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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2013-2023, University of Amsterdam
    7                              VU University Amsterdam
    8                              CWI, Amsterdam
    9                              SWI-Prolog Solutions b.v.
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   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.
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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_unix_daemon,
   39          [ http_daemon/0,
   40            http_daemon/1,                  % +Options
   41            http_opt_type/3,                % ?Flag, ?Option, ?Type
   42            http_opt_help/2,                % ?Option, ?Help
   43            http_opt_meta/2                 % ?Option, ?Meta
   44          ]).   45:- use_module(library(error)).   46:- use_module(library(apply)).   47:- use_module(library(lists)).   48:- use_module(library(debug)).   49:- use_module(library(broadcast)).   50:- use_module(library(socket)).   51:- use_module(library(option)).   52:- use_module(library(uid)).   53:- use_module(library(unix)).   54:- use_module(library(syslog)).   55:- use_module(library(http/thread_httpd)).   56:- use_module(library(http/http_dispatch)).   57:- use_module(library(http/http_host)).   58:- use_module(library(main)).   59:- use_module(library(readutil)).   60
   61:- if(( exists_source(library(http/http_ssl_plugin)),
   62        \+ current_prolog_flag(pldoc_to_tex,true))).   63:- use_module(library(ssl)).   64:- use_module(library(http/http_ssl_plugin)).   65:- endif.   66
   67:- multifile
   68    http_server_hook/1,                     % +Options
   69    http_certificate_hook/3,                % +CertFile, +KeyFile, -Password
   70    http:sni_options/2.                     % +HostName, +SSLOptions
   71
   72:- initialization(http_daemon, main).

Run SWI-Prolog HTTP server as a Unix system daemon

This module provides the logic that is needed to integrate a process into the Unix service (daemon) architecture. It deals with the following aspects, all of which may be used/ignored and configured using commandline options:

The typical use scenario is to write a file that loads the following components:

  1. The application code, including http handlers (see http_handler/3).
  2. This library

In the code below, ?- [load]. loads the remainder of the webserver code. This is often a sequence of use_module/1 directives.

:- use_module(library(http/http_unix_daemon)).

:- [load].

The program entry point is http_daemon/0, declared using initialization/2. This may be overruled using a new declaration after loading this library. The new entry point will typically call http_daemon/1 to start the server in a preconfigured way.

:- use_module(library(http/http_unix_daemon)).
:- initialization(run, main).

run :-
    ...
    http_daemon(Options).

Now, the server may be started using the command below. See http_daemon/0 for supported options.

% [sudo] swipl mainfile.pl [option ...]

Below are some examples. Our first example is completely silent, running on port 80 as user www.

% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid

Our second example logs HTTP interaction with the syslog daemon for debugging purposes. Note that the argument to --debug= is a Prolog term and must often be escaped to avoid misinterpretation by the Unix shell. The debug option can be repeated to log multiple debug topics.

% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid \
        --debug='http(request)' --syslog=http

Broadcasting The library uses broadcast/1 to allow hooking certain events:

http(pre_server_start)
Run after fork, just before starting the HTTP server. Can be used to load additional files or perform additional initialisation, such as starting additional threads. Recall that it is not possible to start threads before forking.
http(post_server_start)
Run after starting the HTTP server.
See also
- The file <swi-home>/doc/packages/examples/http/linux-init-script provides a /etc/init.d script for controlling a server as a normal Unix service. */
To be done
- Cleanup issues wrt. loading and initialization of xpce.
  161:- debug(daemon).  162
  163% Do not run xpce in a thread. This disables forking. The problem here
  164% is that loading library(pce) starts the event dispatching thread. This
  165% should be handled lazily.
  166
  167:- set_prolog_flag(xpce_threaded,   false).  168:- set_prolog_flag(message_ide,     false). % cause xpce to trap messages
  169:- set_prolog_flag(message_context, [thread,time('%F %T.%3f')]).  170:- dynamic interactive/0.
 http_daemon
Start the HTTP server as a daemon process. This predicate processes the commandline arguments below. Commandline arguments that specify servers are processed in the order they appear using the following schema:
  1. Arguments that act as default for all servers.
  2. --http=Spec or --https=Spec is followed by arguments for that server until the next --http=Spec or --https=Spec or the end of the options.
  3. If no --http=Spec or --https=Spec appears, one HTTP server is created from the specified parameters.

