Addition of @@enable_system_settings to vtgate

[frouioui]

Signed-off-by: Florent Poinsard florent.poinsard@outlook.fr

Description

This pull request introduces a new system variable: enable_system_settings. This VitessAware variable is used to define wether the current session is allowed to use reserved connection or not.

The default value of the variable is based on VTGate's -enable_system_settings flag.

Fixes #6860.

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build
• VTAdmin

[frouioui]

The new variable is not tested for all test cases yet. Specifically the following one:

vitess/go/vt/vtgate/executor_select_test.go

Line 319 in 0fa0fc7

func TestSelectSystemVariables(t *testing.T) {

This specific test case relates to the issue mentioned in #7301.

[systay]

It would be great if you could add an end to end test in https://github.com/vitessio/vitess/blob/master/go/test/endtoend/vtgate/reservedconn/sysvar_test.go

[frouioui]

Select tests added in 4bfaf2d, and end-to-end tests added in ebf4d93.

[systay]

Really nice. Welcome as new contributor! 🤗

[frouioui]

Awesome! Thank you 🎉