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POE::Wheel::Run(3pm)           User Contributed Perl Documentation           POE::Wheel::Run(3pm)

NAME
       POE::Wheel::Run - portably run blocking code and programs in subprocesses

SYNOPSIS
         #!/usr/bin/perl

         use warnings;
         use strict;

         use POE qw( Wheel::Run );

         POE::Session->create(
           inline_states => {
             _start           => \&on_start,
             got_child_stdout => \&on_child_stdout,
             got_child_stderr => \&on_child_stderr,
             got_child_close  => \&on_child_close,
             got_child_signal => \&on_child_signal,
           }
         );

         POE::Kernel->run();
         exit 0;

         sub on_start {
           my $child = POE::Wheel::Run->new(
             Program => [ "/bin/ls", "-1", "/" ],
             StdoutEvent  => "got_child_stdout",
             StderrEvent  => "got_child_stderr",
             CloseEvent   => "got_child_close",
           );

           $_[KERNEL]->sig_child($child->PID, "got_child_signal");

           # Wheel events include the wheel's ID.
           $_[HEAP]{children_by_wid}{$child->ID} = $child;

           # Signal events include the process ID.
           $_[HEAP]{children_by_pid}{$child->PID} = $child;

           print(
             "Child pid ", $child->PID,
             " started as wheel ", $child->ID, ".\n"
           );
         }

         # Wheel event, including the wheel's ID.
         sub on_child_stdout {
           my ($stdout_line, $wheel_id) = @_[ARG0, ARG1];
           my $child = $_[HEAP]{children_by_wid}{$wheel_id};
           print "pid ", $child->PID, " STDOUT: $stdout_line\n";
         }

         # Wheel event, including the wheel's ID.
         sub on_child_stderr {
           my ($stderr_line, $wheel_id) = @_[ARG0, ARG1];
           my $child = $_[HEAP]{children_by_wid}{$wheel_id};
           print "pid ", $child->PID, " STDERR: $stderr_line\n";
         }

         # Wheel event, including the wheel's ID.
         sub on_child_close {
           my $wheel_id = $_[ARG0];
           my $child = delete $_[HEAP]{children_by_wid}{$wheel_id};

           # May have been reaped by on_child_signal().
           unless (defined $child) {
             print "wid $wheel_id closed all pipes.\n";
             return;
           }

           print "pid ", $child->PID, " closed all pipes.\n";
           delete $_[HEAP]{children_by_pid}{$child->PID};
         }

         sub on_child_signal {
           print "pid $_[ARG1] exited with status $_[ARG2].\n";
           my $child = delete $_[HEAP]{children_by_pid}{$_[ARG1]};

           # May have been reaped by on_child_close().
           return unless defined $child;

           delete $_[HEAP]{children_by_wid}{$child->ID};
         }

DESCRIPTION
       POE::Wheel::Run executes a program or block of code in a subprocess, created the usual
       way: using fork().  The parent process may exchange information with the child over the
       child's STDIN, STDOUT and STDERR filehandles.

       In the parent process, the POE::Wheel::Run object represents the child process.  It has
       methods such as PID() and kill() to query and manage the child process.

       POE::Wheel::Run's put() method sends data to the child's STDIN.  Child output on STDOUT
       and STDERR may be dispatched as events within the parent, if requested.

       POE::Wheel::Run can also notify the parent when the child has closed its output
       filehandles.  Some programs remain active, but they close their output filehandles to
       indicate they are done writing.

       A more reliable way to detect child exit is to use POE::Kernel's sig_child() method to
       wait for the wheel's process to be reaped.  It is in fact vital to use sig_child() in all
       circumstances since without it, POE will not try to reap child processes.

       Failing to use sig_child() has in the past led to wedged machines.  Long-running programs
       have leaked processes, eventually consuming all available slots in the process table and
       requiring reboots.

       Because process leaks are so severe, POE::Kernel will check for this condition on exit and
       display a notice if it finds that processes are leaking.  Developers should heed these
       warnings.

