1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker, Matt Lilley
    4    E-mail:        J.Wielemaker@cs.vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2006-2024, University of Amsterdam
    7                              VU University Amsterdam
    8                              CWI, Amsterdam
    9                              SWI-Prolog Solutions b.v.
   10    All rights reserved.
   11
   12    Redistribution and use in source and binary forms, with or without
   13    modification, are permitted provided that the following conditions
   14    are met:
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   17       notice, this list of conditions and the following disclaimer.
   18
   19    2. Redistributions in binary form must reproduce the above copyright
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   22       distribution.
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   25    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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   36*/
   37
   38:- module(http_session,
   39          [ http_set_session_options/1, % +Options
   40            http_set_session/1,         % +Option
   41            http_set_session/2,         % +SessionId, +Option
   42            http_session_option/1,      % ?Option
   43
   44            http_session_id/1,          % -SessionId
   45            http_in_session/1,          % -SessionId
   46            http_current_session/2,     % ?SessionId, ?Data
   47            http_close_session/1,       % +SessionId
   48            http_open_session/2,        % -SessionId, +Options
   49
   50            http_session_cookie/1,      % -Cookie
   51
   52            http_session_asserta/1,     % +Data
   53            http_session_assert/1,      % +Data
   54            http_session_retract/1,     % ?Data
   55            http_session_retractall/1,  % +Data
   56            http_session_data/1,        % ?Data
   57
   58            http_session_asserta/2,     % +Data, +SessionId
   59            http_session_assert/2,      % +Data, +SessionId
   60            http_session_retract/2,     % ?Data, +SessionId
   61            http_session_retractall/2,  % +Data, +SessionId
   62            http_session_data/2         % ?Data, +SessionId
   63          ]).   64:- use_module(http_wrapper).   65:- use_module(http_stream).   66:- use_module(library(error)).   67:- use_module(library(debug)).   68:- use_module(library(socket)).   69:- use_module(library(broadcast)).   70:- use_module(library(lists)).   71:- use_module(library(time)).   72:- use_module(library(option)).   73
   74:- predicate_options(http_open_session/2, 2, [renew(boolean)]).

:- listen(http_session(end(SessionId, Peer)),
          end_session(SessionId)).

  112:- dynamic
  113    session_setting/1,              % Name(Value)
  114    current_session/2,              % SessionId, Peer
  115    last_used/2,                    % SessionId, Time
  116    session_data/2.                 % SessionId, Data
  117
  118:- multifile
  119    hooked/0,
  120    hook/1,                         % +Term
  121    session_setting/1,
  122    session_option/2.  123
  124session_setting(timeout(600)).      % timeout in seconds
  125session_setting(granularity(60)).   % granularity for timeout
  126session_setting(cookie('swipl_session')).
  127session_setting(path(/)).
  128session_setting(enabled(true)).
  129session_setting(create(auto)).
  130session_setting(proxy_enabled(false)).
  131session_setting(gc(passive)).
  132session_setting(samesite(lax)).
  133
  134session_option(timeout, integer).
  135session_option(granularity, integer).
  136session_option(cookie, atom).
  137session_option(path, atom).
  138session_option(create, oneof([auto,noauto])).
  139session_option(route, atom).
  140session_option(enabled, boolean).
  141session_option(proxy_enabled, boolean).
  142session_option(gc, oneof([active,passive])).
  143session_option(samesite, oneof([none,lax,strict])).

 http_set_session_options(+Options) is det

Set options for the session library. Provided options are:

timeout(+Seconds)

Session timeout in seconds. Default is 600 (10 min). A timeout of 0 (zero) disables timeout.

cookie(+Cookiekname)

Name to use for the cookie to identify the session. Default swipl_session.

path(+Path)

Path to which the cookie is associated. Default is /. Cookies are only sent if the HTTP request path is a refinement of Path.

route(+Route)

Set the route name. Default is the unqualified hostname. To cancel adding a route, use the empty atom. See route/1.

