feat(move-tables): fix target table creation to use target DB + abort if exists (#8207)

go/logic/applier.go

@@ -596,16 +596,22 @@ func (apl *Applier) createTargetTable(targetTableName string) error {
 }
 
 // createTargetTableFromStatement creates the table on the applier host to which the applier will
-// apply changes.
+// apply changes. In move-tables mode this executes on moveTablesTargetDB (the target cluster);
+// in standard mode it executes on apl.db (the source/applier host).
 func (apl *Applier) createTargetTableFromStatement(targetTableName, createStatement string) error {
 	targetDatabase := apl.migrationContext.GetTargetDatabaseName()
 	apl.migrationContext.Log.Infof("Creating target table %s.%s",
 		sql.EscapeName(targetDatabase),
 		sql.EscapeName(targetTableName),
 	)
 
+	db := apl.db
+	if apl.migrationContext.IsMoveTablesMode() {
+		db = apl.moveTablesTargetDB
+	}
+
 	err := func() error {
-		tx, err := apl.db.Begin()
+		tx, err := db.Begin()
 		if err != nil {
 			return err
 		}
@@ -640,12 +646,35 @@ func (apl *Applier) CreateGhostTable() error {
 	return apl.createTargetTable(apl.migrationContext.GetGhostTableName())
 }
 
-// CreateTargetTable creates the target table on the target host (for move-tables)
+// CreateTargetTable creates the target table on the target host (for move-tables).
+// It aborts with an error if the target table already exists on the target cluster,
+// to prevent silently writing into a table that has unrelated data or a different
+// schema (move_table_mode.md §1.3: "Don't use IF NOT EXISTS for the target table.
+// An existing table is an error condition, not a no-op.").
 func (apl *Applier) CreateTargetTable(createStatement string) error {
 	if !apl.migrationContext.IsMoveTablesMode() {
 		return errors.New("CreateTargetTable is only available in MoveTables mode")
 	}
-	return apl.createTargetTableFromStatement(apl.originalTableName(), createStatement)
+	targetTableName := apl.originalTableName()
+	targetDatabase := apl.migrationContext.GetTargetDatabaseName()
+
+	// Explicit pre-check: abort before any data is copied if the target table
+	// already exists. The CREATE TABLE would also fail (MySQL ERROR 1050), but
+	// this gives operators a clear gh-ost error message explaining what to do.
+	var count int
+	err := apl.moveTablesTargetDB.QueryRow(
+		"SELECT COUNT(*) FROM information_schema.tables WHERE table_schema=? AND table_name=?",
+		targetDatabase, targetTableName,
+	).Scan(&count)
+	if err != nil {
+		return fmt.Errorf("failed to check for existing target table: %w", err)
+	}
+	if count > 0 {
+		return fmt.Errorf("target table %s.%s already exists on the target cluster. Aborting to prevent writing into a table with unrelated data. Drop the table manually if this is intentional",
+			sql.EscapeName(targetDatabase), sql.EscapeName(targetTableName))
+	}
+
+	return apl.createTargetTableFromStatement(targetTableName, createStatement)
 }
 
 // AlterGhost applies `alter` statement on ghost table

go/logic/applier_test.go

@@ -770,6 +770,114 @@ func (suite *ApplierTestSuite) TestCreateGhostTable() {
 	suite.Require().Equal("CREATE TABLE `_testing_gho` (\n  `id` int DEFAULT NULL,\n  `item_id` int DEFAULT NULL\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci", createDDL)
 }
 
