Add ability to change the names of the default handlers

When I wrote the `connected_to` and `connects_to` API's I wrote them
with the idea in mind that it didn't really matter what the
handlers/roles were called as long as those connecting to the roles knew
which one wrote and which one read.

With the introduction of the middleware Rails begins to assume it's
`writing` and `reading` and there's no room for other roles. At GitHub
we've been using this method for a long time so we have a ton of legacy
code that uses different handler names `default` and `readonly`. We
could rename all our code but I think this is better for a few reasons:

- Legacy apps that have been using multiple databases for a long time
  can have an eaiser time switching.
- If we later find this to cause more issues than it's worth we can
  easily deprecate.
- We won't force old apps to rewrite the resolver middleware just to use
  a different handler.

Adding the writing_role/reading_role required that I move the code that
creates the first handler for writing to the railtie. If I didn't move
this the core class would assign the handler before I was able to assign
a new one in my configuration and I'd end up with 3 handlers instead of
2.

1 files changed, 4 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 519acd7605..84604452f7 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -125,6 +125,10 @@ module ActiveRecord
 
       mattr_accessor :connection_handlers, instance_accessor: false, default: {}
 
+      mattr_accessor :writing_role, instance_accessor: false, default: :writing
+
+      mattr_accessor :reading_role, instance_accessor: false, default: :reading
+
       class_attribute :default_connection_handler, instance_writer: false
 
       self.filter_attributes = []
@@ -138,7 +142,6 @@ module ActiveRecord
       end
 
       self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new
-      self.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
     end
 
     module ClassMethods