Add ability to configure cache notifications info

This may seem like an unnecessary refactoring but some apps want / need
to configure the information passed to the query cache logger. In order
to do that we can add a method here that can be easily overridden by the
app itself, rather than hacking the query cache logger to include that
information.

To override apps can call

```
def cache_notifications_info
  super.merge(connected_host: "hostname")
end
```

This will take what's already in the query cache logger and add
`@something="yea"` to the object.

At GitHub we use this to log the number of queries that are cached, the
connection host and the connection url.

2 files changed, 37 insertions, 7 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb b/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb
index 25622e34c8..8aeb934ec2 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb
@@ -110,12 +110,7 @@ module ActiveRecord
               if @query_cache[sql].key?(binds)
                 ActiveSupport::Notifications.instrument(
                   "sql.active_record",
-                  sql: sql,
-                  binds: binds,
-                  type_casted_binds: -> { type_casted_binds(binds) },
-                  name: name,
-                  connection_id: object_id,
-                  cached: true,
+                  cache_notification_info(sql, name, binds)
                 )
                 @query_cache[sql][binds]
               else
@@ -125,6 +120,19 @@ module ActiveRecord
           end
         end
 
+        # Database adapters can override this method to
+        # provide custom cache information.
+        def cache_notification_info(sql, name, binds)
+          {
+            sql: sql,
+            binds: binds,
+            type_casted_binds: -> { type_casted_binds(binds) },
+            name: name,
+            connection_id: object_id,
+            cached: true
+          }
+        end
+
         # If arel is locked this is a SELECT ... FOR UPDATE or somesuch. Such
         # queries should not be cached.
         def locked?(arel)
diff --git a/activerecord/test/cases/query_cache_test.rb b/activerecord/test/cases/query_cache_test.rb
index 0f990bac9d..1c05571f1b 100644
--- a/activerecord/test/cases/query_cache_test.rb
+++ b/activerecord/test/cases/query_cache_test.rb
@@ -13,12 +13,13 @@ class QueryCacheTest < ActiveRecord::TestCase
   fixtures :tasks, :topics, :categories, :posts, :categories_posts
 
   class ShouldNotHaveExceptionsLogger < ActiveRecord::LogSubscriber
-    attr_reader :logger
+    attr_reader :logger, :events
 
     def initialize
       super
       @logger = ::Logger.new File::NULL
       @exception = false
+      @events = []
     end
 
     def exception?
@@ -26,6 +27,7 @@ class QueryCacheTest < ActiveRecord::TestCase
     end
 
     def sql(event)
+      @events << event
       super
     rescue
       @exception = true
@@ -265,6 +267,26 @@ class QueryCacheTest < ActiveRecord::TestCase
     end
   end
 
+  def test_cache_notifications_can_be_overridden
+    logger = ShouldNotHaveExceptionsLogger.new
+    subscriber = ActiveSupport::Notifications.subscribe "sql.active_record", logger
+
+    connection = ActiveRecord::Base.connection.dup
+
+    def connection.cache_notification_info(sql, name, binds)
+      super.merge(neat: true)
+    end
+
+    connection.cache do
+      connection.select_all "select 1"
+      connection.select_all "select 1"
+    end
+
+    assert_equal true, logger.events.last.payload[:neat]
+  ensure
+    ActiveSupport::Notifications.unsubscribe subscriber
+  end
+
   def test_cache_does_not_raise_exceptions
     logger = ShouldNotHaveExceptionsLogger.new
     subscriber = ActiveSupport::Notifications.subscribe "sql.active_record", logger