    Examples:

    --workers=10 --http --https
--http=8080 --https=8443
--http=localhost:8080 --workers=1 --https=8443 --workers=25
--port=Port
Start HTTP server at Port. It requires root permission and the option --user=User to open ports below 1000. The default port is 80. If --https is used, the default port is 443.
--ip=IP
Only listen to the given IP address. Typically used as --ip=localhost to restrict access to connections from localhost if the server itself is behind an (Apache) proxy server running on the same host.
--debug=Topic
Enable debugging Topic. See debug/3.
--syslog=Ident
Write debug messages to the syslog daemon using Ident
--user=User
When started as root to open a port below 1000, this option must be provided to switch to the target user for operating the server. The following actions are performed as root, i.e., before switching to User:
  • open the socket(s)
  • write the pidfile
  • setup syslog interaction
  • Read the certificate, key and password file (--pwfile=File)
--group=Group
May be used in addition to --user. If omitted, the login group of the target user is used.
--pidfile=File
Write the PID of the daemon process to File.
--output=File
Send output of the process to File. By default, all Prolog console output is discarded.
--fork[=Bool]
If given as --no-fork or --fork=false, the process runs in the foreground.
--http[=(Bool|Port|BindTo:Port)]
Create a plain HTTP server. If the argument is missing or true, create at the specified or default address. Else use the given port and interface. Thus, --http creates a server at port 80, --http=8080 creates one at port 8080 and --http=localhost:8080 creates one at port 8080 that is only accessible from localhost.
--https[=(Bool|Port|BindTo:Port)]
As --http, but creates an HTTPS server. Use --certfile, --keyfile, -pwfile, --password and --cipherlist to configure SSL for this server.
--certfile=File
The server certificate for HTTPS.
--keyfile=File
The server private key for HTTPS.
--pwfile=File
File holding the password for accessing the private key. This is preferred over using --password=PW as it allows using file protection to avoid leaking the password. The file is read before the server drops privileges when started with the --user option.
--password=PW
The password for accessing the private key. See also `--pwfile`.
--cipherlist=Ciphers
One or more cipher strings separated by colons. See the OpenSSL documentation for more information. Starting with SWI-Prolog 7.5.11, the default value is always a set of ciphers that was considered secure enough to prevent all critical attacks at the time of the SWI-Prolog release.
--interactive[=Bool]
If true (default false) implies --no-fork and presents the Prolog toplevel after starting the server.
--gtrace=[Bool]
Use the debugger to trace http_daemon/1.
--sighup=Action
Action to perform on kill -HUP <pid>. Default is reload (running make/0). Alternative is quit, stopping the server.

Other options are converted by argv_options/3 and passed to http_server/1. For example, this allows for:

--workers=Count
Set the number of workers for the multi-threaded server.

http_daemon/0 is defined as below. The start code for a specific server can use this as a starting point, for example for specifying defaults or additional options. This uses guided options processing from argv_options/3 from library(main). The option definitions are available as http_opt_type/3, http_opt_help/2 and http_opt_meta/2

