Don't raise exception when each message size is smaller enough

Follow up #3553

In the previous versions, even though each record size is smaller
than chunk limit size, but whole message size exceeds
chunk limit size, BufferChunkOverflowError is raised unexpectedly.

For example, if chunk limit size is 1_280_000, when processing 3 event
stream (every 1_000_000 bytes), it throws an exception like this:

  Fluent::Plugin::Buffer::BufferChunkOverflowError(<a 1000025 bytes
  record (nth: 1) is larger than buffer chunk limit size, a 1000025
  bytes record (nth: 2) is larger than buffer chunk limit size>)

Now changed not to raise exception if it's record size is smaller
enough than chunk limit size. Thus each message is stored into
separated chunks.

The idea is based on that adding byte size is smaller than chunk
limit size, chunk should be unstaged and pushed into queue,
If not, it should be skipped like #3553.

NOTE: This approach depends that chunk implements adding_bytes
interface.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

lib/fluent/plugin/buffer.rb

@@ -780,10 +780,14 @@ def write_step_by_step(metadata, data, format, splits_count, &block)
                     chunk.commit
                   else
                     big_record_size = formatted_split.bytesize
-                    if chunk.bytesize + big_record_size > @chunk_limit_size
+                    if big_record_size > @chunk_limit_size
+                      # Just skip to next split (current split is ignored)
                       errors << "a #{big_record_size} bytes record (nth: #{writing_splits_index}) is larger than buffer chunk limit size (#{@chunk_limit_size})"
                       writing_splits_index += 1
                       next
+                    elsif chunk.bytesize + big_record_size > @chunk_limit_size
+                      # No doubt that the split is expected to cause size over, keep 'split' content here.
+                      chunk.commit
                     end
                   end
                 end
@@ -799,13 +803,23 @@ def write_step_by_step(metadata, data, format, splits_count, &block)
                   chunk.rollback
 
                   if split.size == 1 && original_bytesize == 0
-                    # It is obviously case that BufferChunkOverflowError should be raised here,
-                    # but if it raises here, already processed 'split' or
-                    # the proceeding 'split' will be lost completely.
-                    # so it is a last resort to delay raising such a exception
-                    errors << "concatenated/appended a #{adding_bytes} bytes record (nth: #{writing_splits_index}) is larger than buffer chunk limit size (#{@chunk_limit_size})"
-                    writing_splits_index += 1
-                    next
+                    # FIXME: adding_bytes should be implemented as an official chunk API in the future
+                    if adding_bytes.is_a?(Integer) && adding_bytes < @chunk_limit_size
+                      # already processed content is kept after rollback, then unstaged chunk should be queued.
+                      # After that, re-process current split again (new chunk should be allocated, to do it, modify @stage and so on)
+                      synchronize { @stage.delete(modified_metadata) }
+                      staged_chunk_used = false
+                      chunk.unstaged!
+                      break
+                    else
+                      # It is obviously case that BufferChunkOverflowError should be raised here,
+                      # but if it raises here, already processed 'split' or
+                      # the proceeding 'split' will be lost completely.
+                      # so it is a last resort to delay raising such a exception
+                      errors << "concatenated/appended a #{adding_bytes} bytes record (nth: #{writing_splits_index}) is larger than buffer chunk limit size (#{@chunk_limit_size})"
+                      writing_splits_index += 1
+                      next
+                    end
                   end
 
                   if chunk_size_full?(chunk) || split.size == 1

test/plugin/test_buffer.rb

@@ -990,6 +990,51 @@ def create_chunk_es(metadata, es)
       assert_equal [@dm0], @p.queue.map(&:metadata)
       assert_equal [5000], @p.queue.map(&:size)
     end
+
+    test "confirm that every message which is smaller than chunk threshold does not raise BufferChunkOverflowError" do
+      assert_equal [@dm0], @p.stage.keys
+      assert_equal [], @p.queue.map(&:metadata)
+      timestamp = event_time('2016-04-11 16:00:02 +0000')
+      es = Fluent::ArrayEventStream.new([[timestamp, {"message" => "a" * 1_000_000}],
+                                         [timestamp, {"message" => "b" * 1_000_000}],
+                                         [timestamp, {"message" => "c" * 1_000_000}]])
+
+      # https://github.com/fluent/fluentd/issues/1849
+      # Even though 1_000_000 < 1_280_000 (chunk_limit_size), it raised BufferChunkOverflowError before.
+      # It should not be raised and message a,b,c should be stored into 3 chunks.
+      assert_nothing_raised do
+        @p.write({@dm0 => es}, format: @format)
+      end
+      messages = []
+      # pick up first letter to check whether chunk is queued in expected order
+      3.times do |index|
+        chunk = @p.queue[index]
+        es = Fluent::MessagePackEventStream.new(chunk.chunk)
+        es.ensure_unpacked!
+        records = es.instance_eval{ @unpacked_records }
+        records.each do |record|
+          messages << record["message"][0]
+        end
+      end
+      es = Fluent::MessagePackEventStream.new(@p.stage[@dm0].chunk)
+      es.ensure_unpacked!
+      staged_message = es.instance_eval{ @unpacked_records }.first["message"]
+      # message a and b are queued, message c is staged
+      assert_equal([
+                     [@dm0],
+                     "c" * 1_000_000,
+                     [@dm0, @dm0, @dm0],
+                     [5000, 1, 1],
+                     [["x"] * 5000, "a", "b"].flatten
+                   ],
+                   [
+                     @p.stage.keys,
+                     staged_message,
+                     @p.queue.map(&:metadata),
+                     @p.queue.map(&:size),
+                     messages
+                   ])
+    end
   end
 
   sub_test_case 'custom format with configuration for test with lower chunk limit size' do
@@ -1078,6 +1123,38 @@ def create_chunk_es(metadata, es)
         @p.write({@dm0 => es})
       end
     end
+
+    test 'confirm that every array message which is smaller than chunk threshold does not raise BufferChunkOverflowError' do
+      assert_equal [@dm0], @p.stage.keys
+      assert_equal [], @p.queue.map(&:metadata)
+
+      assert_equal 1_280_000, @p.chunk_limit_size
+
+      es = ["a" * 1_000_000, "b" * 1_000_000, "c" * 1_000_000]
+      assert_nothing_raised do
+        @p.write({@dm0 => es})
+      end
+      queue_messages = @p.queue.collect do |chunk|
+        # collect first character of each message
+        chunk.chunk[0]
+      end
+      assert_equal([
+                     [@dm0],
+                     1,
+                     "c",
+                     [@dm0, @dm0, @dm0],
+                     [5000, 1, 1],
+                     ["x", "a", "b"]
+                   ],
+                   [
+                     @p.stage.keys,
+                     @p.stage[@dm0].size,
+                     @p.stage[@dm0].chunk[0],
+                     @p.queue.map(&:metadata),
+                     @p.queue.map(&:size),
+                     queue_messages
+                   ])
+    end
   end
 
   sub_test_case 'with configuration for test with lower limits' do