Make semi sync extension optional

[morgo]

Fixes #4762

There is a use case to attach Vitess to an existing MySQL topology and then migrate into Vitess. Currently this requires the semi sync extension to be present (irrespective of whether semi-sync is used). This is because of the way the mysql server handles config options: the plugins register them with the server. If the plugin is not present, there is an error because the option is unknown.

Since a user may be moving from a managed platform, or may not want to make too many changes to the source, it is important to not error if semi-sync commands can not be executed.

Signed-off-by: Morgan Tocker tocker@gmail.com

[deepthi]

There are other places where we are setting rpl_semi_sync variables. We need to evaluate whether changes are needed there. E.g.:
https://github.com/vitessio/vitess/blob/master/go/vt/mysqlctl/replication.go#L407

[morgo]

There are other places where we are setting rpl_semi_sync variables. We need to evaluate whether changes are needed there. E.g.:
https://github.com/vitessio/vitess/blob/master/go/vt/mysqlctl/replication.go#L407

I actually think that SetSemiSyncEnabled behaves correctly without modification. i.e. if you try and enable semi-sync but the plugin is not loaded, the error is caught.

But there is another one called SemiSyncEnabled which could benefit. It currently does something very similar to the function I introduce:

https://github.com/vitessio/vitess/blob/master/go/vt/mysqlctl/replication.go#L419-L422

If I modified this to first use SemisyncExtensionLoaded, it would be one extra query though, since it needs the output of the query when the extension is loaded.

[morgo]

I renamed SemisyncExtensionLoaded to SemiSyncExtensionLoaded to be consistent with existing code.

[deepthi]

But there is another one called SemiSyncEnabled which could benefit. It currently does something very similar to the function I introduce:

https://github.com/vitessio/vitess/blob/master/go/vt/mysqlctl/replication.go#L419-L422

If I modified this to first use SemisyncExtensionLoaded, it would be one extra query though, since it needs the output of the query when the extension is loaded.

I don't think that is needed, since it will return false if the extension is not loaded (catch error and return false).

[deepthi]

LGTM