Provided by: libparallel-forkmanager-perl_2.02-1_all bug

NAME

       Parallel::ForkManager - A simple parallel processing fork manager

VERSION

       version 2.02

SYNOPSIS

         use Parallel::ForkManager;

         my $pm = Parallel::ForkManager->new($MAX_PROCESSES);

         DATA_LOOP:
         foreach my $data (@all_data) {
           # Forks and returns the pid for the child:
           my $pid = $pm->start and next DATA_LOOP;

           ... do some work with $data in the child process ...

           $pm->finish; # Terminates the child process
         }

DESCRIPTION

       This module is intended for use in operations that can be done in parallel where the
       number of processes to be forked off should be limited. Typical use is a downloader which
       will be retrieving hundreds/thousands of files.

       The code for a downloader would look something like this:

         use LWP::Simple;
         use Parallel::ForkManager;

         ...

         my @links=(
           ["http://www.foo.bar/rulez.data","rulez_data.txt"],
           ["http://new.host/more_data.doc","more_data.doc"],
           ...
         );

         ...

         # Max 30 processes for parallel download
         my $pm = Parallel::ForkManager->new(30);

         LINKS:
         foreach my $linkarray (@links) {
           $pm->start and next LINKS; # do the fork

           my ($link, $fn) = @$linkarray;
           warn "Cannot get $fn from $link"
             if getstore($link, $fn) != RC_OK;

           $pm->finish; # do the exit in the child process
         }
         $pm->wait_all_children;

       First you need to instantiate the ForkManager with the "new" constructor.  You must
       specify the maximum number of processes to be created. If you specify 0, then NO fork will
       be done; this is good for debugging purposes.

       Next, use $pm->start to do the fork. $pm returns 0 for the child process, and child pid
       for the parent process (see also "fork()" in perlfunc(1p)).  The "and next" skips the
       internal loop in the parent process. NOTE: $pm->start dies if the fork fails.

       $pm->finish terminates the child process (assuming a fork was done in the "start").

       NOTE: You cannot use $pm->start if you are already in the child process.  If you want to
       manage another set of subprocesses in the child process, you must instantiate another
       Parallel::ForkManager object!

METHODS

       The comment letter indicates where the method should be run. P for parent, C for child.

       new $processes
            Instantiate a new Parallel::ForkManager object. You must specify the maximum number
            of children to fork off. If you specify 0 (zero), then no children will be forked.
            This is intended for debugging purposes.

            The optional second parameter, $tempdir, is only used if you want the children to
            send back a reference to some data (see RETRIEVING DATASTRUCTURES below). If not
            provided, it is set via a call to File::Temp::tempdir().

            The new method will die if the temporary directory does not exist or it is not a
            directory.

            Since version 2.00, the constructor can also be called in the typical Moo/Moose
            fashion. I.e.

                my $fm = Parallel::ForkManager->new(
                    max_procs => 4,
                    tempdir => '...',
                    child_role => 'Parallel::ForkManager::CustomChild',
                );

       child_role
            Returns the name of the role consumed by the ForkManager object in child processes.
            Defaults to Parallel::ForkManager::Child and can be set to something else via the
            constructor.

       start [ $process_identifier ]
            This method does the fork. It returns the pid of the child process for the parent,
            and 0 for the child process. If the $processes parameter for the constructor is 0
            then, assuming you're in the child process, $pm->start simply returns 0.

            An optional $process_identifier can be provided to this method... It is used by the
            "run_on_finish" callback (see CALLBACKS) for identifying the finished process.

       start_child [ $process_identifier, ] \&callback
            Like "start", but will run the &callback as the child. If the callback returns
            anything, it'll be passed as the data to transmit back to the parent process via
            "finish()".

       finish [ $exit_code [, $data_structure_reference] ]
            Closes the child process by exiting and accepts an optional exit code (default exit
            code is 0) which can be retrieved in the parent via callback.  If the second optional
            parameter is provided, the child attempts to send its contents back to the parent. If
            you use the program in debug mode ($processes == 0), this method just calls the
            callback.

