(*****************************************************************************)
(* *)
(* Open Source License *)
(* Copyright (c) 2023 Nomadic Labs, <contact@nomadic-labs.com> *)
(* *)
(* Permission is hereby granted, free of charge, to any person obtaining a *)
(* copy of this software and associated documentation files (the "Software"),*)
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(* *)
(* The above copyright notice and this permission notice shall be included *)
(* in all copies or substantial portions of the Software. *)
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(* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR*)
(* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, *)
(* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL *)
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(*****************************************************************************)
open Error_monad
(* This LRU aims to contains ['value Store.t] which handles the
persistent storage for this value. [Store.t] allows to keep in
memory the content of the file. However, because there could be
numerous values to store, we use an LRU to avoid memory leaks. *)
module LRU =
Aches_lwt.Lache.Make_result
(Aches.Rache.Transfer
(Aches.Rache.LRU)
(struct
type t = string
let equal = String.equal
let hash = Hashtbl.hash
end))
type ('key, 'value) t =
| E : {
file_of : 'key -> 'value Stored_data.file;
(* Map a [key] to the file containing the corresponding [value]. *)
read_lru : ('value Stored_data.t, tztrace) LRU.t;
(* LRU which keeps in memory the last [values] stored for reads. *)
write_lru : ('value Stored_data.t, tztrace) LRU.t;
(* LRU which keeps in memory the last [values] stored for writes. *)
pool : unit Lwt_pool.t option;
(* An optional pool to ensure an upper bound of the number
of file descriptors opened by this store. *)
}
-> ('key, 'value) t
(* FIXME https://gitlab.com/tezos/tezos/-/issues/4643
The reason why there are two LRUs and not one, is that in the case
of concurrent reads and writes, the LRU cannot prevent the absence
of race. To prevent that we use two LRUs to be able to discriminate
between the various concurrent accesses. In particular, while
reading a value, we want to wait if there is a write in
progress. Vice versa, if a read fails, we don't want to make the
next write to fail.
In practice, there should not be a duplication in memory of the
values read since values are shared. *)
let with_pool pool f =
match pool with None -> f () | Some pool -> Lwt_pool.use pool f
let init ?pool ~lru_size file_of =
let write_lru = LRU.create lru_size in
let read_lru = LRU.create lru_size in
E {file_of; write_lru; read_lru; pool}
let read_value (E {write_lru; read_lru; file_of; pool}) key =
let open Lwt_result_syntax in
let file = file_of key in
match LRU.take write_lru file.path with
| None ->
LRU.bind_or_put
read_lru
file.path
(fun _path -> with_pool pool (fun () -> Stored_data.load file))
(function
| Error err -> fail err
| Ok store ->
let*! value = Stored_data.get store in
Lwt.return_ok value)
| Some store_promise ->
LRU.put write_lru file.path store_promise ;
let* store = store_promise in
let*! value = Stored_data.get store in
LRU.put read_lru file.path (return store) ;
Lwt.return_ok value
let write_value (type value) ?(override = false)
(E {write_lru; read_lru; file_of; pool}) key (value : value) =
let open Lwt_result_syntax in
let file = file_of key in
let write () =
LRU.bind_or_put
write_lru
file.path
(fun _path ->
with_pool pool (fun () ->
let open Lwt_utils_unix in
let*! () = create_dir (Filename.dirname file.path) in
Stored_data.init file ~initial_data:value))
(function
| Error err -> fail err
| Ok store ->
LRU.put read_lru file.path (return store) ;
let* x =
Stored_data.update_with store (fun (_ : value) ->
Lwt.return value)
in
return x)
in
match LRU.take write_lru file.path with
| None -> write ()
| Some store_promise ->
if override then (
let new_store_promise =
(* The code below is quite tricky. Assume the follow
scenario:
[W(key=0,override=false,value=0);W(key=0,override=true,value=1);
R(0)]. All the three operations can be run in parallel,
but the order is the one given by the list before.
If we keep the current store, it may be possible that
R(0) observes the value 0. This is because the LRU does
not give the guarantee that the `update_with` store will
be run after the one of the first write. *)
let* _store = store_promise in
let* store = Stored_data.load file in
let* () = Stored_data.update_with store (fun _ -> Lwt.return value) in
return store
in
LRU.put write_lru file.path new_store_promise ;
let* _store = new_store_promise in
return_unit)
else (
LRU.put write_lru file.path store_promise ;
let* _store = store_promise in
return_unit)
let write_values ?override t seq =
Seq.ES.iter (fun (key, value) -> write_value ?override t key value) seq
let read_values t seq =
let open Lwt_syntax in
Seq_s.of_seq seq
|> Seq_s.S.map (fun key ->
let* maybe_value = read_value t key in
return (key, maybe_value))