Isolate "truncate_tables" method

* when the unit tests are executed in parallel (typically 4 processes), there can be a race condition when truncating the tables (resulting in "Cannot truncate a table referenced in a foreign key constraint") because "SET FOREIGN_KEY_CHECKS = x" is executed concurrently

Author: jochenehret

spec/support/table_truncator.rb

@@ -1,3 +1,5 @@
+require 'mutex_m'
+
 class TableTruncator
   def initialize(db, tables=nil)
     @db = db
@@ -9,16 +11,19 @@ def self.isolated_tables(db)
   end
 
   def truncate_tables
-    referential_integrity = ReferentialIntegrity.new(db)
-    referential_integrity.without do
-      case db.database_type
-      when :postgres
-        tables.each do |table|
-          db.run("TRUNCATE TABLE #{table} RESTART IDENTITY CASCADE;")
-        end
-      when :mysql
-        tables.each do |table|
-          db.run("TRUNCATE TABLE #{table};")
+    @mutex ||= Mutex.new
+    @mutex.synchronize do
+      referential_integrity = ReferentialIntegrity.new(db)
+      referential_integrity.without do
+        case db.database_type
+        when :postgres
+          tables.each do |table|
+            db.run("TRUNCATE TABLE #{table} RESTART IDENTITY CASCADE;")
+          end
+        when :mysql
+          tables.each do |table|
+            db.run("TRUNCATE TABLE #{table};")
+          end
         end
       end
     end