            If the $data_structure_reference is provided, then it is serialized and passed to the
            parent process. See RETRIEVING DATASTRUCTURES for more info.

       set_max_procs $processes
            Allows you to set a new maximum number of children to maintain.

       wait_all_children
            You can call this method to wait for all the processes which have been forked. This
            is a blocking wait.

       reap_finished_children
            This is a non-blocking call to reap children and execute callbacks independent of
            calls to "start" or "wait_all_children". Use this in scenarios where "start" is
            called infrequently but you would like the callbacks executed quickly.

       is_parent
            Returns "true" if within the parent or "false" if within the child.

       is_child
            Returns "true" if within the child or "false" if within the parent.

       max_procs
            Returns the maximal number of processes the object will fork.

       running_procs
            Returns the pids of the forked processes currently monitored by the
            "Parallel::ForkManager". Note that children are still reported as running until the
            fork manager harvest them, via the next call to "start" or "wait_all_children".

                my @pids = $pm->running_procs;

                my $nbr_children =- $pm->running_procs;

       wait_for_available_procs( $n )
            Wait until $n available process slots are available.  If $n is not given, defaults to
            1.

       waitpid_blocking_sleep
            Returns the sleep period, in seconds, of the pseudo-blocking calls. The sleep period
            can be a fraction of second.

            Returns 0 if disabled.

            Defaults to 1 second.

            See BLOCKING CALLS for more details.

       set_waitpid_blocking_sleep $seconds
            Sets the the sleep period, in seconds, of the pseudo-blocking calls.  Set to 0 to
            disable.

            See BLOCKING CALLS for more details.

CALLBACKS

       You can define callbacks in the code, which are called on events like starting a process
       or upon finish. Declare these before the first call to start().

       The callbacks can be defined with the following methods:

       run_on_finish $code [, $pid ]
           You can define a subroutine which is called when a child is terminated. It is called
           in the parent process.

           The parameters of the $code are the following:

             - pid of the process, which is terminated
             - exit code of the program
             - identification of the process (if provided in the "start" method)
             - exit signal (0-127: signal name)
             - core dump (1 if there was core dump at exit)
             - datastructure reference or undef (see RETRIEVING DATASTRUCTURES)

       run_on_start $code
           You can define a subroutine which is called when a child is started. It called after
           the successful startup of a child in the parent process.

           The parameters of the $code are the following:

             - pid of the process which has been started
             - identification of the process (if provided in the "start" method)

       run_on_wait $code, [$period]
           You can define a subroutine which is called when the child process needs to wait for
           the startup. If $period is not defined, then one call is done per child. If $period is
           defined, then $code is called periodically and the module waits for $period seconds
           between the two calls. Note, $period can be fractional number also. The exact "$period
           seconds" is not guaranteed, signals can shorten and the process scheduler can make it
           longer (on busy systems).

           The $code called in the "start" and the "wait_all_children" method also.

           No parameters are passed to the $code on the call.

BLOCKING CALLS

       When it comes to waiting for child processes to terminate, "Parallel::ForkManager" is
       between a fork and a hard place (if you excuse the terrible pun). The underlying Perl
       "waitpid" function that the module relies on can block until either one specific or any
       child process terminate, but not for a process part of a given group.

       This means that the module can do one of two things when it waits for one of its child
       processes to terminate:

       Only wait for its own child processes
           This is done via a loop using a "waitpid" non-blocking call and a sleep statement.
           The code does something along the lines of

               while(1) {
                   if ( any of the P::FM child process terminated ) {
                       return its pid
                   }

                   sleep $sleep_period
               }

           This is the default behavior that the module will use.  This is not the most efficient
           way to wait for child processes, but it's the safest way to ensure that
           "Parallel::ForkManager" won't interfere with any other part of the codebase.

           The sleep period is set via the method "set_waitpid_blocking_sleep".

       Block until any process terminate
           Alternatively, "Parallel::ForkManager" can call "waitpid" such that it will block
           until any child process terminate. If the child process was not one of the monitored
           subprocesses, the wait will resume. This is more efficient, but mean that "P::FM" can
           captures (and discards) the termination notification that a different part of the code
           might be waiting for.

