Abstraction for irmin-pack's append only files (i.e. suffix and dict).
It is parameterized with Io, a file system abstraction (e.g. unix, mirage, eio_linux).
It comprises a persistent file, an append buffer and take care of automatically shifting offsets to deal with legacy file headers.
module Io : Irmin_pack_unix.Io.SLow level IO abstraction. A typical implementation is unix. This abstraction is meant to be dead simple. Not a lot of documentation is required.
module Errs : Irmin_pack_unix.Io_errors.Sauto_flush_procedure defines behavior when the flush threshold is reached.
`Internal to have the buffer automatically flushed.`External f to have f called when the flush threshold is reached. It is the responsibility of f to call flush, in addition to any other processing it does.val create_rw :
path:string ->
overwrite:bool ->
auto_flush_threshold:int ->
auto_flush_procedure:auto_flush_procedure ->
(t, [> Io.create_error ]) Stdlib.resultCreate a rw instance of t by creating the file at path.
val open_rw :
path:string ->
end_poff:Optint.Int63.t ->
dead_header_size:int ->
auto_flush_threshold:int ->
auto_flush_procedure:auto_flush_procedure ->
(t,
[> Io.open_error
| `Closed
| `Invalid_argument
| `Read_out_of_bounds
| `Inconsistent_store ])
Stdlib.resultCreate 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_poff parameter, and also why refresh_end_poff 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_poff + dead_header_size.
This concept exists in order to keep supporting `V1 and `V2 pack stores with `V3.
Auto Flushes
One of the goals 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. When a call to append_exn fills the buffer, either the buffer will be flushed automatically, if auto_flush_procedure = `Internal, or the supplied external function f will be called, if auto_flush_procedure = `External f.
val open_ro :
path:string ->
end_poff:Optint.Int63.t ->
dead_header_size:int ->
(t,
[> Io.open_error
| `Closed
| `Inconsistent_store
| `Invalid_argument
| `Read_out_of_bounds ])
Stdlib.resultCreate a ro instance of t by opening an existing file at path
val close : t -> (unit, [> Io.close_error | `Pending_flush ]) Stdlib.resultClose the underlying file.
The internal buffer is expected to be in a flushed state when close is called. Otherwise, an error is returned.
val end_poff : t -> Optint.Int63.tend_poff t is the number of bytes of the file. That function doesn't perform IO.
RW mode
It also counts the bytes not flushed yet.
RO mode
This information originates from the latest reload of the control file. Calling refresh_end_poff t updates end_poff.
val read_to_string :
t ->
off:Optint.Int63.t ->
len:int ->
(string, [> Io.read_error ]) Stdlib.resultval read_exn : t -> off:Optint.Int63.t -> len:int -> bytes -> unitread_exn t ~off ~len b puts the len bytes of t at off to b.
read_to_string should always be favored over read_exn, except when performences matter.
It is not possible to read from an offset further than end_poff t.
Raises Io.Read_error and Errors.Pack_error `Read_out_of_bounds.
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.
val append_exn : t -> string -> unitappend_exn t ~off b writes b to the end of t. Might trigger an auto flush.
Grows end_poff, but the parent abstraction is expected to persist this somewhere (e.g. in the control file).
Post-condition: end_poff t - end_poff (old t) = String.length b.
Raises Io.Write_error
RW mode
Always raises Errors.RO_not_allowed
val flush : t -> (unit, [> Io.write_error ]) Stdlib.resultFlush the append buffer. Does not call fsync.
RO mode
Always returns Error `Ro_not_allowed.
val fsync : t -> (unit, [> Io.write_error ]) Stdlib.resultTell the os to fush its internal buffers. Does not call flush.
RO mode
Always returns Error `Ro_not_allowed.
val refresh_end_poff :
t ->
Optint.Int63.t ->
(unit, [> `Rw_not_allowed ]) Stdlib.resultIngest the new end offset of the file. Typically happens in RO mode when the control file has been re-read.
RW mode
Always returns Error `Rw_not_allowed.
val readonly : t -> boolval auto_flush_threshold : t -> int optionval empty_buffer : t -> boolval path : t -> string