process.pl

/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2008-2024, University of Amsterdam
                              VU University Amsterdam
                              CWI, Amsterdam
                              SWI-Prolog Solutions b.v.
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*/

:- module(process,
          [ process_create/3,           % +Exe, +Args, +Options
            process_wait/2,             % +PID, -Status
            process_wait/3,             % +PID, -Status, +Options
            process_id/1,               % -PID
            process_id/2,               % +Process, -PID
            is_process/1,               % +PID
            process_release/1,          % +PID
            process_kill/1,             % +PID
            process_group_kill/1,       % +PID
            process_group_kill/2,       % +PID, +Signal
            process_kill/2,             % +PID, +Signal
            process_which/2,            % +Exe, -AbsoluteFile

            process_set_method/1        % +CreateMethod
          ]).
:- autoload(library(apply),[maplist/3]).
:- autoload(library(error),[must_be/2,existence_error/2]).
:- autoload(library(option),[select_option/3]).


:- use_foreign_library(foreign(process)).

:- predicate_options(process_create/3, 3,
                     [ stdin(any),
                       stdout(any),
                       stderr(any),
                       cwd(atom),
                       env(list(any)),
                       environment(list(any)),
                       priority(+integer),
                       process(-integer),
                       detached(+boolean),
                       window(+boolean)
                     ]).

/** <module> Create processes and redirect I/O

The module library(process) implements interaction  with child processes
and unifies older interfaces such   as  shell/[1,2], open(pipe(command),
...) etc. This library is modelled after SICStus 4.

The main interface is formed by process_create/3.   If the process id is
requested the process must be waited for using process_wait/2. Otherwise
the process resources are reclaimed automatically.

In addition to the predicates, this module   defines  a file search path
(see user:file_search_path/2 and absolute_file_name/3) named =path= that
locates files on the system's  search   path  for  executables. E.g. the
following finds the executable for =ls=:

    ==
    ?- absolute_file_name(path(ls), Path, [access(execute)]).
    ==

*|Incompatibilities and current limitations|*

    * Where SICStus distinguishes between an internal process id and
    the OS process id, this implementation does not make this
    distinction. This implies that is_process/1 is incomplete and
    unreliable.

    * It is unclear what the detached(true) option is supposed to do. Disable
    signals in the child? Use setsid() to detach from the session?  The
    current implementation uses setsid() on Unix systems.

    * An extra option env([Name=Value, ...]) is added to
    process_create/3.  As of version 4.1 SICStus added
    environment(List) which _modifies_ the environment.  A
    compatible option was added to SWI-Prolog 7.7.23.

