Change MigrateLegacyToDescriptor to reopen wallet as BERKELEY_RO

When we reopen the wallet to do the migration, instead of opening using
BDB, open it using the BerkeleyRO implementation.

src/wallet/wallet.cpp

@@ -2923,7 +2923,7 @@ bool CWallet::EraseAddressReceiveRequest(WalletBatch& batch, const CTxDestinatio
     return true;
 }
 
-std::unique_ptr<WalletDatabase> MakeWalletDatabase(const std::string& name, const DatabaseOptions& options, DatabaseStatus& status, bilingual_str& error_string)
+static util::Result<fs::path> GetWalletPath(const std::string& name)
 {
     // Do some checking on wallet path. It should be either a:
     //
@@ -2936,15 +2936,24 @@ std::unique_ptr<WalletDatabase> MakeWalletDatabase(const std::string& name, cons
     if (!(path_type == fs::file_type::not_found || path_type == fs::file_type::directory ||
           (path_type == fs::file_type::symlink && fs::is_directory(wallet_path)) ||
           (path_type == fs::file_type::regular && fs::PathFromString(name).filename() == fs::PathFromString(name)))) {
-        error_string = Untranslated(strprintf(
+        return util::Error{Untranslated(strprintf(
               "Invalid -wallet path '%s'. -wallet path should point to a directory where wallet.dat and "
               "database/log.?????????? files can be stored, a location where such a directory could be created, "
               "or (for backwards compatibility) the name of an existing data file in -walletdir (%s)",
-              name, fs::quoted(fs::PathToString(GetWalletDir()))));
+              name, fs::quoted(fs::PathToString(GetWalletDir()))))};
+    }
+    return wallet_path;
+}
+
+std::unique_ptr<WalletDatabase> MakeWalletDatabase(const std::string& name, const DatabaseOptions& options, DatabaseStatus& status, bilingual_str& error_string)
+{
+    const auto& wallet_path = GetWalletPath(name);
+    if (!wallet_path) {
+        error_string = util::ErrorString(wallet_path);
         status = DatabaseStatus::FAILED_BAD_PATH;
         return nullptr;
     }
-    return MakeDatabase(wallet_path, options, status, error_string);
+    return MakeDatabase(*wallet_path, options, status, error_string);
 }
 
 std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::string& name, std::unique_ptr<WalletDatabase> database, uint64_t wallet_creation_flags, bilingual_str& error, std::vector<bilingual_str>& warnings)
@@ -4346,11 +4355,24 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     // 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;
     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")};
         }
         UnloadWallet(std::move(wallet));
         was_loaded = true;
+    } else {
+        // Check if the wallet is BDB
+        const auto& wallet_path = GetWalletPath(wallet_name);
+        if (!wallet_path) {
+            return util::Error{util::ErrorString(wallet_path)};
+        }
+        if (!IsBDBFile(BDBDataFile(*wallet_path))) {
+            return util::Error{_("Error: This wallet is already a descriptor wallet")};
+        }
     }
 
     // Load the wallet but only in the context of this function.
@@ -4359,6 +4381,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     empty_context.args = context.args;
     DatabaseOptions options;
     options.require_existing = true;
+    options.require_format = DatabaseFormat::BERKELEY_RO;
     DatabaseStatus status;
     std::unique_ptr<WalletDatabase> database = MakeWalletDatabase(wallet_name, options, status, error);
     if (!database) {
@@ -4373,6 +4396,8 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
 
     // Helper to reload as normal for some of our exit scenarios
     const auto& reload_wallet = [&](std::shared_ptr<CWallet>& to_reload) {
+        // Reset options.require_format as wallets of any format may be reloaded.
+        options.require_format = std::nullopt;
         assert(to_reload.use_count() == 1);
         std::string name = to_reload->GetName();
         to_reload.reset();

test/functional/wallet_migration.py

@@ -205,9 +205,13 @@ class WalletMigrationTest(BitcoinTestFramework):
         self.assert_list_txs_equal(basic2.listtransactions(), basic2_txs)
 
         # Now test migration on a descriptor wallet
-        self.log.info("Test \"nothing to migrate\" when the user tries to migrate a wallet with no legacy data")
+        self.log.info("Test \"nothing to migrate\" when the user tries to migrate a loaded wallet with no legacy data")
         assert_raises_rpc_error(-4, "Error: This wallet is already a descriptor wallet", basic2.migratewallet)
 
+        self.log.info("Test \"nothing to migrate\" when the user tries to migrate an unloaded wallet with no legacy data")
+        basic2.unloadwallet()
+        assert_raises_rpc_error(-4, "Error: This wallet is already a descriptor wallet", self.nodes[0].migratewallet, "basic2")
+
     def test_multisig(self):
         default = self.nodes[0].get_wallet_rpc(self.default_wallet_name)