enabled(+Boolean)

Enable/disable session management. Sesion management is enabled by default after loading this file.

create(+Atom)

Defines when a session is created. This is one of auto (default), which creates a session if there is a request whose path matches the defined session path or noauto, in which cases sessions are only created by calling http_open_session/2 explicitely.

proxy_enabled(+Boolean)

Enable/disable proxy session management. Proxy session management associates the originating IP address of the client to the session rather than the proxy IP address. Default is false.

gc(+When)

When is one of active, which starts a thread that performs session cleanup at close to the moment of the timeout or passive, which runs session GC when a new session is created.

samesite(+Restriction)

One of none, lax (default), or strict - The SameSite attribute prevents the CSRF vulnerability. strict has best security, but prevents links from external sites from operating properly. lax stops most CSRF attacks against REST endpoints but rarely interferes with legitimage operations. none removes the samesite attribute entirely. __Caution: The value none exposes the entire site to CSRF attacks.

granularity(+Integer)

Granularity for updating that the session is active. Default is 60 (seconds). Smaller values lead to more precise session timeout at the cost of more database updates. This may notably a problem when using Redis.

In addition, extension libraries can define session_option/2 to make this predicate support more options. In particular, library(http/http_redis_plugin) defines the following additional options:

redis_db(+DB)

Alias name of the redis database to access. See redis_server/3.

redis_ro(+DB)

Alias name of the redis database for read-only access. See redis_server/3.

redis_prefix(+Atom)

Prefix to use for all HTTP session related keys. Default is 'swipl:http:session'

  219http_set_session_options([]) => true.
  220http_set_session_options([H|T]) =>
  221    http_set_session_option(H),
  222    http_set_session_options(T).
  223
  224http_set_session_option(Option), Option =.. [Name,Value] =>
  225    (   session_option(Name, Type)
  226    ->  must_be(Type, Value)
  227    ;   domain_error(http_session_option, Option)
  228    ),
  229    functor(Free, Name, 1),
  230    (   clause(session_setting(Free), _, Ref)
  231    ->  (   Free \== Value
  232        ->  asserta(session_setting(Option)),
  233            erase(Ref),
  234            updated_session_setting(Name, Free, Value)
  235        ;   true
  236        )
  237    ;   asserta(session_setting(Option))
  238    ).

 http_session_option(?Option) is nondet

True if Option is a current option of the session system.

  244http_session_option(Option) :-
  245    session_setting(Option).

 session_setting(+SessionID, ?Setting) is semidet

Find setting for SessionID. It is possible to overrule some session settings using http_session_set(Setting).

  252:- public session_setting/2.  253
  254session_setting(SessionID, Setting) :-
  255    nonvar(Setting),
  256    get_session_option(SessionID, Setting),
  257    !.
  258session_setting(_, Setting) :-
  259    session_setting(Setting).
  260
  261get_session_option(SessionID, Setting) :-
  262    hooked,
  263    !,
  264    hook(get_session_option(SessionID, Setting)).
  265get_session_option(SessionID, Setting) :-
  266    functor(Setting, Name, 1),
  267    local_option(Name, Value, Term),
  268    session_data(SessionID, '$setting'(Term)),
  269    !,
  270    arg(1, Setting, Value).
  271
  272
  273updated_session_setting(gc, _, passive) :-
  274    stop_session_gc_thread, !.
  275updated_session_setting(_, _, _).               % broadcast?

 http_set_session(Setting) is det

 http_set_session(SessionId, Setting) is det

Overrule a setting for the current or specified session. Currently, the only setting that can be overruled is timeout.

Errors

- permission_error(set, http_session, Setting) if setting a setting that is not supported on per-session basis.