       POE::Wheel::Run communicates with the child process in a line-based fashion by default.
       Programs may override this by specifying some other POE::Filter object in "StdinFilter",
       "StdoutFilter", "StdioFilter" and/or "StderrFilter".

PUBLIC METHODS
   Constructor
       POE::Wheel subclasses tend to perform a lot of setup so that they run lighter and faster.
       POE::Wheel::Run's constructor is no exception.

       new

       new() creates and returns a new POE::Wheel::Run object.  If it's successful, the object
       will represent a child process with certain specified qualities.  It also provides an OO-
       and event-based interface for asynchronously interacting with the process.

       Conduit

       Conduit specifies the inter-process communications mechanism that will be used to pass
       data between the parent and child process.  Conduit may be one of "pipe", "socketpair",
       "inet", "pty", or "pty-pipe".  POE::Wheel::Run will use the most appropriate Conduit for
       the run-time (not the compile-time) operating system, but this varies from one OS to the
       next.

       Internally, POE::Wheel::Run passes the Conduit type to POE::Pipe::OneWay and
       POE::Pipe::TwoWay.  These helper classes were created to make IPC portable and reusable.
       They do not require the rest of POE.

       Three Conduit types use pipes or pipelike inter-process communication: "pipe",
       "socketpair" and "inet".  They determine whether the internal IPC uses pipe(),
       socketpair() or Internet sockets.  These Conduit values are passed through to
       POE::Pipe::OneWay or POE::Pipe::TwoWay internally.

       The "pty" conduit type runs the child process under a pseudo-tty, which is created by
       IO::Pty.  Pseudo-ttys (ptys) convince child processes that they are interacting with
       terminals rather than pipes.  This may be used to trick programs like ssh into believing
       it's secure to prompt for a password, although passphraseless identities might be better
       for that.

       The "pty" conduit cannot separate STDERR from STDOUT, but the "pty-pipe" mode can.

       The "pty-pipe" conduit uses a pty for STDIN and STDOUT and a one-way pipe for STDERR.  The
       additional pipe keeps STDERR output separate from STDOUT.

       The IO::Pty module is only loaded if "pty" or "pty-pipe" is used.  It's not a dependency
       until it's actually needed.

       Winsize

       Winsize sets the child process' terminal size.  Its value should be an arrayref with four
       elements.  The first two elements must be the number of lines and columns for the child's
       terminal window, respectively.  The second pair of elements describe the terminal's X and
       Y dimensions in pixels.  If the last pair is missing, they will be calculated from the
       lines and columns using a 9x16 cell size.

         $_[HEAP]{child} = POE::Wheel::Run->new(
           # ... among other things ...
           Winsize => [ 25, 80, 720, 400 ],
         );

       Winsize is only valid for conduits that use pseudo-ttys: "pty" and "pty-pipe".  Other
       conduits don't simulate terminals, so they don't have window sizes.

       Winsize defaults to the parent process' window size, assuming the parent process has a
       terminal to query.

       CloseOnCall

       CloseOnCall, when true, turns on close-on-exec emulation for subprocesses that don't
       actually call exec().  These would be instances when the child is running a block of code
       rather than executing an external program.  For example:

         $_[HEAP]{child} = POE::Wheel::Run->new(
           # ... among other things ...
           CloseOnCall => 1,
           Program => \&some_function,
         );

       CloseOnCall is off (0) by default.

       CloseOnCall works by closing all file descriptors greater than $^F in the child process
       before calling the application's code.  For more details, please the discussion of $^F in
       perlvar.

       StdioDriver

       StdioDriver specifies a single POE::Driver object to be used for both STDIN and STDOUT.
       It's equivalent to setting "StdinDriver" and "StdoutDriver" to the same POE::Driver
       object.

       POE::Wheel::Run will create and use a POE::Driver::SysRW driver of one isn't specified.
       This is by far the most common use case, so it's the default.

       StdinDriver

       "StdinDriver" sets the POE::Driver used to write to the child process' STDIN IPC conduit.
       It is almost never needed.  Omitting it will allow POE::Wheel::Run to use an internally
       created POE::Driver::SysRW object.