@tbd    Implement detached option in process_create/3
@compat SICStus 4
*/


%!  process_create(+Exe, +Args:list, +Options) is det.
%
%   Create a new process running the   file  Exe and using arguments
%   from the given list. Exe is a   file  specification as handed to
%   absolute_file_name/3. Typically one use the =path= file alias to
%   specify an executable file on the current   PATH. Args is a list
%   of arguments that  are  handed  to   the  new  process.  On Unix
%   systems, each element in the list becomes a separate argument in
%   the  new  process.  In  Windows,    the   arguments  are  simply
%   concatenated to form the commandline.   Each  argument itself is
%   either a primitive or  a  list   of  primitives.  A primitive is
%   either atomic or a term file(Spec). Using file(Spec), the system
%   inserts a filename using the OS   filename  conventions which is
%   properly quoted if needed.
%
%   Options:
%
%       * stdin(Spec)
%       * stdout(Spec)
%       * stderr(Spec)
%       Bind the standard streams of the new process. Spec is one of
%       the terms below. If pipe(Pipe) is used, the Prolog stream is
%       a stream in text-mode using the encoding of the default
%       locale.  The encoding can be changed using set_stream/2,
%       or by using the two-argument form of =pipe=, which accepts an
%       encoding(Encoding) option.
%       The options =stdout= and =stderr= may use the same stream,
%       in which case both output streams are connected to the same
%       Prolog stream.
%
%           * std
%           Just share with the Prolog I/O streams.  On Unix,
%           if the `user_input`, etc. are bound to a file handle
%           but not to 0,1,2 the process I/O is bound to the file
%           handles of these streams.
%           * null
%           Bind to a _null_ stream. Reading from such a stream
%           returns end-of-file, writing produces no output
%           * pipe(-Stream)
%           * pipe(-Stream, +StreamOptions)
%           Attach input and/or output to a Prolog stream.
%           The optional StreamOptions argument is a list of options
%           that affect the stream. Currently only the options
%           type(+Type) and encoding(+Encoding) are supported,
%           which have the same meaning as the stream properties
%           of the same name (see stream_property/2).
%           StreamOptions is provided mainly for SICStus compatibility -
%           the SWI-Prolog predicate set_stream/2 can be used
%           for the same purpose.
%           * stream(+Stream)
%           Attach input or output to an existing Prolog stream.
%           This stream must be associated with an OS file
%           handle (see stream_property/2, property `file_no`).
%           This option is __not__ provided by the SICStus
%           implementation.
%
%       * cwd(+Directory)
%       Run the new process in Directory.  Directory can be a
%       compound specification, which is converted using
%       absolute_file_name/3.  See also process_set_method/1.
%       * env(+List)
%       As environment(List), but _only_ the specified variables
%       are passed, i.e., no variables are _inherited_.
%       * environment(+List)
%       Specify _additional_ environment variables for the new process.
%       List is a list of `Name=Value` terms, where `Value` is expanded
%       the same way as the Args argument. If neither `env` nor
%       `environment` is passed the environment is inherited from the
%       Prolog process.  At most one env(List) or environment(List) term
%       may appear in the options. If multiple appear a
%       `permission_error` is raised for the second option.
%       * process(-PID)
%       Unify PID with the process id of the created process.
%       * detached(+Bool)
%       In Unix: If =true=, detach the process from the terminal
%       Currently mapped to setsid();
%       Also creates a new process group for the child
%       In Windows: If =true=, detach the process from the current
%       job via the CREATE_BREAKAWAY_FROM_JOB flag. In Vista and beyond,
%       processes launched from the shell directly have the 'compatibility
%       assistant' attached to them automatically unless they have a UAC
%       manifest embedded in them. This means that you will get a
%       permission denied error if you try and assign the newly-created
%       PID to a job you create yourself.
%       * window(+Bool)
%       If =true=, create a window for the process (Windows only)
%       * priority(+Priority)
%       In Unix: specifies the process priority for the newly
%       created process. Priority must be an integer between -20
%       and 19. Positive values are nicer to others, and negative
%       values are less so. The default is zero. Users are free to
%       lower their own priority. Only the super-user may _raise_ it
%       to less-than zero.
%
%   If the user specifies the process(-PID)   option, he *must* call
%   process_wait/2 to reclaim the process.  Without this option, the
%   system will wait for completion of   the  process after the last
%   pipe stream is closed.
%
%   If the process is not waited for, it must succeed with status 0.
%   If not, an process_error is raised.
%
%   *|Windows notes|*
%
%   On Windows this call is an interface to the CreateProcess() API.
%   The  commandline  consists  of  the  basename  of  Exe  and  the
%   arguments formed from Args. Arguments are  separated by a single
%   space. If all characters satisfy iswalnum()   it is unquoted. If
%   the argument contains a double-quote it   is quoted using single
%   quotes. If both single and double   quotes appear a domain_error
%   is raised, otherwise double-quote are used.
%
%   The CreateProcess() API has  many   options.  Currently only the
%   =CREATE_NO_WINDOW=   options   is   supported     through    the
%   window(+Bool) option. If omitted, the  default   is  to use this
%   option if the application has no   console.  Future versions are
%   likely to support  more  window   specific  options  and replace
%   win_exec/2.
%
%   *Examples*
%
%   First,  a  very  simple  example  that    behaves  the  same  as
%   =|shell('ls -l')|=, except for error handling:
%
%   ==
%   ?- process_create(path(ls), ['-l'], []).
%   ==
%
%   The following example uses grep to find  all matching lines in a
%   file.
%
%   ==
%   grep(File, Pattern, Lines) :-
%           setup_call_cleanup(
%               process_create(path(grep), [ Pattern, file(File) ],
%                              [ stdout(pipe(Out))
%                              ]),
%               read_lines(Out, Lines),
%               close(Out)).
%
%   read_lines(Out, Lines) :-
%           read_line_to_codes(Out, Line1),
%           read_lines(Line1, Out, Lines).
%
%   read_lines(end_of_file, _, []) :- !.
%   read_lines(Codes, Out, [Line|Lines]) :-
%           atom_codes(Line, Codes),
%           read_line_to_codes(Out, Line2),
%           read_lines(Line2, Out, Lines).
%   ==
%
%   @error  process_error(Exe, Status) where Status is one of
%           exit(Code) or killed(Signal).  Raised if the process
%           is waited for (i.e., Options does not include
%           process(-PID)), and does not exit with status 0.
%   @bug    On Windows, environment(List) is handled as env(List),
%           i.e., the environment is not inherited.