  287http_set_session(Setting) :-
  288    http_session_id(SessionId),
  289    http_set_session(SessionId, Setting).
  290
  291http_set_session(SessionId, Setting) :-
  292    functor(Setting, Name, _),
  293    (   local_option(Name, _, _)
  294    ->  true
  295    ;   permission_error(set, http_session, Setting)
  296    ),
  297    arg(1, Setting, Value),
  298    (   session_option(Name, Type)
  299    ->  must_be(Type, Value)
  300    ;   domain_error(http_session_option, Setting)
  301    ),
  302    set_session_option(SessionId, Setting).
  303
  304set_session_option(SessionId, Setting) :-
  305    hooked,
  306    !,
  307    hook(set_session_option(SessionId, Setting)).
  308set_session_option(SessionId, Setting) :-
  309    functor(Setting, Name, Arity),
  310    functor(Free, Name, Arity),
  311    retractall(session_data(SessionId, '$setting'(Free))),
  312    assert(session_data(SessionId, '$setting'(Setting))).
  313
  314local_option(timeout, X, timeout(X)).

 http_session_id(-SessionId) is det

True if SessionId is an identifier for the current session.

Arguments:

SessionId

- is an atom.

Errors

- existence_error(http_session, _)

See also

- http_in_session/1 for a version that fails if there is no session.

  325http_session_id(SessionID) :-
  326    (   http_in_session(ID)
  327    ->  SessionID = ID
  328    ;   throw(error(existence_error(http_session, _), _))
  329    ).

 http_in_session(-SessionId) is semidet

True if SessionId is an identifier for the current session. The current session is extracted from session(ID) from the current HTTP request (see http_current_request/1). The value is cached in a backtrackable global variable http_session_id. Using a backtrackable global variable is safe because continuous worker threads use a failure driven loop and spawned threads start without any global variables. This variable can be set from the commandline to fake running a goal from the commandline in the context of a session.

See also

- http_session_id/1

  345http_in_session(SessionID) :-
  346    nb_current(http_session_id, ID),
  347    ID \== [],
  348    !,
  349    debug(http_session, 'Session id from global variable: ~q', [ID]),
  350    ID \== no_session,
  351    SessionID = ID.
  352http_in_session(SessionID) :-
  353    http_current_request(Request),
  354    http_in_session(Request, SessionID).
  355
  356http_in_session(Request, SessionID) :-
  357    memberchk(session(ID), Request),
  358    !,
  359    debug(http_session, 'Session id from request: ~q', [ID]),
  360    b_setval(http_session_id, ID),
  361    SessionID = ID.
  362http_in_session(Request, SessionID) :-
  363    memberchk(cookie(Cookies), Request),
  364    session_setting(cookie(Cookie)),
  365    member(Cookie=SessionID0, Cookies),
  366    debug(http_session, 'Session id from cookie: ~q', [SessionID0]),
  367    peer(Request, Peer),
  368    valid_session_id(SessionID0, Peer),
  369    !,
  370    b_setval(http_session_id, SessionID0),
  371    SessionID = SessionID0.

 http_session(+RequestIn, -RequestOut, -SessionID) is semidet

Maintain the notion of a session using a client-side cookie. This must be called first when handling a request that wishes to do session management, after which the possibly modified request must be used for further processing.

This predicate creates a session if the setting create is auto. If create is noauto, the application must call http_open_session/1 to create a session.

