 related to legacy wallet dead
code removal made me realize that checking if the legacy
wallet was loaded prior to the start of the migration is not
required ever since legacy wallets can't be loaded in the first
place. I also verified that the `load_on_start` persistent
setting can also not cause the legacy wallets to be loaded, which
further makes the case for removal of the above mentioned checks
during migration.
The current test coverage also shows these lines uncovered.

Author: rkrux

src/bench/wallet_migration.cpp

@@ -66,7 +66,7 @@ static void WalletMigration(benchmark::Bench& bench)
 
     bench.epochs(/*numEpochs=*/1).epochIterations(/*numIters=*/1) // run the migration exactly once
          .run([&] {
-             auto res{MigrateLegacyToDescriptor(std::move(wallet), /*passphrase=*/"", *loader->context(), /*was_loaded=*/false)};
+             auto res{MigrateLegacyToDescriptor(std::move(wallet), /*passphrase=*/"", *loader->context())};
              assert(res);
              assert(res->wallet);
              assert(res->watchonly_wallet);

src/wallet/wallet.cpp

@@ -4174,18 +4174,10 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     std::vector<bilingual_str> warnings;
     bilingual_str error;
 
-    // If the wallet is still loaded, unload it so that nothing else tries to use it while we're changing it
-    bool was_loaded = false;
+    // The only kind of wallets that could be loaded are descriptor ones, which don't need to be migrated.
     if (auto wallet = GetWallet(context, wallet_name)) {
-        if (wallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
-            return util::Error{_("Error: This wallet is already a descriptor wallet")};
-        }
-
-        if (!RemoveWallet(context, wallet, /*load_on_start=*/std::nullopt, warnings)) {
-            return util::Error{_("Unable to unload the wallet before migrating")};
-        }
-        WaitForDeleteWallet(std::move(wallet));
-        was_loaded = true;
+        assert(wallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));
+        return util::Error{_("Error: This wallet is already a descriptor wallet")};
     } else {
         // Check if the wallet is BDB
         const auto& wallet_path = GetWalletPath(wallet_name);
@@ -4219,10 +4211,10 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
         return util::Error{Untranslated("Wallet loading failed.") + Untranslated(" ") + error};
     }
 
-    return MigrateLegacyToDescriptor(std::move(local_wallet), passphrase, context, was_loaded);
+    return MigrateLegacyToDescriptor(std::move(local_wallet), passphrase, context);
 }
 
-util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet> local_wallet, const SecureString& passphrase, WalletContext& context, bool was_loaded)
+util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet> local_wallet, const SecureString& passphrase, WalletContext& context)
 {
     MigrationResult res;
     bilingual_str error;
@@ -4245,9 +4237,6 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
 
     // Before anything else, check if there is something to migrate.
     if (local_wallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
-        if (was_loaded) {
-            reload_wallet(local_wallet);
-        }
         return util::Error{_("Error: This wallet is already a descriptor wallet")};
     }
 
@@ -4256,9 +4245,6 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
     fs::path backup_filename = fs::PathFromString(strprintf("%s_%d.legacy.bak", (wallet_name.empty() ? "default_wallet" : wallet_name), GetTime()));
     fs::path backup_path = this_wallet_dir / backup_filename;
     if (!local_wallet->BackupWallet(fs::PathToString(backup_path))) {
-        if (was_loaded) {
-            reload_wallet(local_wallet);
-        }
         return util::Error{_("Error: Unable to make a backup of your wallet")};
     }
     res.backup_path = backup_path;
@@ -4267,9 +4253,6 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
 
     // Unlock the wallet if needed
     if (local_wallet->IsLocked() && !local_wallet->Unlock(passphrase)) {
-        if (was_loaded) {
-            reload_wallet(local_wallet);
-        }
         if (passphrase.find('\0') == std::string::npos) {
             return util::Error{Untranslated("Error: Wallet decryption failed, the wallet passphrase was not provided or was incorrect.")};
         } else {
@@ -4379,23 +4362,19 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
 
         // Restore the backup
         // Convert the backup file to the wallet db file by renaming it and moving it into the wallet's directory.
-        // Reload it into memory if the wallet was previously loaded.
         bilingual_str restore_error;
-        const auto& ptr_wallet = RestoreWallet(context, temp_backup_location, wallet_name, /*load_on_start=*/std::nullopt, status, restore_error, warnings, /*load_after_restore=*/was_loaded);
+        const auto& ptr_wallet = RestoreWallet(context, temp_backup_location, wallet_name, /*load_on_start=*/std::nullopt, status, restore_error, warnings, /*load_after_restore=*/false);
         if (!restore_error.empty()) {
             error += restore_error + _("\nUnable to restore backup of wallet.");
             return util::Error{error};
         }
+        // Verify that the legacy wallet is not loaded after restoring from the backup.
+        assert(!ptr_wallet);
 
         // The wallet directory has been restored, but just in case, copy the previously created backup to the wallet dir
         fs::copy_file(temp_backup_location, backup_path, fs::copy_options::none);
         fs::remove(temp_backup_location);
 
-        // Verify that there is no dangling wallet: when the wallet wasn't loaded before, expect null.
-        // This check is performed after restoration to avoid an early error before saving the backup.
-        bool wallet_reloaded = ptr_wallet != nullptr;
-        assert(was_loaded == wallet_reloaded);
-
         return util::Error{error};
     }
     return res;

src/wallet/wallet.h

@@ -1137,7 +1137,7 @@ struct MigrationResult {
 //! Do all steps to migrate a legacy wallet to a descriptor wallet
 [[nodiscard]] util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& wallet_name, const SecureString& passphrase, WalletContext& context);
 //! Requirement: The wallet provided to this function must be isolated, with no attachment to the node's context.
-[[nodiscard]] util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet> local_wallet, const SecureString& passphrase, WalletContext& context, bool was_loaded);
+[[nodiscard]] util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet> local_wallet, const SecureString& passphrase, WalletContext& context);
 } // namespace wallet
 
 #endif // BITCOIN_WALLET_WALLET_H