lnd: ensure that native SQL can only be used with a clean KV invoice DB

This commit adds a check to ensure that we don't start LND with native
SQL invoice DB if we already have any invoices in our KV channeldb. This
is needed as native SQL invoices is an experimental feature and we do
not yet support migration.

config_builder.go

@@ -1022,6 +1022,34 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 	// Instantiate a native SQL invoice store if the flag is set.
 	if d.cfg.DB.UseNativeSQL {
+		// KV invoice db resides in the same database as the graph and
+		// channel state DB. Let's query the database to see if we have
+		// any invoices there. If we do, we won't allow the user to
+		// start lnd with native SQL enabled, as we don't currently
+		// migrate the invoices to the new database schema.
+		invoiceSlice, err := dbs.GraphDB.QueryInvoices(
+			ctx, invoices.InvoiceQuery{
+				NumMaxInvoices: 1,
+			},
+		)
+		if err != nil {
+			cleanUp()
+			d.logger.Errorf("Unable to query KV invoice DB: %v",
+				err)
+
+			return nil, nil, err
+		}
+
+		if len(invoiceSlice.Invoices) > 0 {
+			cleanUp()
+			err := fmt.Errorf("found invoices in the KV invoice " +
+				"DB, migration to native SQL is not yet " +
+				"supported")
+			d.logger.Error(err)
+
+			return nil, nil, err
+		}
+
 		executor := sqldb.NewTransactionExecutor(
 			dbs.NativeSQLStore,
 			func(tx *sql.Tx) sqldb.InvoiceQueries {