  385http_session(Request, Request, SessionID) :-
  386    memberchk(session(SessionID0), Request),
  387    !,
  388    SessionID = SessionID0.
  389http_session(Request0, Request, SessionID) :-
  390    memberchk(cookie(Cookies), Request0),
  391    session_setting(cookie(Cookie)),
  392    member(Cookie=SessionID0, Cookies),
  393    peer(Request0, Peer),
  394    valid_session_id(SessionID0, Peer),
  395    !,
  396    SessionID = SessionID0,
  397    Request = [session(SessionID)|Request0],
  398    b_setval(http_session_id, SessionID).
  399http_session(Request0, Request, SessionID) :-
  400    session_setting(create(auto)),
  401    session_setting(path(Path)),
  402    memberchk(path(ReqPath), Request0),
  403    sub_atom(ReqPath, 0, _, _, Path),
  404    !,
  405    create_session(Request0, Request, SessionID).
  406
  407create_session(Request0, Request, SessionID) :-
  408    http_gc_sessions,
  409    http_session_cookie(SessionID),
  410    session_setting(cookie(Cookie)),
  411    session_setting(path(Path)),
  412    session_setting(samesite(SameSite)),
  413    debug(http_session, 'Created session ~q at path=~q', [SessionID, Path]),
  414    (   SameSite == none
  415    ->  format('Set-Cookie: ~w=~w; Path=~w; Version=1\r\n',
  416               [Cookie, SessionID, Path])
  417    ;   format('Set-Cookie: ~w=~w; Path=~w; Version=1; SameSite=~w\r\n',
  418               [Cookie, SessionID, Path, SameSite])
  419    ),
  420    Request = [session(SessionID)|Request0],
  421    peer(Request0, Peer),
  422    open_session(SessionID, Peer).

 http_open_session(-SessionID, +Options) is det

Establish a new session. This is normally used if the create option is set to noauto. Options:

renew(+Boolean)

If true (default false) and the current request is part of a session, generate a new session-id. By default, this predicate returns the current session as obtained with http_in_session/1.

Errors

- permission_error(open, http_session, CGI) if this call is used after closing the CGI header.

See also

- http_set_session_options/1 to control the create option.

- http_close_session/1 for closing the session.

  441http_open_session(SessionID, Options) :-
  442    http_in_session(SessionID0),
  443    \+ option(renew(true), Options, false),
  444    !,
  445    SessionID = SessionID0.
  446http_open_session(SessionID, _Options) :-
  447    (   in_header_state
  448    ->  true
  449    ;   current_output(CGI),
  450        permission_error(open, http_session, CGI)
  451    ),
  452    (   http_in_session(ActiveSession)
  453    ->  http_close_session(ActiveSession, false)
  454    ;   true
  455    ),
  456    http_current_request(Request),
  457    create_session(Request, _, SessionID).
  458
  459
  460:- multifile
  461    http:request_expansion/2.  462
  463http:request_expansion(Request0, Request) :-
  464    session_setting(enabled(true)),
  465    http_session(Request0, Request, _SessionID).

 peer(+Request, -Peer) is det

Find peer for current request. If unknown we leave it unbound. Alternatively we should treat this as an error.

  472peer(Request, Peer) :-
  473    (   session_setting(proxy_enabled(true)),
  474        http_peer(Request, Peer)
  475    ->  true
  476    ;   memberchk(peer(Peer), Request)
  477    ->  true
  478    ;   true
  479    ).

 open_session(+SessionID, +Peer)

Open a new session. Uses broadcast/1 with the term http_session(begin(SessionID, Peer)).

  486open_session(SessionID, Peer) :-
  487    assert_session(SessionID, Peer),
  488    b_setval(http_session_id, SessionID),
  489    broadcast(http_session(begin(SessionID, Peer))).
  490
  491assert_session(SessionID, Peer) :-
  492    hooked,
  493    !,
  494    hook(assert_session(SessionID, Peer)).
  495assert_session(SessionID, Peer) :-
  496    get_time(Now),
  497    assert(current_session(SessionID, Peer)),
  498    assert(last_used(SessionID, Now)).

 valid_session_id(+SessionID, +Peer) is semidet

Check if this sessionID is known. If so, check the idle time and update the last_used for this session.