process_create(Exe, Args, Options) :-
    (   exe_options(ExeOptions),
        absolute_file_name(Exe, PlProg, ExeOptions)
    ->  true
    ),
    must_be(list, Args),
    maplist(map_arg, Args, Av),
    prolog_to_os_filename(PlProg, Prog),
    Term =.. [Prog|Av],
    expand_cwd_option(Options, Options1),
    expand_env_option(env, Options1, Options2),
    expand_env_option(environment, Options2, Options3),
    process_create(Term, Options3).

%!  process_which(+Exe, -Path) is semidet.
%
%   True when Path is an absolute file   name for the specification Exe.
%   This deals with the search path as   well  as extensions used by the
%   OS.

process_which(Exe, Path) :-
    exe_options(ExeOptions),
    absolute_file_name(Exe, Path, [file_errors(fail)|ExeOptions]),
    !.

%!  exe_options(-Options) is multi.
%
%   Get options for absolute_file_name to find   an  executable file. On
%   Windows we first look for a  readable   file,  but  if this does not
%   exist we are happy with a existing file because the file may be a
%   [reparse point](https://docs.microsoft.com/en-us/windows/win32/fileio/reparse-points-and-file-operations)

exe_options(Options) :-
    current_prolog_flag(windows, true),
    !,
    (   Options = [ extensions(['',exe,com]), access(read), file_errors(fail) ]
    ;   Options = [ extensions(['',exe,com]), access(exist) ]
    ).
exe_options(Options) :-
    Options = [ access(execute) ].

expand_cwd_option(Options0, Options) :-
    select_option(cwd(Spec), Options0, Options1),
    !,
    (   compound(Spec)
    ->  absolute_file_name(Spec, PlDir, [file_type(directory), access(read)]),
        prolog_to_os_filename(PlDir, Dir),
        Options = [cwd(Dir)|Options1]
    ;   exists_directory(Spec)
    ->  Options = Options0
    ;   existence_error(directory, Spec)
    ).
expand_cwd_option(Options, Options).

expand_env_option(Name, Options0, Options) :-
    Term =.. [Name,Value0],
    select_option(Term, Options0, Options1),
    !,
    must_be(list, Value0),
    maplist(map_env, Value0, Value),
    NewOption =.. [Name,Value],
    Options = [NewOption|Options1].
expand_env_option(_, Options, Options).

map_env(Name=Value0, Name=Value) :-
    map_arg(Value0, Value).

%!  map_arg(+ArgIn, -Arg) is det.
%
%   Map an individual argument. Primitives  are either file(Spec) or
%   an atomic value (atom, string, number).  If ArgIn is a non-empty
%   list,  all  elements  are   converted    and   the  results  are
%   concatenated.

map_arg([], []) :- !.
map_arg(List, Arg) :-
    is_list(List),
    !,
    maplist(map_arg_prim, List, Prims),
    atomic_list_concat(Prims, Arg).
map_arg(Prim, Arg) :-
    map_arg_prim(Prim, Arg).

map_arg_prim(file(Spec), File) :-
    !,
    (   compound(Spec)
    ->  absolute_file_name(Spec, PlFile)
    ;   PlFile = Spec
    ),
    prolog_to_os_filename(PlFile, File).
map_arg_prim(Arg, Arg).


%!  process_id(-PID) is det.
%
%   True if PID is the process id of the running Prolog process.
%
%   @deprecated     Use current_prolog_flag(pid, PID)

process_id(PID) :-
    current_prolog_flag(pid, PID).

%!  process_id(+Process, -PID) is det.
%
%   PID is the process id of Process.  Given that they are united in
%   SWI-Prolog, this is a simple unify.

process_id(PID, PID).

%!  is_process(+PID) is semidet.
%
%   True if PID might  be  a   process.  Succeeds  for  any positive
%   integer.

is_process(PID) :-
    integer(PID),
    PID > 0.

%!  process_release(+PID)
%
%   Release process handle.  In this implementation this is the same
%   as process_wait(PID, _).

process_release(PID) :-
    process_wait(PID, _).

