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.