Correct offsets in nested message sets

CHANGELOG.md

@@ -4,6 +4,8 @@ Changes and additions to the library will be listed here.
 
 ## Unreleased
 
+- Fix bug when using compression (#458).
+
 ## v0.5.0
 
 - Drops support for Kafka 0.9 in favor of Kafka 0.10 (#381)!

lib/kafka/protocol/message.rb

@@ -56,8 +56,20 @@ def decompress
         # For some weird reason we need to cut out the first 20 bytes.
         data = codec.decompress(value)
         message_set_decoder = Decoder.from_string(data)
+        message_set = MessageSet.decode(message_set_decoder)
+
+        # The contained messages need to have their offset corrected.
+        messages = message_set.messages.each_with_index.map do |message, i|
+          Message.new(
+            offset: offset + i,
+            value: message.value,
+            key: message.key,
+            create_time: message.create_time,
+            codec_id: message.codec_id
+          )
+        end
 
-        MessageSet.decode(message_set_decoder)
+        MessageSet.new(messages: messages)
       end
 
       def self.decode(decoder)

spec/compressor_spec.rb

@@ -15,8 +15,14 @@
       decoder = Kafka::Protocol::Decoder.from_string(data)
       decoded_message = Kafka::Protocol::Message.decode(decoder)
       decoded_message_set = decoded_message.decompress
+      messages = decoded_message_set.messages
 
-      expect(decoded_message_set.messages.map(&:value)).to eq ["hello1", "hello2"]
+      expect(messages.map(&:value)).to eq ["hello1", "hello2"]
+
+      # When decoding a compressed message, the offsets are calculated relative to that
+      # of the container message. The broker will set the offset in normal operation,
+      # but at the client-side we set it to -1.
+      expect(messages.map(&:offset)).to eq [-1, 0]
     end
 
     it "only compresses the messages if there are at least the configured threshold" do