Space in the ring that the writer has claimed with writer_reserve_message and not committed yet.
Write the message through user_write_pointer, as many bytes as were asked for, then pass the record to
writer_commit_message. The rest of the fields are the channel's bookkeeping and must be left as they are. Only one
reservation can be open at a time: reserving again before committing gives the same space.
Fields
- channel: *Write_Channel
- offset: s64
- total_bytes: s64
- user_write_pointer: *u8