Ensure that table_options respect "Policy Validation" and "Skipping Policy Validation"

[morgo]

Enhancement

Subtask of #18030

We need to ensure that table_options respect "Policy Validation" and "Skipping Policy Validation" sections of the design document.

This involves adding a new sysvar to skip validation for reloading, which might be used by logical dump tools to prevent restore errors.

[sylzd]

/assign

[morgo]

Here is an example bug:

mysql> create placement policy myplacement followers=4;
Query OK, 0 rows affected (0.08 sec)

mysql> show placement;
+--------------------+-------------+
| Target             | Placement   |
+--------------------+-------------+
| POLICY myplacement | FOLLOWERS=4 |
+--------------------+-------------+
1 row in set (0.00 sec)

mysql> create table t1 (a int) placement policy=myplacement;
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 {7480000000000000FF3800000000000000F8, 7480000000000000FF3900000000000000F8}"

The expectation is that the error should be returned in the create placement policy statement, not the create table statement.

Edit: I think this placement policy should be legal though. It just says 4 followers, no constraints. It should be implied there is a leader.

[AilinKid]

Edit: I think this placement policy should be legal though. It just says 4 followers, no constraints. It should be implied there is a leader.

This means we need to infer those implicit syntax logic, too many cases for auto complete, I'm wary about.

[morgo]

We discussed how to handle validation in our weekly meeting today. The notes are summarized at: https://docs.google.com/document/d/1_A-bU2JoXZ0Fllhh0xJUdEtDXJQmz52toeNItW42fF0/edit?n=Placement_Rules_in_SQL_Weekly_Report#heading=h.ti4gpl7ary6h

The short answer is: we will try and do validation in the tidb-server for now. This may lead to some cases where there are errors on create table, but we can handle these as bugs and try to improve our validation. If this proves ineffective, we can add the create-a-range-in-pd behavior.

[sylzd]

What does create-a-range-in-pd behavior mean?