Task: Change TabletType MASTER to PRIMARY

[deepthi]

There has been discussion in the past on exactly which term to replace master with. Since vitess is a consensus-based system, primary seems like the most appropriate choice (even though source is popular for replication use cases).

TabletType is everywhere in the code base, so we will need to first determine the scope of this change and how to do it in a backwards compatible way.

Website docs should be updated along with code.

[ajm188]

I think that if we just rename the field in the enum, then anyone dependent on that should not notice, since it'll still be int(1) on the wire. Then, the code change becomes atomic; we do a global find/replace to topodatapb.TabletType_PRIMARY, and code compiled on the old version still works with the old field name, and code compiled on the new version works with the new field name.

[derekperkins]

There are a few edge cases where the string representation is used, specifically when json marshaling, so we'd have to check for those types of usage.

[derekperkins]

While I personally prefer primary, I'd vote to match MySQL and call it source
https://mysqlhighavailability.com/mysql-terminology-updates/
https://dev.mysql.com/doc/refman/8.0/en/replication.html

[ajm188]

There are a few edge cases where the string representation is used, specifically when json marshaling, so we'd have to check for those types of usage.

Ah, nice catch! I think we can use an alias in that case https://developers.google.com/protocol-buffers/docs/proto3#enum (I can't link precisely enough)

[deepthi]

Fixed by #8511