From 6a5a814eaa0d4f98d6151df5bd96cc8ec1ae5675 Mon Sep 17 00:00:00 2001
From: Matthew Draper <matthew@trebex.net>
Date: Thu, 25 May 2017 09:08:45 +0930
Subject: Add a Monitor to ModelSchema#load_schema

[Vikrant Chaudhary, David Abdemoulaie, Matthew Draper]
---
 activerecord/lib/active_record/model_schema.rb | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

(limited to 'activerecord/lib/active_record/model_schema.rb')

diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index 54216caaaf..5095cdfe9f 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -1,3 +1,5 @@
+require "monitor"
+
 module ActiveRecord
   module ModelSchema
     extend ActiveSupport::Concern
@@ -152,6 +154,8 @@ module ActiveRecord
       self.inheritance_column = "type"
 
       delegate :type_for_attribute, to: :class
+
+      initialize_load_schema_monitor
     end
 
     # Derives the join table name for +first_table+ and +second_table+. The
@@ -435,15 +439,27 @@ module ActiveRecord
         initialize_find_by_cache
       end
 
+      protected
+
+        def initialize_load_schema_monitor
+          @load_schema_monitor = Monitor.new
+        end
+
       private
 
+        def inherited(child_class)
+          super
+          child_class.initialize_load_schema_monitor
+        end
+
         def schema_loaded?
-          defined?(@columns_hash) && @columns_hash
+          defined?(@schema_loaded) && @schema_loaded
         end
 
         def load_schema
-          unless schema_loaded?
-            load_schema!
+          return if schema_loaded?
+          @load_schema_monitor.synchronize do
+            load_schema! unless defined?(@columns_hash) && @columns_hash
           end
         end
 
@@ -457,6 +473,8 @@ module ActiveRecord
               user_provided_default: false
             )
           end
+
+          @schema_loaded = true
         end
 
         def reload_schema_from_cache
@@ -470,6 +488,7 @@ module ActiveRecord
           @attributes_builder = nil
           @columns = nil
           @columns_hash = nil
+          @schema_loaded = false
           @attribute_names = nil
           @yaml_encoder = nil
           direct_descendants.each do |descendant|
-- 
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