Cache Capability to indicate that CAS is read-only

[sluongng]

We need to signal to the client that in some configurations, the CAS will only accept read requests. All write requests will result in errors.

Given that we are currently advertising

// Describes the server/instance capabilities for updating the action cache.
message ActionCacheUpdateCapabilities {
  bool update_enabled = 1;
}

in CacheCapabilities proto. I was thinking that we could simply add another identical field for CASUpdateCapabilities.

However, such a field would default to false, which is troublesome for backward compatibility. And doing bool is_readonly = 1; feels out of spec. Looking for feedback on this design.

[EdSchouten]

Use an enum where 0 indicates it’s writable? Or place it in a nested message. If the message is absent, clients may assume legacy behaviour where the CAS is writable.

[bergsieker]

I think the right course is an enum with UNKNOWN=0, WRITEABLE, READ-ONLY as the values. It's possible there are other CAS read-only CAS implementations (e.g., local caches), and we shouldn't extend the API in a way that invalidates their position by default.