add test

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

test/plugin/test_buffer.rb

@@ -901,6 +901,65 @@ def create_chunk_es(metadata, es)
 
       assert_equal 2, purge_count
     end
+
+    # https://github.com/fluent/fluentd/issues/4446
+    test "#write_step_by_step keeps chunks kept in locked in entire #write process" do
+      assert_equal 8 * 1024 * 1024, @p.chunk_limit_size
+      assert_equal 0.95, @p.chunk_full_threshold
+
+      mon_enter_counts_by_chunk = {}
+      mon_exit_counts_by_chunk = {}
+
+      stub.proxy(@p).generate_chunk(anything) do |chunk|
+        stub(chunk).mon_enter do
+          enter_count = 1 + mon_enter_counts_by_chunk.fetch(chunk, 0)
+          exit_count = mon_exit_counts_by_chunk.fetch(chunk, 0)
+          mon_enter_counts_by_chunk[chunk] = enter_count
+
+          # Assert that chunk is passed to &block of write_step_by_step before exiting the lock.
+          # (i.e. The lock count must be 2 greater than the exit count).
+          # Since ShouldRetry occurs once, the staged chunk takes the lock 3 times when calling the block.
+          if chunk.staged?
+            lock_in_block = enter_count == 3
+            assert_equal(enter_count - 2, exit_count) if lock_in_block
+          else
+            lock_in_block = enter_count == 2
+            assert_equal(enter_count - 2, exit_count) if lock_in_block
+          end
+        end
+        stub(chunk).mon_exit do
+          exit_count = 1 + mon_exit_counts_by_chunk.fetch(chunk, 0)
+          mon_exit_counts_by_chunk[chunk] = exit_count
+        end
+        chunk
+      end
+
+      m = @p.metadata(timekey: Time.parse('2016-04-11 16:40:00 +0000').to_i)
+      small_row = "x" * 1024 * 400
+      big_row = "x" * 1024 * 1024 * 8 # just `chunk_size_limit`, it does't cause BufferOverFlowError.
+
+      # Write 42 events in 1 event stream, last one is for triggering `ShouldRetry`
+      @p.write({m => [small_row] * 40 + [big_row] + ["x"]})
+
+      # Above event strem will be splitted twice by `Buffer#write_step_by_step`
+      #
+      # 1. `write_once`: 42 [events] * 1 [stream]
+      # 2. `write_step_by_step`: 4 [events]* 10 [streams] + 2 [events] * 1 [stream]
+      # 3. `write_step_by_step` (by `ShouldRetry`): 1 [event] * 42 [streams]
+      # 
+      # Example of staged chunk lock behavior:
+      # 
+      # 1. mon_enter in write_step_by_step
+      # 2. ShouldRetry occurs
+      # 3. mon_exit in write_step_by_step
+      # 4. mon_enter again in write_step_by_step (retry)
+      # 5. passed to &block of write_step_by_step
+      # 6. mon_enter in the block (write)
+      # 7. mon_exit in write_step_by_step
+      # 8. mon_exit in write
+
+      assert_equal(mon_enter_counts_by_chunk.values, mon_exit_counts_by_chunk.values)
+    end
   end
 
   sub_test_case 'standard format with configuration for test with lower chunk limit size' do