Make the Resolver template cache threadsafe - closes #6404

The Template cache in the Resolver can be accessed by multiple threads
similtaneously in multi-threaded environments. The cache is implemented
using a Hash, which isn't threadsafe in all VMs (notably JRuby).

This commit extracts the cache to a new Cache class and adds mutexes to
prevent concurrent access.

actionpack/lib/action_view/template/resolver.rb

@@ -2,6 +2,7 @@ require "pathname"
 require "active_support/core_ext/class"
 require "active_support/core_ext/class/attribute_accessors"
 require "action_view/template"
+require "thread"
 
 module ActionView
   # = Action View Resolver
@@ -24,6 +25,46 @@ module ActionView
       end
     end
 
+    # Threadsafe template cache
+    class Cache #:nodoc:
+      def initialize
+        @data = Hash.new { |h1,k1| h1[k1] = Hash.new { |h2,k2|
+                  h2[k2] = Hash.new { |h3,k3| h3[k3] = Hash.new { |h4,k4| h4[k4] = {} } } } }
+        @mutex = Mutex.new
+      end
+
+      # Cache the templates returned by the block
+      def cache(key, name, prefix, partial, locals)
+        @mutex.synchronize do
+          if Resolver.caching?
+            # all templates are cached forever the first time they are accessed
+            @data[key][name][prefix][partial][locals] ||= yield
+          else
+            # templates are still cached, but are only returned if they are
+            # all still current
+            fresh = yield
+
+            cache = @data[key][name][prefix][partial][locals]
+            mtime = cache && cache.map(&:updated_at).max
+
+            newer = !mtime || fresh.empty?  || fresh.any? { |t| t.updated_at > mtime }
+
+            if newer
+              @data[key][name][prefix][partial][locals] = fresh
+            else
+              @data[key][name][prefix][partial][locals]
+            end
+          end
+        end
+      end
+
+      def clear
+        @mutex.synchronize do
+          @data.clear
+        end
+      end
+    end
+
     cattr_accessor :caching
     self.caching = true
 
@@ -32,12 +73,11 @@ module ActionView
     end
 
     def initialize
-      @cached = Hash.new { |h1,k1| h1[k1] = Hash.new { |h2,k2|
-        h2[k2] = Hash.new { |h3,k3| h3[k3] = Hash.new { |h4,k4| h4[k4] = {} } } } }
+      @cache = Cache.new
     end
 
     def clear_cache
-      @cached.clear
+      @cache.clear
     end
 
     # Normalizes the arguments and passes it on to find_template.
@@ -65,27 +105,18 @@ module ActionView
 
     # Handles templates caching. If a key is given and caching is on
     # always check the cache before hitting the resolver. Otherwise,
-    # it always hits the resolver but check if the resolver is fresher
-    # before returning it.
+    # it always hits the resolver but if the key is present, check if the
+    # resolver is fresher before returning it.
     def cached(key, path_info, details, locals) #:nodoc:
       name, prefix, partial = path_info
       locals = locals.map { |x| x.to_s }.sort!
 
-      if key && caching?
-        @cached[key][name][prefix][partial][locals] ||= decorate(yield, path_info, details, locals)
-      else
-        fresh = decorate(yield, path_info, details, locals)
-        return fresh unless key
-
-        scope = @cached[key][name][prefix][partial]
-        cache = scope[locals]
-        mtime = cache && cache.map(&:updated_at).max
-
-        if !mtime || fresh.empty?  || fresh.any? { |t| t.updated_at > mtime }
-          scope[locals] = fresh
-        else
-          cache
+      if key
+        @cache.cache(key, name, prefix, partial, locals) do
+          decorate(yield, path_info, details, locals)
         end
+      else
+        decorate(yield, path_info, details, locals)
       end
     end
 

actionpack/test/template/lookup_context_test.rb

@@ -169,7 +169,7 @@ class LookupContextTest < ActiveSupport::TestCase
 
     assert_not_equal template, old_template
   end
-  
+
   test "responds to #prefixes" do
     assert_equal [], @lookup_context.prefixes
     @lookup_context.prefixes = ["foo"]
@@ -180,7 +180,7 @@ end
 class LookupContextWithFalseCaching < ActiveSupport::TestCase
   def setup
     @resolver = ActionView::FixtureResolver.new("test/_foo.erb" => ["Foo", Time.utc(2000)])
-    @resolver.stubs(:caching?).returns(false)
+    ActionView::Resolver.stubs(:caching?).returns(false)
     @lookup_context = ActionView::LookupContext.new(@resolver, {})
   end
 
@@ -247,6 +247,6 @@ class TestMissingTemplate < ActiveSupport::TestCase
       @lookup_context.view_paths.find("foo", "parent", true, details)
     end
     assert_match %r{Missing partial parent/foo with .* Searched in:\n  \* "/Path/to/views"\n}, e.message
-  end 
-  
+  end
+
 end