           If this is a race condition that doesn't apply to your codebase, you can set the
           waitpid_blocking_sleep period to 0, which will enable "waitpid" call blocking.

               my $pm = Parallel::ForkManager->new( 4 );

               $pm->set_waitpid_blocking_sleep(0);  # true blocking calls enabled

               for ( 1..100 ) {
                   $pm->start and next;

                   ...; # do work

                   $pm->finish;
               }

RETRIEVING DATASTRUCTURES from child processes

       The ability for the parent to retrieve data structures is new as of version 0.7.6.

       Each child process may optionally send 1 data structure back to the parent.  By data
       structure, we mean a reference to a string, hash or array. The contents of the data
       structure are written out to temporary files on disc using the Storable modules' store()
       method. The reference is then retrieved from within the code you send to the run_on_finish
       callback.

       The data structure can be any scalar perl data structure which makes sense: string,
       numeric value or a reference to an array, hash or object.

       There are 2 steps involved in retrieving data structures:

       1) A reference to the data structure the child wishes to send back to the parent is
       provided as the second argument to the finish() call. It is up to the child to decide
       whether or not to send anything back to the parent.

       2) The data structure reference is retrieved using the callback provided in the
       run_on_finish() method.

       Keep in mind that data structure retrieval is not the same as returning a data structure
       from a method call. That is not what actually occurs. The data structure referenced in a
       given child process is serialized and written out to a file by Storable. The file is
       subsequently read back into memory and a new data structure belonging to the parent
       process is created. Please consider the performance penalty it can imply, so try to keep
       the returned structure small.

EXAMPLES

   Parallel get
       This small example can be used to get URLs in parallel.

         use Parallel::ForkManager;
         use LWP::Simple;

         my $pm = Parallel::ForkManager->new(10);

         LINKS:
         for my $link (@ARGV) {
           $pm->start and next LINKS;
           my ($fn) = $link =~ /^.*\/(.*?)$/;
           if (!$fn) {
             warn "Cannot determine filename from $fn\n";
           } else {
             $0 .= " " . $fn;
             print "Getting $fn from $link\n";
             my $rc = getstore($link, $fn);
             print "$link downloaded. response code: $rc\n";
           };
           $pm->finish;
         };

   Callbacks
       Example of a program using callbacks to get child exit codes:

         use strict;
         use Parallel::ForkManager;

         my $max_procs = 5;
         my @names = qw( Fred Jim Lily Steve Jessica Bob Dave Christine Rico Sara );
         # hash to resolve PID's back to child specific information

         my $pm = Parallel::ForkManager->new($max_procs);

         # Setup a callback for when a child finishes up so we can
         # get it's exit code
         $pm->run_on_finish( sub {
             my ($pid, $exit_code, $ident) = @_;
             print "** $ident just got out of the pool ".
               "with PID $pid and exit code: $exit_code\n";
         });

         $pm->run_on_start( sub {
             my ($pid, $ident)=@_;
             print "** $ident started, pid: $pid\n";
         });

         $pm->run_on_wait( sub {
             print "** Have to wait for one children ...\n"
           },
           0.5
         );

         NAMES:
         foreach my $child ( 0 .. $#names ) {
           my $pid = $pm->start($names[$child]) and next NAMES;

           # This code is the child process
           print "This is $names[$child], Child number $child\n";
           sleep ( 2 * $child );
           print "$names[$child], Child $child is about to get out...\n";
           sleep 1;
           $pm->finish($child); # pass an exit code to finish
         }

         print "Waiting for Children...\n";
         $pm->wait_all_children;
         print "Everybody is out of the pool!\n";

   Data structure retrieval
       In this simple example, each child sends back a string reference.

         use Parallel::ForkManager 0.7.6;
         use strict;

         my $pm = Parallel::ForkManager->new(2, '/server/path/to/temp/dir/');

         # data structure retrieval and handling
         $pm -> run_on_finish ( # called BEFORE the first call to start()
           sub {
             my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $data_structure_reference) = @_;

             # retrieve data structure from child
             if (defined($data_structure_reference)) {  # children are not forced to send anything
               my $string = ${$data_structure_reference};  # child passed a string reference
               print "$string\n";
             }
             else {  # problems occurring during storage or retrieval will throw a warning
               print qq|No message received from child process $pid!\n|;
             }
           }
         );

         # prep random statement components
         my @foods = ('chocolate', 'ice cream', 'peanut butter', 'pickles', 'pizza', 'bacon', 'pancakes', 'spaghetti', 'cookies');
         my @preferences = ('loves', q|can't stand|, 'always wants more', 'will walk 100 miles for', 'only eats', 'would starve rather than eat');