http_daemon :-
    current_prolog_flag(argv, Argv),
    argv_options(Argv, _RestArgv, Options),
    http_daemon(Options).
See also
- http_daemon/1
  307http_daemon :-
  308    current_prolog_flag(argv, Argv),
  309    argv_options(Argv, _RestArgv, Options),
  310    http_daemon(Options).
  311
  312% Option declarations for argv_options/3 from library(main).
  313
  314opt_type(port,               port,               nonneg).
  315opt_type(p,                  port,               nonneg).
  316opt_type(ip,                 ip,                 atom).
  317opt_type(debug,              debug,              term).
  318opt_type(syslog,             syslog,             atom).
  319opt_type(user,               user,               atom).
  320opt_type(group,              group,              atom).
  321opt_type(pidfile,            pidfile,            file(write)).
  322opt_type(output,             output,             file(write)).
  323opt_type(fork,               fork,               boolean).
  324opt_type(http,               http,               nonneg|boolean).
  325opt_type(https,              https,              nonneg|boolean).
  326opt_type(certfile,           certfile,           file(read)).
  327opt_type(keyfile,            keyfile,            file(read)).
  328opt_type(pwfile,             pwfile,             file(read)).
  329opt_type(password,           password,           string).
  330opt_type(cipherlist,         cipherlist,         string).
  331opt_type(redirect,           redirect,           string).
  332opt_type(interactive,        interactive,        boolean).
  333opt_type(i,                  interactive,        boolean).
  334opt_type(gtrace,             gtrace,             boolean).
  335opt_type(sighup,             sighup,             oneof([reload,quit])).
  336opt_type(workers,            workers,            natural).
  337opt_type(timeout,            timeout,            number).
  338opt_type(keep_alive_timeout, keep_alive_timeout, number).
  339
  340opt_help(port,               "HTTP port to listen to").
  341opt_help(ip,                 "Only listen to this ip (--ip=localhost)").
  342opt_help(debug,              "Print debug message for topic").
  343opt_help(syslog,             "Send output to syslog daemon as ident").
  344opt_help(user,               "Run server under this user").
  345opt_help(group,              "Run server under this group").
  346opt_help(pidfile,            "Write PID to path").
  347opt_help(output,             "Send output to file (instead of syslog)").
  348opt_help(fork,               "Do (default) or do not fork").
  349opt_help(http,               "Create HTTP server").
  350opt_help(https,              "Create HTTPS server").
  351opt_help(certfile,           "The server certificate").
  352opt_help(keyfile,            "The server private key").
  353opt_help(pwfile,             "File holding password for the private key").
  354opt_help(password,           "Password for the private key").
  355opt_help(cipherlist,         "Cipher strings separated by colons").
  356opt_help(redirect,           "Redirect all requests to a URL or port").
  357opt_help(interactive,        "Enter Prolog toplevel after starting server").
  358opt_help(gtrace,             "Start (graphical) debugger").
  359opt_help(sighup,             "Action on SIGHUP: reload (default) or quit").
  360opt_help(workers,            "Number of HTTP worker threads").
  361opt_help(timeout,            "Time to wait for client to complete request").
  362opt_help(keep_alive_timeout, "Time to wait for a new request").
  363
  364opt_meta(port,               'PORT').
  365opt_meta(ip,                 'IP').
  366opt_meta(debug,              'TERM').
  367opt_meta(http,               'PORT').
  368opt_meta(https,              'PORT').
  369opt_meta(syslog,             'IDENT').
  370opt_meta(user,               'NAME').
  371opt_meta(group,              'NAME').
  372opt_meta(redirect,           'URL').
  373opt_meta(sighup,             'ACTION').
  374opt_meta(workers,            'COUNT').
  375opt_meta(timeout,            'SECONDS').
  376opt_meta(keep_alive_timeout, 'SECONDS').
 http_opt_type(?Flag, ?Option, ?Type)
 http_opt_help(?Option, ?Help)
 http_opt_meta(?Option, ?Meta)
Allow reusing http option processing
  384http_opt_type(Flag, Option, Type) :-
  385    opt_type(Flag, Option, Type).
  386
  387http_opt_help(Option, Help) :-
  388    opt_help(Option, Help),
  389    Option \= help(_).
  390
  391http_opt_meta(Option, Meta) :-
  392    opt_meta(Option, Meta).
 http_daemon(+Options)
Start the HTTP server as a daemon process. This predicate processes a Prolog option list. It is normally called from http_daemon/0, which derives the option list from the command line arguments.

