ddl: support the rolling upgrade.

ddl/column.go

@@ -460,6 +460,13 @@ func (d *ddl) doModifyColumn(t *meta.Meta, job *model.Job, newCol *model.ColumnI
 		}
 	}
 
+	// gofail: var uninitializedOffsetAndState bool
+	// if uninitializedOffsetAndState {
+	// if newCol.State != model.StatePublic {
+	//      return ver, errors.New("the column state is wrong")
+	// }
+	// }
+
 	// We need the latest column's offset and state. This information can be obtained from the store.
 	newCol.Offset = oldCol.Offset
 	newCol.State = oldCol.State

ddl/ddl_api.go

@@ -1372,7 +1372,16 @@ func (d *ddl) getModifiableColumnJob(ctx sessionctx.Context, ident ast.Ident, or
 	}
 
 	newCol := table.ToColumn(&model.ColumnInfo{
-		ID:                 col.ID,
+		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,
+		State:              col.State,
 		OriginDefaultValue: col.OriginDefaultValue,
 		FieldType:          *specNewColumn.Tp,
 		Name:               newColName,

ddl/fail_db_test.go

@@ -0,0 +1,35 @@
+// Copyright 2018 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ddl_test
+
+import (
+	gofail "github.com/coreos/gofail/runtime"
+	. "github.com/pingcap/check"
+	"golang.org/x/net/context"
+)
+
+// TestInitializeOffsetAndState tests the case that the column's offset and state don't be initialized in the file of ddl_api.go when
+// doing the operation of 'modify column'.
+func (s *testStateChangeSuite) TestInitializeOffsetAndState(c *C) {
+	_, err := s.se.Execute(context.Background(), "use test_db_state")
+	c.Assert(err, IsNil)
+	_, err = s.se.Execute(context.Background(), "create table t(a int, b int, c int)")
+	c.Assert(err, IsNil)
+	defer s.se.Execute(context.Background(), "drop table t")
+
+	gofail.Enable("github.com/pingcap/tidb/ddl/uninitializedOffsetAndState", `return(true)`)
+	_, err = s.se.Execute(context.Background(), "ALTER TABLE t MODIFY COLUMN b int FIRST;")
+	c.Assert(err, IsNil)
+	gofail.Disable("github.com/pingcap/tidb/ddl/uninitializedOffsetAndState")
+}