  505valid_session_id(SessionID, Peer) :-
  506    active_session(SessionID, SessionPeer, LastUsed),
  507    get_time(Now),
  508    (   session_setting(SessionID, timeout(Timeout)),
  509        Timeout > 0
  510    ->  Idle is Now - LastUsed,
  511        (   Idle =< Timeout
  512        ->  true
  513        ;   http_close_session(SessionID),
  514            fail
  515        )
  516    ;   Peer \== SessionPeer
  517    ->  http_close_session(SessionID),
  518        fail
  519    ;   true
  520    ),
  521    set_last_used(SessionID, Now, Timeout).
  522
  523active_session(SessionID, Peer, LastUsed) :-
  524    hooked,
  525    !,
  526    hook(active_session(SessionID, Peer, LastUsed)).
  527active_session(SessionID, Peer, LastUsed) :-
  528    current_session(SessionID, Peer),
  529    get_last_used(SessionID, LastUsed).
  530
  531get_last_used(SessionID, Last) :-
  532    atom(SessionID),
  533    !,
  534    once(last_used(SessionID, Last)).
  535get_last_used(SessionID, Last) :-
  536    last_used(SessionID, Last).

 set_last_used(+SessionID, +Now, +TimeOut)

Set the last-used notion for SessionID from the current time stamp. The time is rounded down to 10 second intervals to avoid many updates and simplify the scheduling of session GC.

  544set_last_used(SessionID, Now, TimeOut) :-
  545    hooked,
  546    !,
  547    hook(set_last_used(SessionID, Now, TimeOut)).
  548set_last_used(SessionID, Now, _TimeOut) :-
  549    session_setting(granularity(TimeGranularity)),
  550    LastUsed is floor(Now/TimeGranularity)*TimeGranularity,
  551    (   clause(last_used(SessionID, CurrentLast), _, Ref)
  552    ->  (   CurrentLast == LastUsed
  553        ->  true
  554        ;   asserta(last_used(SessionID, LastUsed)),
  555            erase(Ref)
  556        )
  557    ;   asserta(last_used(SessionID, LastUsed))
  558    ).
  559
  560
  561                 /*******************************
  562                 *         SESSION DATA         *
  563                 *******************************/

 http_session_asserta(+Data) is det

 http_session_assert(+Data) is det

 http_session_retract(?Data) is nondet

 http_session_retractall(?Data) is det

Versions of assert/1, retract/1 and retractall/1 that associate data with the current HTTP session.

  573http_session_asserta(Data) :-
  574    http_session_id(SessionId),
  575    (   hooked
  576    ->  hook(asserta(session_data(SessionId, Data)))
  577    ;   asserta(session_data(SessionId, Data))
  578    ).
  579
  580http_session_assert(Data) :-
  581    http_session_id(SessionId),
  582    (   hooked
  583    ->  hook(assertz(session_data(SessionId, Data)))
  584    ;   assertz(session_data(SessionId, Data))
  585    ).
  586
  587http_session_retract(Data) :-
  588    http_session_id(SessionId),
  589    (   hooked
  590    ->  hook(retract(session_data(SessionId, Data)))
  591    ;   retract(session_data(SessionId, Data))
  592    ).
  593
  594http_session_retractall(Data) :-
  595    http_session_id(SessionId),
  596    (   hooked
  597    ->  hook(retractall(session_data(SessionId, Data)))
  598    ;   retractall(session_data(SessionId, Data))
  599    ).

 http_session_data(?Data) is nondet

True if Data is associated using http_session_assert/1 to the current HTTP session.

Errors

- existence_error(http_session,_)

  608http_session_data(Data) :-
  609    http_session_id(SessionId),
  610    (   hooked
  611    ->  hook(session_data(SessionId, Data))
  612    ;   session_data(SessionId, Data)
  613    ).

 http_session_asserta(+Data, +SessionID) is det

 http_session_assert(+Data, +SessionID) is det

 http_session_retract(?Data, +SessionID) is nondet

 http_session_retractall(@Data, +SessionID) is det

 http_session_data(?Data, +SessionID) is det

Versions of assert/1, retract/1 and retractall/1 that associate data with an explicit HTTP session.

See also

- http_current_session/2.