Error handling depends on whether or not interactive(true) is in effect. If so, the error is printed before entering the toplevel. In non-interactive mode this predicate calls halt(1).

  405http_daemon(Options) :-
  406    Error = error(_,_),
  407    catch(http_daemon_guarded(Options), Error, start_failed(Error)).
  408
  409start_failed(Error) :-
  410    interactive,
  411    !,
  412    print_message(warning, Error).
  413start_failed(Error) :-
  414    print_message(error, Error),
  415    halt(1).
 http_daemon_guarded(+Options)
Helper that is started from http_daemon/1. See http_daemon/1 for options that are processed.
  422http_daemon_guarded(Options) :-
  423    setup_debug(Options),
  424    kill_x11(Options),
  425    option_servers(Options, Servers0),
  426    maplist(make_socket, Servers0, Servers),
  427    (   option(fork(true), Options, true),
  428        option(interactive(false), Options, false),
  429        can_switch_user(Options)
  430    ->  fork(Who),
  431        (   Who \== child
  432        ->  halt
  433        ;   disable_development_system,
  434            setup_syslog(Options),
  435            write_pid(Options),
  436            setup_output(Options),
  437            switch_user(Options),
  438            setup_signals(Options),
  439            start_servers(Servers),
  440            wait(Options)
  441        )
  442    ;   write_pid(Options),
  443        switch_user(Options),
  444        setup_signals(Options),
  445        start_servers(Servers),
  446        wait(Options)
  447    ).
 option_servers(+Options, -Sockets:list)
Find all sockets that must be created according to Options. Each socket is a term server(Scheme, Address, Opts), where Address is either a plain port (integer) or Host:Port. The latter binds the port to the interface belonging to Host. For example: socket(http, localhost:8080, Opts) creates an HTTP socket that binds to the localhost interface on port 80. Opts are the options specific for the given server.
  459option_servers(Options, Sockets) :-
  460    opt_sockets(Options, [], [], Sockets).
  461
  462opt_sockets([], Options, [], [Socket]) :-
  463    !,
  464    make_server(http(true), Options, Socket).
  465opt_sockets([], _, Sockets, Sockets).
  466opt_sockets([H|T], OptsH, Sockets0, Sockets) :-
  467    server_option(H),
  468    !,
  469    append(OptsH, [H], OptsH1),
  470    opt_sockets(T, OptsH1, Sockets0, Sockets).
  471opt_sockets([H|T0], Opts, Sockets0, Sockets) :-
  472    server_start_option(H),
  473    !,
  474    server_options(T0, T, Opts, SOpts),
  475    make_server(H, SOpts, Socket),
  476    append(Sockets0, [Socket], Sockets1),
  477    opt_sockets(T, Opts, Sockets1, Sockets).
  478opt_sockets([_|T], Opts, Sockets0, Sockets) :-
  479    opt_sockets(T, Opts, Sockets0, Sockets).
  480
  481server_options([], [], Options, Options).
  482server_options([H|T], Rest, Options0, Options) :-
  483    server_option(H),
  484    !,
  485    generalise_option(H, G),
  486    delete(Options0, G, Options1),
  487    append(Options1, [H], Options2),
  488    server_options(T, Rest, Options2, Options).
  489server_options([H|T], [H|T], Options, Options) :-
  490    server_start_option(H),
  491    !.
  492server_options([_|T0], Rest, Options0, Options) :-
  493    server_options(T0, Rest, Options0, Options).
  494
  495generalise_option(H, G) :-
  496    H =.. [Name,_],
  497    G =.. [Name,_].
  498
  499server_start_option(http(_)).
  500server_start_option(https(_)).
  501
  502server_option(port(_)).
  503server_option(ip(_)).
  504server_option(certfile(_)).
  505server_option(keyfile(_)).
  506server_option(pwfile(_)).
  507server_option(password(_)).
  508server_option(cipherlist(_)).
  509server_option(workers(_)).
  510server_option(redirect(_)).
  511server_option(timeout(_)).
  512server_option(keep_alive_timeout(_)).