         # run the parallel processes
         PERSONS:
         foreach my $person (qw(Fred Wilma Ernie Bert Lucy Ethel Curly Moe Larry)) {
           $pm->start() and next PERSONS;

           # generate a random statement about food preferences
           my $statement = $person . ' ' . $preferences[int(rand @preferences)] . ' ' . $foods[int(rand @foods)];

           # send it back to the parent process
           $pm->finish(0, \$statement);  # note that it's a scalar REFERENCE, not the scalar itself
         }
         $pm->wait_all_children;

       A second datastructure retrieval example demonstrates how children decide whether or not
       to send anything back, what to send and how the parent should process whatever is
       retrieved.

         use Parallel::ForkManager 0.7.6;
         use Data::Dumper;  # to display the data structures retrieved.
         use strict;

         my $pm = Parallel::ForkManager->new(20);  # using the system temp dir $L<File::Temp::tempdir()

         # data structure retrieval and handling
         my %retrieved_responses = ();  # for collecting responses
         $pm -> run_on_finish (
           sub {
             my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $data_structure_reference) = @_;

             # see what the child sent us, if anything
             if (defined($data_structure_reference)) {  # test rather than assume child sent anything
               my $reftype = ref($data_structure_reference);
               print qq|ident "$ident" returned a "$reftype" reference.\n\n|;
               if (1) {  # simple on/off switch to display the contents
                 print &Dumper($data_structure_reference) . qq|end of "$ident" sent structure\n\n|;
               }

               # we can also collect retrieved data structures for processing after all children have exited
               $retrieved_responses{$ident} = $data_structure_reference;
             } else {
               print qq|ident "$ident" did not send anything.\n\n|;
             }
           }
         );

         # generate a list of instructions
         my @instructions = (  # a unique identifier and what the child process should send
           {'name' => '%ENV keys as a string', 'send' => 'keys'},
           {'name' => 'Send Nothing'},  # not instructing the child to send anything back to the parent
           {'name' => 'Childs %ENV', 'send' => 'all'},
           {'name' => 'Child chooses randomly', 'send' => 'random'},
           {'name' => 'Invalid send instructions', 'send' => 'Na Na Nana Na'},
           {'name' => 'ENV values in an array', 'send' => 'values'},
         );

         INSTRUCTS:
         foreach my $instruction (@instructions) {
           $pm->start($instruction->{'name'}) and next INSTRUCTS;  # this time we are using an explicit, unique child process identifier

           # last step in child processing
           $pm->finish(0) unless $instruction->{'send'};  # no data structure is sent unless this child is told what to send.

           if ($instruction->{'send'} eq 'keys') {
             $pm->finish(0, \join(', ', keys %ENV));

           } elsif ($instruction->{'send'} eq 'values') {
             $pm->finish(0, [values %ENV]);  # kinda useless without knowing which keys they belong to...

           } elsif ($instruction->{'send'} eq 'all') {
             $pm->finish(0, \%ENV);  # remember, we are not "returning" anything, just copying the hash to disc

           # demonstrate clearly that the child determines what type of reference to send
           } elsif ($instruction->{'send'} eq 'random') {
             my $string = q|I'm just a string.|;
             my @array = qw(I am an array);
             my %hash = (type => 'associative array', synonym => 'hash', cool => 'very :)');
             my $return_choice = ('string', 'array', 'hash')[int(rand 3)];  # randomly choose return data type
             $pm->finish(0, \$string) if ($return_choice eq 'string');
             $pm->finish(0, \@array) if ($return_choice eq 'array');
             $pm->finish(0, \%hash) if ($return_choice eq 'hash');

           # as a responsible child, inform parent that their instruction was invalid
           } else {
             $pm->finish(0, \qq|Invalid instructions: "$instruction->{'send'}".|);  # ordinarily I wouldn't include invalid input in a response...
           }
         }
         $pm->wait_all_children;  # blocks until all forked processes have exited

         # post fork processing of returned data structures
         for (sort keys %retrieved_responses) {
           print qq|Post processing "$_"...\n|;
         }

