Merge pull request rails#51222 from Shopify/ar-clear-pinned-connectio…

…n-cache

Clear all threads query cache when a connection is pinned

activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb

@@ -129,7 +129,14 @@ def query_cache_enabled
         end
 
         def clear_query_cache
-          query_cache.clear
+          if @pinned_connection
+            # With transactional fixtures, and especially systems test
+            # another thread may use the same connection, but with a different
+            # query cache. So we must clear them all.
+            @thread_query_caches.each_value(&:clear)
+          else
+            query_cache.clear
+          end
         end
 
         private

activerecord/test/cases/query_cache_test.rb

@@ -672,6 +672,40 @@ def test_clear_query_cache_is_called_on_all_connections
     end
   end
 
+  test "query cache is cleared for all thread when a connection is shared" do
+    ActiveRecord::Base.connection_pool.pin_connection!(ActiveSupport::IsolatedExecutionState.context)
+
+    begin
+      assert_cache :off
+      ActiveRecord::Base.connection.enable_query_cache!
+      assert_cache :clean
+
+      Post.first
+      assert_cache :dirty
+
+      thread_a = Thread.new do
+        middleware { |env|
+          assert_cache :dirty # The cache is shared with the main thread
+
+          Post.first
+          assert_cache :dirty
+
+          Post.delete_all
+
+          assert_cache :clean
+
+          [200, {}, nil]
+        }.call({})
+      end
+
+      thread_a.join
+
+      assert_cache :clean
+    ensure
+      ActiveRecord::Base.connection_pool.unpin_connection!
+    end
+  end
+
   private
     def with_temporary_connection_pool(&block)
       pool_config = ActiveRecord::Base.connection.pool.pool_config