Protobuf Codec: Fails to decode lists of lists

[thinkharderdev]

Currently, we fail to decode empty lists of lists. Issue can replicated with the following test case

  case class WithPiggyback(
    conversationId: Long,
    message: Chunk[Byte],
    gossip: List[Chunk[Byte]]
  )

  object WithPiggyback {
    implicit val schema: Schema[WithPiggyback] = DeriveSchema.gen

    implicit val gen: Gen[Random with Sized, WithPiggyback] =
      for {
        conversationId <- Gen.anyLong
        message        <- Gen.chunkOf(Gen.anyByte)
        gossip         <- Gen.listOf(Gen.chunkOf(Gen.anyByte))
      } yield WithPiggyback(conversationId, message, gossip)
  }

      testM("record with array fields") {
        checkM(WithPiggyback.gen) { value =>
          encodeAndDecodeNS(Schema[WithPiggyback], value, print = true).map { ed =>
            assert(ed)(equalTo(value))
          }
        }
      }

The issue seems to be in composing Decoders where flatMap errors out if there are no bytes remaining in the chunk after running the "first" decoder. This causes an issue when the final field in a Record is a collection. In that case, the key will be decoded as a length delimited field with zero length. As expected there are no more bytes to decode since the list is empty but when flatMaping the Decoder will error out.

[jupposessho]

The problem occurs with a single empty list as well:

testM("record with array fields") {
  val value = WithPiggyback(List(), 0)
  encodeAndDecodeNS(Schema[WithPiggyback], value, print = true).map { ed =>
    assert(ed)(equalTo(value))
  }
}

case class WithPiggyback(
  gossip: List[String],
  conversationId: Long
)

Even without the conversationId field.

I tried to decode the field as an EmptyCollection but the problem is the same, so not sure how to handle this.

[ashprakasan]

I'll work on this.