Support for tidb_enable_noop_sysvars

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, it is hard to know if setting a variable in TiDB has an effect. There are a number of sysvars that are included for compatibility with MySQL, and their inclusion helps compatibility but hurts usability (I have caught myself a couple of times, only to read the manual or the source code and see something is not implemented).

Describe the feature you'd like:

Similar to tidb_enable_noop_functions, there can be tidb_enable_noop_sysvars. When enabled (default) the behavior is the same as currently. When disabled:

• SHOW [GLOBAL] VARIABLES omits the variable
• Setting the variable returns a warning
• SELECT @@variable returns a warning

Because most ORMs that depend on noops will SELECT @@ the variable (for performance reasons they should be doing this), it will actually still work in most instances. But for compatibility, at least for now the default is still enabled.

I have a cleanup proposal here: https://docs.google.com/document/d/1i181NttnnQm-B7iq1GCfEFlFccZmZCMBXziqybLoTu8/edit?usp=sharing

Describe alternatives you've considered:

It could error instead of warn, but warn seems sufficient.

Teachability, Documentation, Adoption, Migration Strategy:

The name being similar to noop_funcs makes it very approachable. Unfortunately adoption will be difficult since the default still shows noops, but this is a starting point. It might be hard to completely remove noops because the compatibility testing requirements are so difficult.

[morgo]

I discussed this with the TiDB team following devcon this year, and we proposed a slightly different behavior: off by default, but produce a warning when updating a sysvar that is unsupported (irrespective of tidb_mysql_compatible_variable_info value). I will update the proposal to incorporate shortly.