Function GetModifiableColumnJob

pkg/ddl/modify_column.go:1450–1731

GetModifiableColumnJob returns a DDL job of model.ActionModifyColumn.

(
	ctx context.Context,
	sctx sessionctx.Context,
	is infoschema.InfoSchema, // WARN: is maybe nil here.
	ident ast.Ident,
	originalColName pmodel.CIStr,
	schema *model.DBInfo,
	t table.Table,
	spec *ast.AlterTableSpec,
)

// GetModifiableColumnJob returns a DDL job of model.ActionModifyColumn.
func GetModifiableColumnJob(
	ctx context.Context,
	sctx sessionctx.Context,
	is infoschema.InfoSchema, // WARN: is maybe nil here.
	ident ast.Ident,
	originalColName pmodel.CIStr,
	schema *model.DBInfo,
	t table.Table,
	spec *ast.AlterTableSpec,
) (*JobWrapper, error) {
	var err error
	specNewColumn := spec.NewColumns[0]

	col := table.FindCol(t.Cols(), originalColName.L)
	if col == nil {
		return nil, infoschema.ErrColumnNotExists.GenWithStackByArgs(originalColName, ident.Name)
	}
	newColName := specNewColumn.Name.Name
	if newColName.L == model.ExtraHandleName.L {
		return nil, dbterror.ErrWrongColumnName.GenWithStackByArgs(newColName.L)
	}
	errG := checkModifyColumnWithGeneratedColumnsConstraint(t.Cols(), originalColName)

	// If we want to rename the column name, we need to check whether it already exists.
	if newColName.L != originalColName.L {
		c := table.FindCol(t.Cols(), newColName.L)
		if c != nil {
			return nil, infoschema.ErrColumnExists.GenWithStackByArgs(newColName)
		}

		// And also check the generated columns dependency, if some generated columns
		// depend on this column, we can't rename the column name.
		if errG != nil {
			return nil, errors.Trace(errG)
		}
	}

	// Constraints in the new column means adding new constraints. Errors should thrown,
	// which will be done by `processColumnOptions` later.
	if specNewColumn.Tp == nil {
		// Make sure the column definition is simple field type.
		return nil, errors.Trace(dbterror.ErrUnsupportedModifyColumn)
	}

	if err = checkColumnAttributes(specNewColumn.Name.OrigColName(), specNewColumn.Tp); err != nil {
		return nil, errors.Trace(err)
	}

	newCol := table.ToColumn(&model.ColumnInfo{
		ID: col.ID,
		// We use this PR(https://github.com/pingcap/tidb/pull/6274) as the dividing line to define whether it is a new version or an old version TiDB.
		// The old version TiDB initializes the column's offset and state here.
		// The new version TiDB doesn't initialize the column's offset and state, and it will do the initialization in run DDL function.
		// When we do the rolling upgrade the following may happen:
		// a new version TiDB builds the DDL job that doesn't be set the column's offset and state,
		// and the old version TiDB is the DDL owner, it doesn't get offset and state from the store. Then it will encounter errors.
		// So here we set offset and state to support the rolling upgrade.
		Offset:                col.Offset,