       StdoutDriver

       "StdoutDriver" sets the POE::Driver object that will be used to read from the child
       process' STDOUT conduit.  It's almost never needed.  If omitted, POE::Wheel::Run will
       internally create and use a POE::Driver::SysRW object.

       StderrDriver

       "StderrDriver" sets the driver that will be used to read from the child process' STDERR
       conduit.  As with "StdoutDriver", it's almost always preferable to let POE::Wheel::Run
       instantiate its own driver.

       CloseEvent

       CloseEvent contains the name of an event that the wheel will emit when the child process
       closes its last open output handle.  This is a consistent notification that the child is
       done sending output.  Please note that it does not signal when the child process has
       exited.  Programs should use sig_child() to detect that.

       While it is impossible for ErrorEvent or StdoutEvent to happen after CloseEvent, there is
       no such guarantee for CHLD, which may happen before or after CloseEvent.

       In addition to the usual POE parameters, each CloseEvent comes with one of its own:

       "ARG0" contains the wheel's unique ID.  This can be used to keep several child processes
       separate when they're managed by the same session.

       A sample close event handler:

         sub close_state {
           my ($heap, $wheel_id) = @_[HEAP, ARG0];

           my $child = delete $heap->{child}->{$wheel_id};
           print "Child ", $child->PID, " has finished.\n";
         }

       ErrorEvent

       ErrorEvent contains the name of an event to emit if something fails.  It is optional; if
       omitted, the wheel will not notify its session if any errors occur.  However,
       POE::Wheel::Run->new() will still throw an exception if it fails.

       "ARG0" contains the name of the operation that failed.  It may be 'read', 'write', 'fork',
       'exec' or the name of some other function or task.  The actual values aren't yet defined.
       They will probably not correspond so neatly to Perl builtin function names.

       "ARG1" and "ARG2" hold numeric and string values for $!, respectively.  "$!" will eq ""
       for read error 0 (child process closed the file handle).

       "ARG3" contains the wheel's unique ID.

       "ARG4" contains the name of the child filehandle that has the error.  It may be "STDIN",
       "STDOUT", or "STDERR".  The sense of "ARG0" will be the opposite of what you might
       normally expect for these handles.  For example, POE::Wheel::Run will report a "read"
       error on "STDOUT" because it tried to read data from the child's STDOUT handle.

       A sample error event handler:

         sub error_state {
           my ($operation, $errnum, $errstr, $wheel_id) = @_[ARG0..ARG3];
           $errstr = "remote end closed" if $operation eq "read" and !$errnum;
           warn "Wheel $wheel_id generated $operation error $errnum: $errstr\n";
         }

       Note that unless you deactivate the signal pipe, you might also see "EIO" (5) error during
       read operations.

       StdinEvent

       StdinEvent contains the name of an event that Wheel::Run emits whenever everything queued
       by its put() method has been flushed to the child's STDIN handle.  It is the equivalent to
       POE::Wheel::ReadWrite's FlushedEvent.

       StdinEvent comes with only one additional parameter: "ARG0" contains the unique ID for the
       wheel that sent the event.

       StdoutEvent

       StdoutEvent contains the name of an event  that Wheel::Run emits whenever the child
       process writes something to its STDOUT filehandle.  In other words, whatever the child
       prints to STDOUT, the parent receives a StdoutEvent---provided that the child prints
       something compatible with the parent's StdoutFilter.

       StdoutEvent comes with two parameters.  "ARG0" contains the information that the child
       wrote to STDOUT.  "ARG1" holds the unique ID of the wheel that read the output.

         sub stdout_state {
           my ($heap, $input, $wheel_id) = @_[HEAP, ARG0, ARG1];
           print "Child process in wheel $wheel_id wrote to STDOUT: $input\n";
         }

       StderrEvent

       StderrEvent behaves exactly as StdoutEvent, except for data the child process writes to
       its STDERR filehandle.

