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|
(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2017 *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
(* *)
(* A persistent hashtable *)
(* *)
(* Frédéric Besson (Inria Rennes) 2009-2014 *)
(* *)
(************************************************************************)
module type PHashtable =
sig
type 'a t
type key
val create : int -> string -> 'a t
(** [create i f] creates an empty persistent table
with initial size i associated with file [f] *)
val open_in : string -> 'a t
(** [open_in f] rebuilds a table from the records stored in file [f].
As marshaling is not type-safe, it migth segault.
*)
val find : 'a t -> key -> 'a
(** find has the specification of Hashtable.find *)
val add : 'a t -> key -> 'a -> unit
(** [add tbl key elem] adds the binding [key] [elem] to the table [tbl].
(and writes the binding to the file associated with [tbl].)
If [key] is already bound, raises KeyAlreadyBound *)
val close : 'a t -> unit
(** [close tbl] is closing the table.
Once closed, a table cannot be used.
i.e, find,add will raise UnboundTable *)
val memo : string -> (key -> 'a) -> (key -> 'a)
(** [memo cache f] returns a memo function for [f] using file [cache] as persistent table.
Note that the cache will only be loaded when the function is used for the first time *)
end
open Hashtbl
module PHashtable(Key:HashedType) : PHashtable with type key = Key.t =
struct
open Unix
type key = Key.t
module Table = Hashtbl.Make(Key)
exception InvalidTableFormat
exception UnboundTable
type mode = Closed | Open
type 'a t =
{
outch : out_channel ;
mutable status : mode ;
htbl : 'a Table.t
}
let create i f =
let flags = [O_WRONLY; O_TRUNC;O_CREAT] in
{
outch = out_channel_of_descr (openfile f flags 0o666);
status = Open ;
htbl = Table.create i
}
let finally f rst =
try
let res = f () in
rst () ; res
with reraise ->
(try rst ()
with any -> raise reraise
); raise reraise
let read_key_elem inch =
try
Some (Marshal.from_channel inch)
with
| End_of_file -> None
| e when CErrors.noncritical e -> raise InvalidTableFormat
(**
We used to only lock/unlock regions.
Is-it more robust/portable to lock/unlock a fixed region e.g. [0;1]?
In case of locking failure, the cache is not used.
**)
type lock_kind = Read | Write
let lock kd fd =
let pos = lseek fd 0 SEEK_CUR in
let success =
try
ignore (lseek fd 0 SEEK_SET);
let lk = match kd with
| Read -> F_RLOCK
| Write -> F_LOCK in
lockf fd lk 1; true
with Unix.Unix_error(_,_,_) -> false in
ignore (lseek fd pos SEEK_SET) ;
success
let unlock fd =
let pos = lseek fd 0 SEEK_CUR in
try
ignore (lseek fd 0 SEEK_SET) ;
lockf fd F_ULOCK 1
with
Unix.Unix_error(_,_,_) -> ()
(* Here, this is really bad news --
there is a pending lock which could cause a deadlock.
Should it be an anomaly or produce a warning ?
*);
ignore (lseek fd pos SEEK_SET)
(* We make the assumption that an acquired lock can always be released *)
let do_under_lock kd fd f =
if lock kd fd
then
finally f (fun () -> unlock fd)
else f ()
let open_in f =
let flags = [O_RDONLY ; O_CREAT] in
let finch = openfile f flags 0o666 in
let inch = in_channel_of_descr finch in
let htbl = Table.create 100 in
let rec xload () =
match read_key_elem inch with
| None -> ()
| Some (key,elem) ->
Table.replace htbl key elem ;
xload () in
try
(* Locking of the (whole) file while reading *)
do_under_lock Read finch xload ;
close_in_noerr inch ;
{
outch = out_channel_of_descr (openfile f [O_WRONLY;O_APPEND;O_CREAT] 0o666) ;
status = Open ;
htbl = htbl
}
with InvalidTableFormat ->
(* The file is corrupted *)
begin
close_in_noerr inch ;
let flags = [O_WRONLY; O_TRUNC;O_CREAT] in
let out = (openfile f flags 0o666) in
let outch = out_channel_of_descr out in
do_under_lock Write out
(fun () ->
Table.iter
(fun k e -> Marshal.to_channel outch (k,e) [Marshal.No_sharing]) htbl;
flush outch) ;
{ outch = outch ;
status = Open ;
htbl = htbl
}
end
let close t =
let {outch = outch ; status = status ; htbl = tbl} = t in
match t.status with
| Closed -> () (* don't do it twice *)
| Open ->
close_out outch ;
Table.clear tbl ;
t.status <- Closed
let add t k e =
let {outch = outch ; status = status ; htbl = tbl} = t in
if status == Closed
then raise UnboundTable
else
let fd = descr_of_out_channel outch in
begin
Table.replace tbl k e ;
do_under_lock Write fd
(fun _ ->
Marshal.to_channel outch (k,e) [Marshal.No_sharing] ;
flush outch
)
end
let find t k =
let {outch = outch ; status = status ; htbl = tbl} = t in
if status == Closed
then raise UnboundTable
else
let res = Table.find tbl k in
res
let memo cache f =
let tbl = lazy (try Some (open_in cache) with _ -> None) in
fun x ->
match Lazy.force tbl with
| None -> f x
| Some tbl ->
try
find tbl x
with
Not_found ->
let res = f x in
add tbl x res ;
res
end
(* Local Variables: *)
(* coding: utf-8 *)
(* End: *)
|