+// TestCreateTargetTable_HappyPath exercises #8207 AC #1:
+// "A move table run targeting a clean target schema creates the migrated table
+// with a SHOW CREATE TABLE output equivalent to the source's."
+//
+// It calls CreateTargetTable (not just IsMoveTablesMode()), asserts the table
+// exists on the target database, verifies schema equivalence via SHOW CREATE TABLE,
+// and confirms no table was accidentally created on the source.
+func (suite *ApplierTestSuite) TestCreateTargetTable_HappyPath() {
+	ctx := context.Background()
+
+	_, err := suite.db.ExecContext(ctx, fmt.Sprintf("CREATE TABLE %s (id INT PRIMARY KEY, name VARCHAR(64), updated_at DATETIME);", getTestTableName()))
+	suite.Require().NoError(err)
+
+	connectionConfig, err := getTestConnectionConfig(ctx, suite.mysqlContainer)
+	suite.Require().NoError(err)
+
+	migrationContext := newTestMigrationContext()
+	migrationContext.MoveTables.TableNames = []string{testMysqlTableName}
+	migrationContext.MoveTables.TargetDatabase = testMysqlDatabaseOther
+	migrationContext.ApplierConnectionConfig = connectionConfig
+	migrationContext.MoveTables.ConnectionConfig = connectionConfig
+	migrationContext.SetConnectionConfig("innodb")
+	migrationContext.OriginalTableColumns = sql.NewColumnList([]string{"id", "name", "updated_at"})
+
+	applier := NewApplier(migrationContext)
+	defer applier.Teardown()
+
+	err = applier.InitDBConnections()
+	suite.Require().NoError(err)
+
+	var dummy, sourceCreateDDL string
+	err = suite.db.QueryRow(fmt.Sprintf("SHOW CREATE TABLE %s", getTestTableName())).Scan(&dummy, &sourceCreateDDL)
+	suite.Require().NoError(err)
+
+	var count int
+	err = suite.otherDB.QueryRow(
+		"SELECT COUNT(*) FROM information_schema.tables WHERE table_schema=? AND table_name=?",
+		testMysqlDatabaseOther, testMysqlTableName,
+	).Scan(&count)
+	suite.Require().NoError(err)
+	suite.Require().Equal(0, count, "precondition: target table must not exist before CreateTargetTable")
+
+	err = applier.CreateTargetTable(sourceCreateDDL)
+	suite.Require().NoError(err)
+
+	var targetTableName, targetCreateDDL string
+	err = suite.otherDB.QueryRow(fmt.Sprintf("SHOW CREATE TABLE `%s`.`%s`", testMysqlDatabaseOther, testMysqlTableName)).Scan(&targetTableName, &targetCreateDDL)
+	suite.Require().NoError(err)
+	suite.Require().Equal(testMysqlTableName, targetTableName)
+	suite.Require().Equal(sourceCreateDDL, targetCreateDDL, "target table schema must be equivalent to source")
+
+	err = suite.otherDB.QueryRow(
+		"SELECT COUNT(*) FROM information_schema.tables WHERE table_schema=? AND table_name=?",
+		testMysqlDatabaseOther, testMysqlTableName,
+	).Scan(&count)
+	suite.Require().NoError(err)
+	suite.Require().Equal(1, count, "target table must exist exactly once on the target database")
+}
+
+// TestCreateTargetTable_AbortsIfExists exercises #8207 AC #2:
+// "A move table run aborts before any data is copied if the target table already exists."
+//
+// It pre-creates the target table, then calls CreateTargetTable and asserts it
+// returns a descriptive error (not just MySQL's raw ERROR 1050).
+func (suite *ApplierTestSuite) TestCreateTargetTable_AbortsIfExists() {
+	ctx := context.Background()
+
+	_, err := suite.db.ExecContext(ctx, fmt.Sprintf("CREATE TABLE %s (id INT PRIMARY KEY);", getTestTableName()))
+	suite.Require().NoError(err)
+
+	_, err = suite.otherDB.ExecContext(ctx, fmt.Sprintf("CREATE TABLE `%s`.`%s` (id INT PRIMARY KEY);", testMysqlDatabaseOther, testMysqlTableName))
+	suite.Require().NoError(err)
+
+	var count int
+	err = suite.otherDB.QueryRow(
+		"SELECT COUNT(*) FROM information_schema.tables WHERE table_schema=? AND table_name=?",
+		testMysqlDatabaseOther, testMysqlTableName,
+	).Scan(&count)
+	suite.Require().NoError(err)
+	suite.Require().Equal(1, count, "precondition: target table must exist before CreateTargetTable")
+
+	connectionConfig, err := getTestConnectionConfig(ctx, suite.mysqlContainer)
+	suite.Require().NoError(err)
+
+	migrationContext := newTestMigrationContext()
+	migrationContext.MoveTables.TableNames = []string{testMysqlTableName}
+	migrationContext.MoveTables.TargetDatabase = testMysqlDatabaseOther
+	migrationContext.ApplierConnectionConfig = connectionConfig
+	migrationContext.MoveTables.ConnectionConfig = connectionConfig
+	migrationContext.SetConnectionConfig("innodb")
+	migrationContext.OriginalTableColumns = sql.NewColumnList([]string{"id"})
+
+	applier := NewApplier(migrationContext)
+	defer applier.Teardown()
+
+	err = applier.InitDBConnections()
+	suite.Require().NoError(err)
+
+	var dummy, sourceCreateDDL string
+	err = suite.db.QueryRow(fmt.Sprintf("SHOW CREATE TABLE %s", getTestTableName())).Scan(&dummy, &sourceCreateDDL)
+	suite.Require().NoError(err)
+
+	err = applier.CreateTargetTable(sourceCreateDDL)
+	suite.Require().Error(err, "CreateTargetTable must return an error when target table already exists")
+	suite.Require().Contains(err.Error(), "already exists", "error message must mention 'already exists'")
+	suite.Require().Contains(err.Error(), testMysqlTableName, "error message must name the table")
+}
+
 func (suite *ApplierTestSuite) TestPanicOnWarningsInApplyIterationInsertQuerySucceedsWithUniqueKeyWarningInsertedByDMLEvent() {
 	ctx := context.Background()