Skip the `:race_condition_ttl` branch if the option is 0 or nil. This fixes an issue with the redis cache, where this code will sometimes throw an error out of SETEX when passing 0 as the `expires_at`.

activesupport/lib/active_support/cache.rb

@@ -566,8 +566,8 @@ module ActiveSupport
         def handle_expired_entry(entry, key, options)
           if entry && entry.expired?
             race_ttl = options[:race_condition_ttl].to_i
-            if race_ttl && (Time.now.to_f - entry.expires_at <= race_ttl)
-              # When an entry has :race_condition_ttl defined, put the stale entry back into the cache
+            if (race_ttl > 0) && (Time.now.to_f - entry.expires_at <= race_ttl)
+              # When an entry has a positive :race_condition_ttl defined, put the stale entry back into the cache
               # for a brief period while the entry is being recalculated.
               entry.expires_at = Time.now + race_ttl
               write_entry(key, entry, :expires_in => race_ttl * 2)

activesupport/test/caching_test.rb

@@ -399,15 +399,16 @@ module CacheStoreBehavior
     assert_nil @cache.read('foo')
   end
 
-  def test_race_condition_protection
-    time = Time.now
-    @cache.write('foo', 'bar', :expires_in => 60)
-    Time.stubs(:now).returns(time + 61)
-    result = @cache.fetch('foo', :race_condition_ttl => 10) do
-      assert_equal 'bar', @cache.read('foo')
-      "baz"
+  def test_race_condition_protection_skipped_if_not_defined
+    @cache.write('foo', 'bar')
+    time = @cache.send(:read_entry, 'foo', {}).expires_at
+    Time.stubs(:now).returns(Time.at(time))
+
+    result = @cache.fetch('foo') do
+      assert_equal nil, @cache.read('foo')
+      'baz'
     end
-    assert_equal "baz", result
+    assert_equal 'baz', result
   end
 
   def test_race_condition_protection_is_limited
@@ -437,6 +438,17 @@ module CacheStoreBehavior
     assert_nil @cache.read('foo')
   end
 
+  def test_race_condition_protection
+    time = Time.now
+    @cache.write('foo', 'bar', :expires_in => 60)
+    Time.stubs(:now).returns(time + 61)
+    result = @cache.fetch('foo', :race_condition_ttl => 10) do
+      assert_equal 'bar', @cache.read('foo')
+      "baz"
+    end
+    assert_equal "baz", result
+  end
+
   def test_crazy_key_characters
     crazy_key = "#/:*(<+=> )&$%@?;'\"\'`~-"
     assert @cache.write(crazy_key, "1", :raw => true)