  513
  514make_server(http(Address0), Options0, server(http, Address, Options)) :-
  515    make_address(Address0, 80, Address, Options0, Options).
  516make_server(https(Address0), Options0, server(https, Address, SSLOptions)) :-
  517    make_address(Address0, 443, Address, Options0, Options),
  518    merge_https_options(Options, SSLOptions).
  519
  520make_address(true, DefPort, Address, Options0, Options) :-
  521    !,
  522    option(port(Port), Options0, DefPort),
  523    (   option(ip(Bind), Options0)
  524    ->  Address = (Bind:Port)
  525    ;   Address = Port
  526    ),
  527    merge_options([port(Port)], Options0, Options).
  528make_address(Bind:Port, _, Bind:Port, Options0, Options) :-
  529    !,
  530    must_be(atom, Bind),
  531    must_be(integer, Port),
  532    merge_options([port(Port), ip(Bind)], Options0, Options).
  533make_address(Port, _, Address, Options0, Options) :-
  534    integer(Port),
  535    !,
  536    (   option(ip(Bind), Options0)
  537    ->  Address = (Bind:Port)
  538    ;   Address = Port,
  539        merge_options([port(Port)], Options0, Options)
  540    ).
  541make_address(Spec, _, Address, Options0, Options) :-
  542    atomic(Spec),
  543    split_string(Spec, ":", "", [BindString, PortString]),
  544    number_string(Port, PortString),
  545    !,
  546    atom_string(Bind, BindString),
  547    Address = (Bind:Port),
  548    merge_options([port(Port), ip(Bind)], Options0, Options).
  549make_address(Spec, _, _, _, _) :-
  550    domain_error(address, Spec).
  551
  552:- dynamic sni/3.  553
  554merge_https_options(Options, [SSL|Options]) :-
  555    (   option(certfile(CertFile), Options),
  556        option(keyfile(KeyFile), Options)
  557    ->  prepare_https_certificate(CertFile, KeyFile, Passwd0),
  558        read_file_to_string(CertFile, Certificate, []),
  559        read_file_to_string(KeyFile, Key, []),
  560        Pairs = [Certificate-Key]
  561    ;   Pairs = []
  562    ),
  563    ssl_secure_ciphers(SecureCiphers),
  564    option(cipherlist(CipherList), Options, SecureCiphers),
  565    (   string(Passwd0)
  566    ->  Passwd = Passwd0
  567    ;   options_password(Options, Passwd)
  568    ),
  569    findall(HostName-HostOptions, http:sni_options(HostName, HostOptions), SNIs),
  570    maplist(sni_contexts, SNIs),
  571    SSL = ssl([ certificate_key_pairs(Pairs),
  572                cipher_list(CipherList),
  573                password(Passwd),
  574                sni_hook(http_unix_daemon:sni)
  575              ]).
  576
  577sni_contexts(Host-Options) :-
  578    ssl_context(server, SSL, Options),
  579    assertz(sni(_, Host, SSL)).
 http_certificate_hook(+CertFile, +KeyFile, -Password) is semidet
Hook called before starting the server if the --https option is used. This hook may be used to create or refresh the certificate. If the hook binds Password to a string, this string will be used to decrypt the server private key as if the --password=Password option was given.
  589prepare_https_certificate(CertFile, KeyFile, Password) :-
  590    http_certificate_hook(CertFile, KeyFile, Password),
  591    !.
  592prepare_https_certificate(_, _, _).
  593
  594
  595options_password(Options, Passwd) :-
  596    option(password(Passwd), Options),
  597    !.
  598options_password(Options, Passwd) :-
  599    option(pwfile(File), Options),
  600    !,
  601    read_file_to_string(File, String, []),
  602    split_string(String, "", "\r\n\t ", [Passwd]).
  603options_password(_, '').
 start_servers(+Servers) is det
Start the HTTP server. It performs the following steps:
  1. Call broadcast(http(pre_server_start))
  2. For each server a. Call broadcast(http(pre_server_start(Port))) b. Call http_server(http_dispatch, Options) c. Call broadcast(http(post_server_start(Port)))
  3. Call broadcast(http(post_server_start))