  626http_session_asserta(Data, SessionId) :-
  627    must_be(atom, SessionId),
  628    (   hooked
  629    ->  hook(asserta(session_data(SessionId, Data)))
  630    ;   asserta(session_data(SessionId, Data))
  631    ).
  632
  633http_session_assert(Data, SessionId) :-
  634    must_be(atom, SessionId),
  635    (   hooked
  636    ->  hook(assertz(session_data(SessionId, Data)))
  637    ;   assertz(session_data(SessionId, Data))
  638    ).
  639
  640http_session_retract(Data, SessionId) :-
  641    must_be(atom, SessionId),
  642    (   hooked
  643    ->  hook(retract(session_data(SessionId, Data)))
  644    ;   retract(session_data(SessionId, Data))
  645    ).
  646
  647http_session_retractall(Data, SessionId) :-
  648    must_be(atom, SessionId),
  649    (   hooked
  650    ->  hook(retractall(session_data(SessionId, Data)))
  651    ;   retractall(session_data(SessionId, Data))
  652    ).
  653
  654http_session_data(Data, SessionId) :-
  655    must_be(atom, SessionId),
  656    (   hooked
  657    ->  hook(session_data(SessionId, Data))
  658    ;   session_data(SessionId, Data)
  659    ).
  660
  661
  662                 /*******************************
  663                 *           ENUMERATE          *
  664                 *******************************/

 http_current_session(?SessionID, ?Data) is nondet

Enumerate the current sessions and associated data. There are two pseudo data elements:

idle(Seconds)

Session has been idle for Seconds.

peer(Peer)

Peer of the connection.

  677http_current_session(SessionID, Data) :-
  678    hooked,
  679    !,
  680    hook(current_session(SessionID, Data)).
  681http_current_session(SessionID, Data) :-
  682    get_time(Now),
  683    get_last_used(SessionID, Last), % binds SessionID
  684    Idle is Now - Last,
  685    (   session_setting(SessionID, timeout(Timeout)),
  686        Timeout > 0
  687    ->  Idle =< Timeout
  688    ;   true
  689    ),
  690    (   Data = idle(Idle)
  691    ;   Data = peer(Peer),
  692        current_session(SessionID, Peer)
  693    ;   session_data(SessionID, Data)
  694    ).
  695
  696
  697                 /*******************************
  698                 *          GC SESSIONS         *
  699                 *******************************/

 http_close_session(+SessionID) is det

Closes an HTTP session. This predicate can be called from any thread to terminate a session. It uses the broadcast/1 service with the message below.

http_session(end(SessionId, Peer))

The broadcast is done before the session data is destroyed and the listen-handlers are executed in context of the session that is being closed. Here is an example that destroys a Prolog thread that is associated to a thread:

:- listen(http_session(end(SessionId, _Peer)),
          kill_session_thread(SessionID)).

kill_session_thread(SessionID) :-
        http_session_data(thread(ThreadID)),
        thread_signal(ThreadID, throw(session_closed)).

Succeed without any effect if SessionID does not refer to an active session.

If http_close_session/1 is called from a handler operating in the current session and the CGI stream is still in state header, this predicate emits a Set-Cookie to expire the cookie.

Errors

- type_error(atom, SessionID)

See also

- listen/2 for acting upon closed sessions

  734http_close_session(SessionId) :-
  735    http_close_session(SessionId, true).
  736
  737http_close_session(SessionId, Expire) :-
  738    hooked,
  739    !,
  740    forall(hook(close_session(SessionId)),
  741           expire_session_cookie(Expire)).
  742http_close_session(SessionId, Expire) :-
  743    must_be(atom, SessionId),
  744    (   current_session(SessionId, Peer),
  745        (   b_setval(http_session_id, SessionId),
  746            broadcast(http_session(end(SessionId, Peer))),
  747            fail
  748        ;   true
  749        ),
  750        expire_session_cookie(Expire),
  751        retractall(current_session(SessionId, _)),
  752        retractall(last_used(SessionId, _)),
  753        retractall(session_data(SessionId, _)),
  754        fail
  755    ;   true
  756    ).

 expire_session_cookie(+Expire) is det

Emit a request to delete a session cookie. This is only done if http_close_session/1 is still in `header mode'.

