The wording of the RFC has improved since that draft and the flexibility could be useful in some scenarios.
I ended up with an API which can be asked to deliver complete messages 'if possible' given the buffers provided by the client of the API. If it's not possible and the buffer becomes full then the API simply gives you the fragment of data and tells you if it knows how much more there is to come or not.
The wording of the RFC has improved since that draft and the flexibility could be useful in some scenarios.
I ended up with an API which can be asked to deliver complete messages 'if possible' given the buffers provided by the client of the API. If it's not possible and the buffer becomes full then the API simply gives you the fragment of data and tells you if it knows how much more there is to come or not.