This predicate can be hooked using http_server_hook/1. This predicate is executed after

  624start_servers(Servers) :-
  625    broadcast(http(pre_server_start)),
  626    maplist(start_server, Servers),
  627    broadcast(http(post_server_start)).
  628
  629start_server(server(_Scheme, Socket, Options)) :-
  630    option(redirect(To), Options),
  631    !,
  632    http_server(server_redirect(To), [tcp_socket(Socket)|Options]).
  633start_server(server(_Scheme, Socket, Options)) :-
  634    http_server_hook([tcp_socket(Socket)|Options]),
  635    !.
  636start_server(server(_Scheme, Socket, Options)) :-
  637    option(port(Port), Options),
  638    broadcast(http(pre_server_start(Port))),
  639    http_server(http_dispatch, [tcp_socket(Socket)|Options]),
  640    broadcast(http(post_server_start(Port))).
  641
  642make_socket(server(Scheme, Address, Options),
  643            server(Scheme, Socket, Options)) :-
  644    tcp_socket(Socket),
  645    catch(bind_socket(Socket, Address), Error,
  646          make_socket_error(Error, Address)),
  647    debug(daemon(socket),
  648          'Created socket ~p, listening on ~p', [Socket, Address]).
  649
  650bind_socket(Socket, Address) :-
  651    tcp_setopt(Socket, reuseaddr),
  652    tcp_bind(Socket, Address),
  653    tcp_listen(Socket, 5).
  654
  655make_socket_error(error(socket_error(_,_), _), Address) :-
  656    address_port(Address, Port),
  657    integer(Port),
  658    Port =< 1000,
  659    !,
  660    verify_root(open_port(Port)).
  661make_socket_error(Error, _) :-
  662    throw(Error).
  663
  664address_port(_:Port, Port) :- !.
  665address_port(Port, Port).
 disable_development_system
Disable some development stuff.
  671disable_development_system :-
  672    set_prolog_flag(editor, '/bin/false').
 enable_development_system
Re-enable the development environment. Currently re-enables xpce if this was loaded, but not initialised and causes the interactive toplevel to be re-enabled.
  680enable_development_system :-
  681    assertz(interactive),
  682    set_prolog_flag(xpce_threaded, true),
  683    set_prolog_flag(message_ide, true),
  684    (   current_prolog_flag(xpce_version, _)
  685    ->  call(pce_dispatch([]))
  686    ;   true
  687    ),
  688    set_prolog_flag(toplevel_goal, prolog).
 setup_syslog(+Options) is det
Setup syslog interaction.
  694setup_syslog(Options) :-
  695    option(syslog(Ident), Options),
  696    !,
  697    openlog(Ident, [pid], user).
  698setup_syslog(_).
 setup_output(+Options) is det
Setup output from the daemon process. The default is to send all output to a null-stream (see open_null_stream/1). With the option output(File), all output is written to File.
  707setup_output(Options) :-
  708    option(output(File), Options),
  709    !,
  710    open(File, write, Out, [encoding(utf8)]),
  711    set_stream(Out, buffer(line)),
  712    detach_IO(Out).
  713setup_output(_) :-
  714    open_null_stream(Out),
  715    detach_IO(Out).
 write_pid(+Options) is det
If the option pidfile(File) is present, write the PID of the daemon to this file.
  723write_pid(Options) :-
  724    option(pidfile(File), Options),
  725    current_prolog_flag(pid, PID),
  726    !,
  727    setup_call_cleanup(
  728        open(File, write, Out),
  729        format(Out, '~d~n', [PID]),
  730        close(Out)),
  731    at_halt(catch(delete_file(File), _, true)).
  732write_pid(_).
 switch_user(+Options) is det
Switch to the target user and group. If the server is started as root, this option must be present.
  740switch_user(Options) :-
  741    option(user(User), Options),
  742    !,
  743    verify_root(switch_user(User)),
  744    (   option(group(Group), Options)