  764expire_session_cookie(true) :-
  765    !,
  766    expire_session_cookie.
  767expire_session_cookie(_).
  768
  769expire_session_cookie :-
  770    in_header_state,
  771    session_setting(cookie(Cookie)),
  772    session_setting(path(Path)),
  773    !,
  774    format('Set-Cookie: ~w=; \c
  775                expires=Tue, 01-Jan-1970 00:00:00 GMT; \c
  776                path=~w\r\n',
  777           [Cookie, Path]).
  778expire_session_cookie.
  779
  780in_header_state :-
  781    current_output(CGI),
  782    is_cgi_stream(CGI),
  783    cgi_property(CGI, state(header)),
  784    !.

 http_gc_sessions is det

 http_gc_sessions(+TimeOut) is det

Delete dead sessions. There are two modes. if gc(passive) is active (default), session GC is executed if a new session is created and if the last GC was more than granularity ago. If gc(active) is on, a thread is created that runs the session GC every granularity seconds.

http_gc_sessions/0 is called each time when a session is created.

  798:- dynamic
  799    last_gc/1.  800
  801http_gc_sessions :-
  802    session_setting(gc(active)),
  803    !,
  804    start_session_gc_thread.
  805http_gc_sessions :-
  806    session_setting(granularity(TimeGranularity)),
  807    http_gc_sessions(TimeGranularity).
  808
  809http_gc_sessions(TimeOut) :-
  810    (   with_mutex(http_session_gc, need_sesion_gc(TimeOut))
  811    ->  do_http_gc_sessions
  812    ;   true
  813    ).
  814
  815need_sesion_gc(TimeOut) :-
  816    get_time(Now),
  817    (   last_gc(LastGC),
  818        Now-LastGC < TimeOut
  819    ->  fail
  820    ;   retractall(last_gc(_)),
  821        asserta(last_gc(Now))
  822    ).
  823
  824do_http_gc_sessions :-
  825    hooked,
  826    !,
  827    hook(gc_sessions).
  828do_http_gc_sessions :-
  829    debug(http_session(gc), 'Running HTTP session GC', []),
  830    get_time(Now),
  831    (   session_setting(SessionID, timeout(Timeout)),
  832        last_used(SessionID, Last),
  833        Timeout > 0,
  834        Idle is Now - Last,
  835        Idle > Timeout,
  836        http_close_session(SessionID, false),
  837        fail
  838    ;   true
  839    ).

 start_session_gc_thread is det

 stop_session_gc_thread is det

Create/stop a thread that listens for timeout-at timing and wakes up to run http_gc_sessions/1 shortly after a session is scheduled to timeout.

  848:- dynamic
  849    session_gc_queue/1.  850
  851start_session_gc_thread :-
  852    session_gc_queue(_),
  853    !.
  854start_session_gc_thread :-
  855    session_setting(gc(active)),
  856    !,
  857    catch(thread_create(session_gc_loop, _,
  858                        [ alias('__http_session_gc'),
  859                          at_exit(retractall(session_gc_queue(_))),
  860                          inherit_from(main)
  861                        ]),
  862          error(permission_error(create, thread, _),_),
  863          true).
  864start_session_gc_thread.
  865
  866stop_session_gc_thread :-
  867    retract(session_gc_queue(Id)),
  868    !,
  869    thread_send_message(Id, done),
  870    thread_join(Id, _).
  871stop_session_gc_thread.
  872
  873session_gc_loop :-
  874    thread_self(GcQueue),
  875    asserta(session_gc_queue(GcQueue)),
  876    session_gc_loop_.
  877
  878session_gc_loop_ :-
  879    session_setting(gc(active)),
  880    session_setting(granularity(TimeGranularity)),
  881    get_time(Now),
  882    At is Now+TimeGranularity,
  883    thread_self(GcQueue),
  884    repeat,
  885    (   thread_get_message(GcQueue, Message, [deadline(At)])
  886    ->  (   Message == done
  887        ->  !
  888        ;   fail
  889        )
  890    ;   !,
  891        http_gc_sessions(10),       % short time, so it runs
  892        session_gc_loop_
  893    ).
  894
  895
  896                 /*******************************
  897                 *             UTIL             *
  898                 *******************************/