       StderrEvent comes with two parameters.  "ARG0" contains the information that the child
       wrote to STDERR.  "ARG1" holds the unique ID of the wheel that read the output.

         sub stderr_state {
           my ($heap, $input, $wheel_id) = @_[HEAP, ARG0, ARG1];
           print "Child process in wheel $wheel_id wrote to STDERR: $input\n";
         }

       RedirectStdout

       This is a filehandle or filename to which standard output will be redirected.  It is an
       error to use this option together with StdoutEvent. This is useful in case your program
       needs to have standard I/O, but do not actually care for its contents to be visible to the
       parent.

       RedirectStderr

       Just like RedirectStdout, but with standard error. It is an error to use this together
       with StderrEvent

       RedirectStdin

       This is a filehandle or filename which the child process will use as its standard input.
       It is an error to use this option with StdinEvent

       RedirectOutput

       This will redirect stderr and stdout to the same filehandle. This is equivalent to do
       doing something like

         $ something > /path/to/output 2>&1

       in bourne shell.

       NoStdin

       While output filehandles will be closed if there are no events to be received on them,
       stdin is open by default - because lack of an event handler does not necessarily mean
       there is no desired input stream. This option explicitly disables the creation of an IPC
       stdin conduit.

       StdioFilter

       StdioFilter, if used, must contain an instance of a POE::Filter subclass.  This filter
       describes how the parent will format put() data for the child's STDIN, and how the parent
       will parse the child's STDOUT.

       If STDERR will also be parsed, then a separate StderrFilter will also be needed.

       StdioFilter defaults to a POE::Filter::Line instance, but only if both StdinFilter and
       StdoutFilter are not specified.  If either StdinFilter or StdoutFilter is used, then
       StdioFilter is illegal.

       StdinFilter

       StdinFilter may be used to specify a particular STDIN serializer that is different from
       the STDOUT parser.  If specified, it conflicts with StdioFilter.  StdinFilter's value, if
       specified, must be an instance of a POE::Filter subclass.

       Without a StdinEvent, StdinFilter is illegal.

       StdoutFilter

       StdoutFilter may be used to specify a particular STDOUT parser that is different from the
       STDIN serializer.  If specified, it conflicts with StdioFilter.  StdoutFilter's value, if
       specified, must be an instance of a POE::Filter subclass.

       Without a StdoutEvent, StdoutFilter is illegal.

       StderrFilter

       StderrFilter may be used to specify a filter for a child process' STDERR output.  If
       omitted, POE::Wheel::Run will create and use its own POE::Filter::Line instance, but only
       if a StderrEvent is specified.

       Without a StderrEvent, StderrFilter is illegal.

       Group

       Group contains a numeric group ID that the child process should run within.  By default,
       the child process will run in the same group as the parent.

       Group is not fully portable.  It may not work on systems that have no concept of user
       groups.  Also, the parent process may need to run with elevated privileges for the child
       to be able to change groups.

       User

       User contains a numeric user ID that should own the child process.  By default, the child
       process will run as the same user as the parent.

       User is not fully portable.  It may not work on systems that have no concept of users.
       Also, the parent process may need to run with elevated privileges for the child to be able
       to change users.

       NoSetSid

       When true, NoSetSid disables setsid() in the child process.  By default, the child process
       calls setsid() is called so that it may execute in a separate UNIX session.

       NoSetPgrp

       When true, NoSetPgrp disables setprgp() in the child process. By default, the child
       process calls setpgrp() to change its process group, if the OS supports that.

       setsid() is used instead of setpgrp() if Conduit is pty or pty-pipe.  See "NoSetSid".

       Priority

       Priority adjusts the child process' niceness or priority level, depending on which (if
       any) the underlying OS supports.  Priority contains a numeric offset which will be added
       to the parent's priority to determine the child's.

       The priority offset may be negative, which in UNIX represents a higher priority.  However
       UNIX requires elevated privileges to increase a process' priority.

       Program

       Program specifies the program to exec() or the block of code to run in the child process.
       Program's type is significant.

       If Program holds a scalar, its value will be executed as exec($program).  Shell
       metacharacters are significant, per exec(SCALAR) semantics.

       If Program holds an array reference, it will executed as exec(@$program).  As per
       exec(ARRAY), shell metacharacters will not be significant.

