wallet: migrate wallet, exit early if no legacy data exist

otherwise the process will create a backup file then return
an error when notices that the db is already running sqlite.

Author: furszy

src/wallet/wallet.cpp

@@ -3839,10 +3839,7 @@ std::optional<MigrationData> CWallet::GetDescriptorsForLegacy(bilingual_str& err
     AssertLockHeld(cs_wallet);
 
     LegacyScriptPubKeyMan* legacy_spkm = GetLegacyScriptPubKeyMan();
-    if (!legacy_spkm) {
-        error = _("Error: This wallet is already a descriptor wallet");
-        return std::nullopt;
-    }
+    assert(legacy_spkm);
 
     std::optional<MigrationData> res = legacy_spkm->MigrateToDescriptor();
     if (res == std::nullopt) {
@@ -4138,6 +4135,11 @@ bool DoMigration(CWallet& wallet, WalletContext& context, bilingual_str& error,
 
 util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>&& wallet, WalletContext& context)
 {
+    // Before anything else, check if there is something to migrate.
+    if (!wallet->GetLegacyScriptPubKeyMan()) {
+        return util::Error{_("Error: This wallet is already a descriptor wallet")};
+    }
+
     MigrationResult res;
     bilingual_str error;
     std::vector<bilingual_str> warnings;

test/functional/wallet_migration.py

@@ -163,6 +163,10 @@ def test_basic(self):
         assert_equal(basic2.getbalance(), basic2_balance)
         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")
+        assert_raises_rpc_error(-4, "Error: This wallet is already a descriptor wallet", basic2.migratewallet)
+
     def test_multisig(self):
         default = self.nodes[0].get_wallet_rpc(self.default_wallet_name)