Remove unused ActiveRecord::Base.connection_id

This method appears to have been partially used in connection pool
caching, but it was introduced without much reasoning or any tests. One
edge case test was added later on, but it was focused on implementation
details. This method is no longer used outside of tests, and as such is
removed.

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Removed the unused methods `ActiveRecord::Base.connection_id` and
+    `ActiveRecord::Base.connection_id=`
+
+    *Sean Griffin*
+
 *   Ensure hashes can be assigned to attributes created using `composed_of`.
     Fixes #25210.
 

activerecord/lib/active_record/connection_handling.rb

@@ -98,14 +98,6 @@ module ActiveRecord
       @connection_specification_name
     end
 
-    def connection_id
-      ActiveRecord::RuntimeRegistry.connection_id ||= Thread.current.object_id
-    end
-
-    def connection_id=(connection_id)
-      ActiveRecord::RuntimeRegistry.connection_id = connection_id
-    end
-
     # Returns the configuration of the associated connection as a hash:
     #
     #  ActiveRecord::Base.connection_config

activerecord/lib/active_record/query_cache.rb

@@ -26,16 +26,12 @@ module ActiveRecord
     def self.run
       connection    = ActiveRecord::Base.connection
       enabled       = connection.query_cache_enabled
-      connection_id = ActiveRecord::Base.connection_id
       connection.enable_query_cache!
 
-      [enabled, connection_id]
+      enabled
     end
 
-    def self.complete(state)
-      enabled, connection_id = state
-
-      ActiveRecord::Base.connection_id = connection_id
+    def self.complete(enabled)
       ActiveRecord::Base.connection.clear_query_cache
       ActiveRecord::Base.connection.disable_query_cache! unless enabled
     end

activerecord/lib/active_record/runtime_registry.rb

@@ -12,9 +12,9 @@ module ActiveRecord
   class RuntimeRegistry # :nodoc:
     extend ActiveSupport::PerThreadRegistry
 
-    attr_accessor :connection_handler, :sql_runtime, :connection_id
+    attr_accessor :connection_handler, :sql_runtime
 
-    [:connection_handler, :sql_runtime, :connection_id].each do |val|
+    [:connection_handler, :sql_runtime].each do |val|
       class_eval %{ def self.#{val}; instance.#{val}; end }, __FILE__, __LINE__
       class_eval %{ def self.#{val}=(x); instance.#{val}=x; end }, __FILE__, __LINE__
     end

activerecord/test/cases/query_cache_test.rb

@@ -39,18 +39,6 @@ class QueryCacheTest < ActiveRecord::TestCase
     assert ActiveRecord::Base.connection.query_cache_enabled, 'cache on'
   end
 
-  def test_exceptional_middleware_assigns_original_connection_id_on_error
-    connection_id = ActiveRecord::Base.connection_id
-
-    mw = middleware { |env|
-      ActiveRecord::Base.connection_id = self.object_id
-      raise "lol borked"
-    }
-    assert_raises(RuntimeError) { mw.call({}) }
-
-    assert_equal connection_id, ActiveRecord::Base.connection_id
-  end
-
   def test_middleware_delegates
     called = false
     mw = middleware { |env|