       If Program holds a code reference, that code will be called in the child process.  This
       mode allows POE::Wheel::Run to execute long-running internal code asynchronously, while
       the usual modes execute external programs.  The child process will exit after that code is
       finished, in such a way as to avoid DESTROY and END block execution.  See "Coderef
       Execution Side Effects" for more details.

       perlfunc has more information about exec() and the different ways to call it.

       Please avoid calling exit() explicitly when executing a subroutine.  The child process
       inherits all objects from the parent, including ones that may perform side effects.
       POE::Wheel::Run takes special care to avoid object destructors and END blocks in the child
       process, but calling exit() will trigger them.

       ProgramArgs

       If specified, ProgramArgs should refer to a list of parameters for the program being run.

         my @parameters = qw(foo bar baz);  # will be passed to Program
         ProgramArgs => \@parameters;

   event EVENT_TYPE => EVENT_NAME, ...
       event() allows programs to change the events that Wheel::Run emits when certain activities
       occurs.  EVENT_TYPE may be one of the event parameters described in POE::Wheel::Run's
       constructor.

       This example changes the events that $wheel emits for STDIN flushing and STDOUT activity:

         $wheel->event(
           StdinEvent  => 'new-stdin-event',
           StdoutEvent => 'new-stdout-event',
         );

       Undefined EVENT_NAMEs disable events.

   put RECORDS
       put() queues up a list of RECORDS that will be sent to the child process' STDIN
       filehandle.  These records will first be serialized according to the wheel's StdinFilter.
       The serialized RECORDS will be flushed asynchronously once the current event handler
       returns.

   get_stdin_filter
       get_stind_filter() returns the POE::Filter object currently being used to serialize put()
       records for the child's STDIN filehandle.  The return object may be used according to its
       own interface.

   get_stdout_filter
       get_stdout_filter() returns the POE::Filter object currently being used to parse what the
       child process writes to STDOUT.

   get_stderr_filter
       get_stderr_filter() returns the POE::Filter object currently being used to parse what the
       child process writes to STDERR.

   set_stdio_filter FILTER_OBJECT
       Set StdinFilter and StdoutFilter to the same new FILTER_OBJECT.  Unparsed STDOUT data will
       be parsed later by the new FILTER_OBJECT.  However, data already put() will remain
       serialized by the old filter.

   set_stdin_filter FILTER_OBJECT
       Set StdinFilter to a new FILTER_OBJECT.  Data already put() will remain serialized by the
       old filter.

   set_stdout_filter FILTER_OBJECT
       Set StdoutFilter to a new FILTER_OBJECT.  Unparsed STDOUT data will be parsed later by the
       new FILTER_OBJECT.

   set_stderr_filter FILTER_OBJECT
       Set StderrFilter to a new FILTER_OBJECT.  Unparsed STDERR data will be parsed later by the
       new FILTER_OBJECT.

   pause_stdout
       Pause reading of STDOUT from the child.  The child process may block if the STDOUT IPC
       conduit fills up.  Reading may be resumed with resume_stdout().

   pause_stderr
       Pause reading of STDERR from the child.  The child process may block if the STDERR IPC
       conduit fills up.  Reading may be resumed with resume_stderr().

   resume_stdout
       Resume reading from the child's STDOUT filehandle.  This is only meaningful if
       pause_stdout() has been called and remains in effect.

   resume_stderr
       Resume reading from the child's STDERR filehandle.  This is only meaningful if
       pause_stderr() has been called and remains in effect.

   shutdown_stdin
       shutdown_stdin() closes the child process' STDIN and stops the wheel from reporting
       StdinEvent.  It is extremely useful for running utilities that expect to receive EOF on
       STDIN before they respond.

   ID
       ID() returns the wheel's unique ID.  Every event generated by a POE::Wheel::Run object
       includes a wheel ID so that it can be matched to the wheel that emitted it.  This lets a
       single session manage several wheels without becoming confused about which one generated
       what event.

       ID() is not the same as PID().

