Create a rw instance of t by opening an existing file at path.
End Offset
The file has an end offset at which new data will be saved. While this information could be computed by looking at the size of the file, we prefer storing that information elsewhere (i.e. in the control file). This is why open_rw and open_ro take an end_offset parameter, and also why refresh_end_offset exists. The abstractions above Append_only_file are responsible for reading/writing the offsets from/to the control file.
dead_header_size
Designates a small area at the beginning of the file that should be ignored. The offsets start after that area.
The actual persisted size of a file is end_offset + dead_header_size.
This concept exists in order to keep supporting `V1 and `V2 pack stores with `V3.
Auto Flushes
One of the goal of the Append_only_file abstraction is to provide buffered appends. auto_flush_threshold is the soft cap after which the buffer should be flushed. If a call to append_exn fills the buffer, auto_flush_callback will be called so that the parent abstraction takes care of the flush procedure.
val read_exn : t->off:Optint.Int63.t->len:int ->bytes -> unit
read_exn t ~off ~len b puts the len bytes of t at off to b.
Raises Io.Read_error
RW mode
Attempting to read from the append buffer results in an `Read_out_of_bounds error. This feature could easily be implemented in the future if ever needed. It was not needed with io_legacy.
RO mode
It is not possible to read from an offset further than end_offset t.