DecodeArgs relies on a wrong behavior of json.Unmarshal

[bobotu]

Bug Report

In Go 1.15 (master branch), TiDB's DDL cannot work correctly and DDL related unit-test all failed.

The root cause is Job.DecodeArgs relies on the wrong behavior of json.Unmarshal, and this bug is fixed in Go 1.15.

The docs read:

To unmarshal a JSON array into a slice, Unmarshal resets the slice length to zero and then appends each element to the slice.

This means the value in args doesn't matter, and the value in args will be replaced by an interface{}(nil).

parser/model/ddl.go

Lines 284 to 288 in fb51991

	func (job *Job) DecodeArgs(args ...interface{}) error {
	job.Args = args
	err := json.Unmarshal(job.RawArgs, &job.Args)
	return errors.Trace(err)
	}

See golang/go#39427 for a more detailed discussion.