Placement option of FOLLOWERS=4 not supported

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> CREATE TABLE t1 (a int) FOLLOWERS=4;
ERROR 1105 (HY000): failed to notify PD the placement rules: "[PD:placement:ErrBuildRuleList]build rule list failed, needs at least one leader or voter for range {7480000000000000FF5000000000000000F8, 7480000000000000FF5100000000000000F8}"

2. What did you expect to see? (Required)

This should work. It implies there is a leader, and now 4 followers with no constraints.

3. What did you see instead (Required)

Error

4. What is your TiDB version? (Required)

mysql> SELECT tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v5.3.0-alpha-1064-gfd42198c3-dirty
Edition: Community
Git Commit Hash: fd42198c3916cbe666c6339784418db646d4eef6
Git Branch: revoke-all-with-role-syntax
UTC Build Time: 2021-10-12 00:13:16
GoVersion: go1.16.7
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[morgo]

This was first commented on #27974 under the subject of validation. But it should be a legal case that is accepted/translated by the rules engine.

[AilinKid]

inference about the unmentioned constraints, what do you think @xhebox @lcwangchao

[xhebox]

inference about the unmentioned constraints, what do you think @xhebox @lcwangchao

I think the thing is that morgo thought it is an sugar option, where primaryRegion is omitted. But in my implementation, it is the non-sugar syntax where every thing needs to be explicit. @morgo

If that is the case, it is easy to fix.

[morgo]

I think we should consider it both sugar/non-sugar, although it can't be specified in non-sugar with dict constraints.

Yes, let's fix it :-) The leader can be implied as 1, and constraints can be implied as none.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.