lnd: plug in graph SQL migration into dev build

This commit plugs in the graph kvdb-to-sql migration for builds
containing the `test_native_sql` tag. This will allow us to perform
local tests and write itests for the migration without exposing it to
the production release build.

config_test_native_sql.go

@@ -3,11 +3,15 @@
 package lnd
 
 import (
+	"context"
 	"database/sql"
+	"fmt"
 
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/sqldb"
+	"github.com/lightningnetwork/lnd/sqldb/sqlc"
 )
 
 // getGraphStore returns a graphdb.V1Store backed by a graphdb.SQLStore
@@ -29,3 +33,32 @@ func (d *DefaultDatabaseBuilder) getGraphStore(baseDB *sqldb.BaseDB,
 		graphExecutor, opts...,
 	)
 }
+
+// graphSQLMigration is the version number for the graph migration
+// that migrates the KV graph to the native SQL schema.
+const graphSQLMigration = 9
+
+// getSQLMigration returns a migration function for the given version.
+func getSQLMigration(ctx context.Context, version int,
+	kvBackend kvdb.Backend,
+	chain chainhash.Hash) (func(tx *sqlc.Queries) error, bool) {
+
+	switch version {
+	case graphSQLMigration:
+		return func(tx *sqlc.Queries) error {
+			err := graphdb.MigrateGraphToSQL(
+				ctx, kvBackend, tx, chain,
+			)
+			if err != nil {
+				return fmt.Errorf("failed to migrate graph "+
+					"to SQL: %w", err)
+			}
+
+			return nil
+		}, true
+	}
+
+	// No version was matched, so we return false to indicate that no
+	// migration is known for the given version.
+	return nil, false
+}