Move while_preventing_writes from conn to handler

[eileencodes]

If we put the while_preventing_writes on the connection then the
middleware that sends reads to the primary and ensures they can't write
will not work. The while_preventing_writes will only be applied to the
connection which it's called on - which in the case of the middleware is
Ar::Base.

This worked fine if you called it directly like
OtherDbConn.connection.while_preventing_writes but Rails didn't have a
way of knowing you wanted to call it on all the connections.

The change here moves the while_preventing_writes method from the
connection to the handler so that it can block writes to all queries for
that handler. This will apply to all the connections associated with
that handler.

cc/ @matthewd @tenderlove @jhawthorn @rafaelfranca

[rafaelfranca]

I never remember this but, does each model have its different connection handler or all models always share the ActiveRecord::Base connection handler?

[eileencodes]

Each type of connection has it's own handler depending on what you put in connects_to.

Given the following:

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :primary_replica }
end

class Person < ApplicationRecord
end

class AnimalsBase < ApplicationRecord
  self.abstract_class = true

  connects_to database: { writing: :animals, reading: :animals_replica }
end

class Dog < AnimalsBase
end

This will create 2 handlers a :writing handler and a :reading handler (aka role). Each of these handlers has 2 connections: one for application record (person) and one for animals base (dog). Because we use connected_to to swap the handler out:

ActiveRecord::Base.connected_to(role: :reading) do
  # do something on reading handler
end

means that ActiveRecord::Base.connection_handler.prevent_writes will ask the handler whether it should prevent writes instead of the connection. Since connected_to operates on the handler we can be confident it's passing the value down correctly.

[eileencodes]

I added a test that demonstrates this is working correctly here https://github.com/rails/rails/pull/36469/files#diff-3ad790de5c6c5fbd6d956e443bcc0490R1579

[kamipo]

We don't need public attr_writer :prevent_writes.

Suggested change

	attr_accessor :prevent_writes
	attr_reader :prevent_writes