  745    ->  set_user_and_group(User, Group)
  746    ;   set_user_and_group(User)
  747    ),
  748    prctl(set_dumpable(true)).      % re-enable core dumps on Linux
  749switch_user(_Options) :-
  750    verify_no_root.
 can_switch_user(Options) is det
Verify the user options before forking, so we can print the message in time.
  757can_switch_user(Options) :-
  758    option(user(User), Options),
  759    !,
  760    verify_root(switch_user(User)).
  761can_switch_user(_Options) :-
  762    verify_no_root.
  763
  764verify_root(_Task) :-
  765    geteuid(0),
  766    !.
  767verify_root(Task) :-
  768    print_message(error, http_daemon(no_root(Task))),
  769    halt(1).
  770
  771verify_no_root :-
  772    geteuid(0),
  773    !,
  774    throw(error(permission_error(open, server, http),
  775                context('Refusing to run HTTP server as root', _))).
  776verify_no_root.
  777
  778:- if(\+current_predicate(prctl/1)).  779prctl(_).
  780:- endif.
 server_redirect(+To, +Request)
Redirect all requests for this server to the specified server. To is one of:
A port (integer)
Redirect to the server running on that port in the same Prolog process.
true
Results from just passing --redirect. Redirects to an HTTPS server in the same Prolog process.
A URL
Redirect to the the given URL + the request uri. This can be used if the server cannot find its public address. For example: 
--http --redirect=https://myhost.org --https
  802server_redirect(Port, Request) :-
  803    integer(Port),
  804    http_server_property(Port, scheme(Scheme)),
  805    http_public_host(Request, Host, _Port, []),
  806    memberchk(request_uri(Location), Request),
  807    (   default_port(Scheme, Port)
  808    ->  format(string(To), '~w://~w~w', [Scheme, Host, Location])
  809    ;   format(string(To), '~w://~w:~w~w', [Scheme, Host, Port, Location])
  810    ),
  811    throw(http_reply(moved_temporary(To))).
  812server_redirect(true, Request) :-
  813    !,
  814    http_server_property(P, scheme(https)),
  815    server_redirect(P, Request).
  816server_redirect(URI, Request) :-
  817    memberchk(request_uri(Location), Request),
  818    atom_concat(URI, Location, To),
  819    throw(http_reply(moved_temporary(To))).
  820
  821default_port(http, 80).
  822default_port(https, 443).
 setup_debug(+Options) is det
Initialise debug/3 topics. The --debug option may be used multiple times.
  830setup_debug(Options) :-
  831    setup_trace(Options),
  832    nodebug(_),
  833    debug(daemon),
  834    enable_debug(Options).
  835
  836enable_debug([]).
  837enable_debug([debug(Topic)|T]) :-
  838    !,
  839    atom_to_term(Topic, Term, _),
  840    debug(Term),
  841    enable_debug(T).
  842enable_debug([_|T]) :-
  843    enable_debug(T).
  844
  845setup_trace(Options) :-
  846    option(gtrace(true), Options),
  847    !,
  848    gtrace.
  849setup_trace(_).
 kill_x11(+Options) is det
Get rid of X11 access if interactive is false.
  856kill_x11(Options) :-
  857    getenv('DISPLAY', Display),
  858    Display \== '',
  859    option(interactive(false), Options, false),
  860    !,
  861    setenv('DISPLAY', ''),
  862    set_prolog_flag(gui, false).
  863kill_x11(_).
 setup_signals(+Options)
Prepare the server for signal handling. By default SIGINT and SIGTERM terminate the server. SIGHUP causes the server to run make/0.
  872setup_signals(Options) :-
  873    option(interactive(true), Options, false),
  874    !.
  875setup_signals(Options) :-
  876    on_signal(int,  _, quit),
  877    on_signal(term, _, quit),
  878    option(sighup(Action), Options, reload),
  879    must_be(oneof([reload,quit]), Action),
  880    on_signal(usr1, _, logrotate),
  881    on_signal(hup,  _, Action).
  882
