Prevent concurrent compilation of templates - closes rails#6400

actionpack/lib/action_view/template.rb

@@ -2,6 +2,7 @@
 require 'active_support/core_ext/object/blank'
 require 'active_support/core_ext/object/try'
 require 'active_support/core_ext/kernel/singleton_class'
+require 'thread'
 
 module ActionView
   # = Action View Template
@@ -123,6 +124,7 @@ def initialize(source, identifier, handler, details)
       @virtual_path      = details[:virtual_path]
       @updated_at        = details[:updated_at] || Time.now
       @formats = Array.wrap(format).map { |f| f.is_a?(Mime::Type) ? f.ref : f }
+      @compile_mutex     = Mutex.new
     end
 
     # Returns if the underlying handler supports streaming. If so,
@@ -224,18 +226,28 @@ def encode!
       def compile!(view) #:nodoc:
         return if @compiled
 
-        if view.is_a?(ActionView::CompiledTemplates)
-          mod = ActionView::CompiledTemplates
-        else
-          mod = view.singleton_class
-        end
+        # Templates can be used concurrently in threaded environments
+        # so compilation and any instance variable modification must
+        # be synchronized
+        @compile_mutex.synchronize do
+          # Any thread holding this lock will be compiling the template needed
+          # by the threads waiting. So re-check the @compiled flag to avoid
+          # re-compilation
+          return if @compiled
+
+          if view.is_a?(ActionView::CompiledTemplates)
+            mod = ActionView::CompiledTemplates
+          else
+            mod = view.singleton_class
+          end
 
-        compile(view, mod)
+          compile(view, mod)
 
-        # Just discard the source if we have a virtual path. This
-        # means we can get the template back.
-        @source = nil if @virtual_path
-        @compiled = true
+          # Just discard the source if we have a virtual path. This
+          # means we can get the template back.
+          @source = nil if @virtual_path
+          @compiled = true
+        end
       end
 
       # Among other things, this method is responsible for properly setting