 http_session_cookie(-Cookie) is det

Generate a random cookie that can be used by a browser to identify the current session. The cookie has the format XXXX-XXXX-XXXX-XXXX[.<route>], where XXXX are random hexadecimal numbers and [.<route>] is the optionally added routing information.

  908http_session_cookie(Cookie) :-
  909    route(Route),
  910    !,
  911    random_4(R1,R2,R3,R4),
  912    format(atom(Cookie),
  913            '~`0t~16r~4|-~`0t~16r~9|-~`0t~16r~14|-~`0t~16r~19|.~w',
  914            [R1,R2,R3,R4,Route]).
  915http_session_cookie(Cookie) :-
  916    random_4(R1,R2,R3,R4),
  917    format(atom(Cookie),
  918            '~`0t~16r~4|-~`0t~16r~9|-~`0t~16r~14|-~`0t~16r~19|',
  919            [R1,R2,R3,R4]).
  920
  921:- thread_local
  922    route_cache/1.

 route(-RouteID) is semidet

Fetch the route identifier. This value is added as .<route> to the session cookie and used by -for example- the apache load balanching module. The default route is the local name of the host. Alternatives may be provided using http_set_session_options/1.

  932route(Route) :-
  933    route_cache(Route),
  934    !,
  935    Route \== ''.
  936route(Route) :-
  937    route_no_cache(Route),
  938    assert(route_cache(Route)),
  939    Route \== ''.
  940
  941route_no_cache(Route) :-
  942    session_setting(route(Route)),
  943    !.
  944route_no_cache(Route) :-
  945    gethostname(Host),
  946    (   sub_atom(Host, Before, _, _, '.')
  947    ->  sub_atom(Host, 0, Before, _, Route)
  948    ;   Route = Host
  949    ).
  950
  951:- if(\+current_prolog_flag(windows, true)).

 urandom(-Handle) is semidet

Handle is a stream-handle for /dev/urandom. Originally, this simply tried to open /dev/urandom, failing if this device does not exist. It turns out that trying to open /dev/urandom can block indefinitely on some Windows installations, so we no longer try this on Windows.

  960:- dynamic
  961    urandom_handle/1.  962
  963urandom(Handle) :-
  964    urandom_handle(Handle),
  965    !,
  966    Handle \== [].
  967urandom(Handle) :-
  968    catch(open('/dev/urandom', read, In, [type(binary)]), _, fail),
  969    !,
  970    assert(urandom_handle(In)),
  971    Handle = In.
  972urandom(_) :-
  973    assert(urandom_handle([])),
  974    fail.
  975
  976get_pair(In, Value) :-
  977    get_byte(In, B1),
  978    get_byte(In, B2),
  979    Value is B1<<8+B2.
  980:- endif.

 random_4(-R1, -R2, -R3, -R4) is det

Generate 4 2-byte random numbers. Uses /dev/urandom when available to make prediction of the session IDs hard.

  987:- if(current_predicate(urandom/1)).  988random_4(R1,R2,R3,R4) :-
  989    urandom(In),
  990    !,
  991    get_pair(In, R1),
  992    get_pair(In, R2),
  993    get_pair(In, R3),
  994    get_pair(In, R4).
  995:- endif.  996random_4(R1,R2,R3,R4) :-
  997    R1 is random(65536),
  998    R2 is random(65536),
  999    R3 is random(65536),
 1000    R4 is random(65536).

 hooked is semidet

 hook(+Goal)

These multifile predicates may be used to hook the data storage of this library. An example is implemented by library(http/http_redis_plugin), storing all session data in a redis database.