  883:- public
  884    quit/1,
  885    reload/1,
  886    logrotate/1.  887
  888quit(Signal) :-
  889    debug(daemon, 'Dying on signal ~w', [Signal]),
  890    thread_send_message(main, quit(Signal)).
  891
  892reload(Signal) :-
  893    debug(daemon, 'Reload on signal ~w', [Signal]),
  894    thread_send_message(main, reload).
  895
  896logrotate(Signal) :-
  897    debug(daemon, 'Closing log files on signal ~w', [Signal]),
  898    thread_send_message(main, logrotate).
 wait(+Options)
This predicate runs in the main thread, waiting for messages send by signal handlers to control the server. In addition, it broadcasts maintenance(Interval, Deadline) messages every Interval seconds. These messages may be trapped using listen/2 for performing scheduled maintenance such as rotating log files, cleaning stale data, etc.
  909wait(Options) :-
  910    option(interactive(true), Options, false),
  911    !,
  912    enable_development_system.
  913wait(Options) :-
  914    thread_self(Me),
  915    option(maintenance_interval(Interval), Options, 300),
  916    Interval > 0,
  917    !,
  918    first_deadline(Interval, FirstDeadline),
  919    State = deadline(0),
  920    repeat,
  921        State = deadline(Count),
  922        Deadline is FirstDeadline+Count*Interval,
  923        (   thread_idle(thread_get_message(Me, Msg, [deadline(Deadline)]),
  924                        long)
  925        ->  catch(ignore(handle_message(Msg)), E,
  926                  print_message(error, E)),
  927            Msg = quit(Signal),
  928            catch(broadcast(http(shutdown)), E,
  929                  print_message(error, E)),
  930            halt(Signal)
  931        ;   Count1 is Count + 1,
  932            nb_setarg(1, State, Count1),
  933            catch(broadcast(maintenance(Interval, Deadline)), E,
  934                  print_message(error, E)),
  935            fail
  936        ).
  937wait(_) :-
  938    thread_self(Me),
  939    repeat,
  940        thread_idle(thread_get_message(Me, Msg), long),
  941        catch(ignore(handle_message(Msg)), E,
  942              print_message(error, E)),
  943        Msg == quit,
  944        !,
  945        halt(0).
  946
  947handle_message(reload) :-
  948    make,
  949    broadcast(logrotate).
  950handle_message(logrotate) :-
  951    broadcast(logrotate).
  952
  953first_deadline(Interval, Deadline) :-
  954    get_time(Now),
  955    Deadline is ((integer(Now) + Interval - 1)//Interval)*Interval.
  956
  957
  958                 /*******************************
  959                 *            HOOKS             *
  960                 *******************************/
 http_server_hook(+Options) is semidet
Hook that is called to start the HTTP server. This hook must be compatible to http_server(Handler, Options). The default is provided by start_server/1.



 http:sni_options(-HostName, -SSLOptions) is multi
Hook to provide Server Name Indication (SNI) for TLS
servers. When starting an HTTPS server, all solutions of this
predicate are collected and a suitable sni_hook/1 is defined
for ssl_context/3 to use different contexts depending on the
host name of the client request. This hook is executed
before privileges are dropped.


  979                 /*******************************
  980                 *           MESSAGES           *
  981                 *******************************/
  982
  983:- multifile
  984    prolog:message//1.  985
  986prolog:message(http_daemon(no_root(switch_user(User)))) -->
  987    [ 'Program must be started as root to use --user=~w.'-[User] ].
  988prolog:message(http_daemon(no_root(open_port(Port)))) -->
  989    [ 'Cannot open port ~w.  Only root can open ports below 1000.'-[Port] ]