USING RAND() IN FORKED PROCESSES

       A caveat worth noting is that all forked processes will use the same random seed, so
       potentially providing the same results (see
       <http://blogs.perl.org/users/brian_phillips/2010/06/when-rand-isnt-random.html>).  If you
       are using "rand()" and want each forked child to use a different seed, you can add the
       following to your program:

           $pm->run_on_start(sub { srand });

EXTENDING

       As of version 2.0.0, "Parallel::ForkManager" uses Moo under the hood. When a process is
       being forked from the parent object, the forked instance of the object will be modified to
       consume the Parallel::ForkManager::Child role. All of this makes extending
       Parallel::ForkManager to implement any storing/retrieving mechanism or any other behavior
       fairly easy.

   Example: store and retrieve data via a web service
           {
               package Parallel::ForkManager::Web;

               use HTTP::Tiny;

               use Moo;
               extends 'Parallel::ForkManager';

               has ua => (
                   is => 'ro',
                   lazy => 1,
                   default => sub {
                       HTTP::Tiny->new;
                   }
               );

               sub store {
                   my( $self, $data ) = @_;

                   $self->ua->post( "http://.../store/$$", { body => $data } );
               }

               sub retrieve {
                   my( $self, $kid_id ) = @_;

                   $self->ua->get( "http://.../store/$kid_id" )->{content};
               }

           }

           my $fm = Parallel::ForkManager::Web->new(2);

           $fm->run_on_finish(sub{
               my $retrieved = $_[5];

               print "got ", $retrieved, "\n";
           });

           $fm->start_child(sub {
               return $_**2;
           }) for 1..3;

           $fm->wait_all_children;

   Example: have the child processes exit differently
           use Parallel::ForkManager;

           package Parallel::ForkManager::Child::PosixExit {
               use Moo::Role;
               with 'Parallel::ForkManager::Child';

               sub finish  { POSIX::_exit() };
           }

           my $fm = Parallel::ForkManager->new(
               max_proc   => 1,
               child_role => 'Parallel::ForkManager::Child::PosixExit'
           );

SECURITY

       Parallel::ForkManager uses temporary files when a child process returns information to its
       parent process. The filenames are based on the process of the parent and child processes,
       so they are fairly easy to guess. So if security is a concern in your environment, make
       sure the directory used by Parallel::ForkManager is restricted to the current user only
       (the default behavior is to create a directory, via File::Temp's "tempdir", which does
       that).

TROUBLESHOOTING

   PerlIO::gzip and Parallel::ForkManager do not play nice together
       If you are using PerlIO::gzip in your child processes, you may end up with garbled files.
       This is not really P::FM's fault, but rather a problem between PerlIO::gzip and "fork()"
       (see <https://rt.cpan.org/Public/Bug/Display.html?id=114557>).

       Fortunately, it seems there is an easy way to fix the problem by adding the "unix" layer?
       I.e.,

           open(IN, '<:unix:gzip', ...

BUGS AND LIMITATIONS

       Do not use Parallel::ForkManager in an environment where other child processes can affect
       the run of the main program; using this module is not recommended in an environment where
       fork() / wait() is already used.

       If you want to use more than one copies of the Parallel::ForkManager, then you have to
       make sure that all children processes are terminated, before you use the second object in
       the main program.

       You are free to use a new copy of Parallel::ForkManager in the child processes, although I
       don't think it makes sense.

CREDITS

         Michael Gang (bug report)
         Noah Robin <sitz@onastick.net> (documentation tweaks)
         Chuck Hirstius <chirstius@megapathdsl.net> (callback exit status, example)
         Grant Hopwood <hopwoodg@valero.com> (win32 port)
         Mark Southern <mark_southern@merck.com> (bugfix)
         Ken Clarke <www.perlprogrammer.net>  (datastructure retrieval)

AUTHORS

       ·   dLux (Szabó, Balázs) <dlux@dlux.hu>

       ·   Yanick Champoux <yanick@cpan.org>

       ·   Gabor Szabo <gabor@szabgab.com>

COPYRIGHT AND LICENSE

       This software is copyright (c) 2018, 2016, 2015 by Balázs Szabó.

       This is free software; you can redistribute it and/or modify it under the same terms as
       the Perl 5 programming language system itself.