Add a disable_query_cache option to with_advisory_lock (#70)

Author: muxcmux

Diff for: README.md

@@ -94,6 +94,14 @@ If you want to see if the current Thread is holding a lock, you can call
 `Tag.current_advisory_lock` which will return the name of the current lock. If
 no lock is currently held, `.current_advisory_lock` returns `nil`.
 
+### ActiveRecord Query Cache
+
+You can optionally pass `disable_query_cache: true` to the options hash of
+`with_advisory_lock` in order to disable ActiveRecord's query cache. This can
+prevent problems when you query the database from within the lock and it returns
+stale results. More info on why this can be a problem can be
+[found here](https://github.com/ClosureTree/with_advisory_lock/issues/52)
+
 ## Installation
 
 Add this line to your application's Gemfile:

Diff for: lib/with_advisory_lock/base.rb

@@ -21,17 +21,18 @@ def lock_was_acquired?
   LockStackItem = Struct.new(:name, :shared)
 
   class Base
-    attr_reader :connection, :lock_name, :timeout_seconds, :shared, :transaction
+    attr_reader :connection, :lock_name, :timeout_seconds, :shared, :transaction, :disable_query_cache
 
     def initialize(connection, lock_name, options)
       options = { timeout_seconds: options } unless options.respond_to?(:fetch)
-      options.assert_valid_keys :timeout_seconds, :shared, :transaction
+      options.assert_valid_keys :timeout_seconds, :shared, :transaction, :disable_query_cache
 
       @connection = connection
       @lock_name = lock_name
       @timeout_seconds = options.fetch(:timeout_seconds, nil)
       @shared = options.fetch(:shared, false)
       @transaction = options.fetch(:transaction, false)
+      @disable_query_cache = options.fetch(:disable_query_cache, false)
     end
 
     def lock_str
@@ -53,6 +54,16 @@ def already_locked?
     end
 
     def with_advisory_lock_if_needed(&block)
+      if disable_query_cache
+        return lock_and_yield do
+          ActiveRecord::Base.uncached(&block)
+        end
+      end
+
+      lock_and_yield(&block)
+    end
+
+    def lock_and_yield(&block)
       if already_locked?
         Result.new(true, yield)
       elsif timeout_seconds == 0

Diff for: test/concern_test.rb

@@ -19,3 +19,17 @@
     assert_respond_to(Label.new, :advisory_lock_exists?)
   end
 end
+
+describe 'ActiveRecord query cache' do
+  it 'does not disable quary cache by default' do
+    ActiveRecord::Base.expects(:uncached).never
+
+    Tag.with_advisory_lock('lock') { Tag.first }
+  end
+
+  it 'can disable ActiveRecord query cache' do
+    ActiveRecord::Base.expects(:uncached).once
+
+    Tag.with_advisory_lock('a-lock', disable_query_cache: true) { Tag.first }
+  end
+end