%!  process_wait(+PID, -Status) is det.
%!  process_wait(+PID, -Status, +Options) is det.
%
%   True if PID completed with  Status.   This  call normally blocks
%   until the process is finished.  Options:
%
%       * timeout(+Timeout)
%       Default: =infinite=.  If this option is a number, the
%       waits for a maximum of Timeout seconds and unifies Status
%       with =timeout= if the process does not terminate within
%       Timeout.  In this case PID is _not_ invalidated.  On Unix
%       systems only timeout 0 and =infinite= are supported.  A
%       0-value can be used to poll the status of the process.
%
%       * release(+Bool)
%       Do/do not release the process.  We do not support this flag
%       and a domain_error is raised if release(false) is provided.
%
%   @arg  Status is one of exit(Code) or killed(Signal), where
%         Code and Signal are integers.  If the `timeout` option
%         is used Status is unified with `timeout` after the wait
%         timed out.

process_wait(PID, Status) :-
    process_wait(PID, Status, []).

%!  process_kill(+PID) is det.
%!  process_kill(+PID, +Signal) is det.
%
%   Send signal to process PID.  Default   is  =term=.  Signal is an
%   integer, Unix signal name (e.g. =SIGSTOP=)   or  the more Prolog
%   friendly variation one gets after   removing  =SIG= and downcase
%   the result: =stop=. On Windows systems,   Signal  is ignored and
%   the process is terminated using   the TerminateProcess() API. On
%   Windows systems PID must  be   obtained  from  process_create/3,
%   while any PID is allowed on Unix systems.
%
%   @compat SICStus does not accept the prolog friendly version.  We
%           choose to do so for compatibility with on_signal/3.

process_kill(PID) :-
    process_kill(PID, term).


%!  process_group_kill(+PID) is det.
%!  process_group_kill(+PID, +Signal) is det.
%
%   Send signal to the group containing process PID.  Default   is
%   =term=.   See process_wait/1  for  a  description  of  signal
%   handling. In Windows, the same restriction on PID applies: it
%   must have been created from process_create/3, and the the group
%   is terminated via the TerminateJobObject API.

process_group_kill(PID) :-
    process_group_kill(PID, term).


%!  process_set_method(+Method) is det.
%
%   Determine how the process is created on  Unix systems. Method is one
%   of `spawn` (default), `fork` or `vfork`.   If  the method is `spawn`
%   but this cannot be used because it is either not supported by the OS
%   or the cwd(Dir) option is given `fork` is used.
%
%   The problem is to be understood   as  follows. The official portable
%   and safe method to create a process is using the fork() system call.
%   This call however copies the process   page tables and get seriously
%   slow  as  the  (Prolog)  process  is   multiple  giga  bytes  large.
%   Alternatively, we may use vfork() which   avoids copying the process
%   space. But, the safe usage as guaranteed   by  the POSIX standard of
%   vfork() is insufficient for our purposes.  On practical systems your
%   mileage may vary. Modern posix   systems also provide posix_spawn(),
%   which provides a safe and portable   alternative  for the fork() and
%   exec() sequence that may be implemented using   fork()  or may use a
%   fast  but  safe  alternative.  Unfortunately  posix_spawn()  doesn't
%   support the option to specify the   working  directory for the child
%   and we cannot use working_directory/2 as   the  working directory is
%   shared between threads.
%
%   Summarizing, the default is  safe  and  tries   to  be  as  fast  as
%   possible. On some scenarios and on some   OSes  it is possible to do
%   better. It is generally a good  idea   to  avoid  using the cwd(Dir)
%   option of process_create/3 as without we can use posix_spawn().


                 /*******************************
                 *            MESSAGES          *
                 *******************************/

:- multifile
    prolog:error_message/3.

prolog:error_message(process_error(File, exit(Status))) -->
    [ 'Process "~w": exit status: ~w'-[File, Status] ].
prolog:error_message(process_error(File, killed(Signal))) -->
    [ 'Process "~w": killed by signal ~w'-[File, Signal] ].
prolog:error_message(existence_error(source_sink, path(Exe))) -->
    [ 'Could not find executable file "~p" in '-[Exe] ],
    path_var.

path_var -->
    (   { current_prolog_flag(windows, true) }
    ->  [ '%PATH%'-[] ]
    ;   [ '$PATH'-[] ]
    ).