   PID
       PID() returns the process ID for the child represented by the POE::Wheel::Run object.
       It's often used as a parameter to sig_child().

       PID() is not the same as ID().

   kill SIGNAL
       POE::Wheel::Run's kill() method sends a SIGNAL to the child process the object represents.
       kill() is often used to force a reluctant program to terminate.  SIGNAL is one of the
       operating signal names present in %SIG.

       kill() returns the number of processes successfully signaled: 1 on success, or 0 on
       failure, since the POE::Wheel::Run object only affects at most a single process.

       kill() sends SIGTERM if SIGNAL is undef or omitted.

   get_driver_out_messages
       get_driver_out_messages() returns the number of put() records remaining in whole or in
       part in POE::Wheel::Run's POE::Driver output queue.  It is often used to tell whether the
       wheel has more input for the child process.

       In most cases, StdinEvent may be used to trigger activity when all data has been sent to
       the child process.

   get_driver_out_octets
       get_driver_out_octets() returns the number of serialized octets remaining in
       POE::Wheel::Run's POE::Driver output queue.  It is often used to tell whether the wheel
       has more input for the child process.

TIPS AND TRICKS
   MSWin32 Support
       In the past POE::Wheel::Run did not support MSWin32 and users had to use custom work-
       arounds. Then Chris Williams ( BINGOS ) arrived and saved the day with his
       POE::Wheel::Run::Win32 module. After some testing, it was decided to merge the win32 code
       into POE::Wheel::Run.  Everyone was happy!

       However, after some investigation Apocalypse ( APOCAL ) found out that in some situations
       it still didn't behave properly. The root cause was that the win32 code path in
       POE::Wheel::Run didn't exit cleanly. This means DESTROY and END blocks got executed! After
       talking with more people, the solution was not pretty.

       The problem is that there is no equivalent of POSIX::_exit() for MSWin32.  Hopefully, in a
       future version of Perl this can be fixed! In the meantime, POE::Wheel::Run will use
       CORE::kill() to terminate the child. However, this comes with a caveat: you will leak
       around 1KB per exec. The code has been improved so the chance of this happening has been
       reduced.

       As of now the most reliable way to trigger this is to exec an invalid binary. The
       definition of "invalid binary" depends on different things, but what it means is that
       Win32::Job->spawn() failed to run. This will force POE::Wheel::Run to use the workaround
       to exit the child. If this happens, a very big warning will be printed to the STDERR of
       the child and the parent process will receive it.

       If you are a Perl MSWin32 hacker, PLEASE help us with this situation! Go read rt.cpan.org
       bug #56417 and talk with us/p5p to see where you can contribute.

       Thanks again for your patience as we continue to improve POE::Wheel::Run on MSWin32!

       kill() and ClosedEvent on Windows

       Windows will often fail to report EOF on pipes when subprocesses are killed.  The work-
       around is to catch the signal in the subprocess, and exit normally:

         my $child = POE::Wheel::Run->new(
           Program => sub {
             $SIG{INT} = sub { exit };
             ...;
           },
           ...,
         );

       Be sure to kill() the subprocess using the same signal that it catches and exits upon.
       Remember, not all signals can be caught by user code.

         $child->kill("INT");

   Execution Environment
       It's common to scrub a child process' environment, so that only required, secure values
       exist.  This amounts to clearing the contents of %ENV and repopulating it.

       Environment scrubbing is easy when the child process is running a subroutine, but it's not
       so easy---or at least not as intuitive---when executing external programs.

       The way we do it is to run a small subroutine in the child process that performs the
       exec() call for us.

         Program => \&exec_with_scrubbed_env,

         sub exec_with_scrubbed_env {
           delete @ENV{keys @ENV};
           $ENV{PATH} = "/bin";
           exec(@program_and_args);
         }

       That deletes everything from the environment and sets a simple, secure PATH before
       executing a program.

   Coderef Execution Side Effects
       The child process is created by fork(), which duplicates the parent process including a
       copy of POE::Kernel, all running Session instances, events in the queue, watchers, open
       filehandles, and so on.

       When executing an external program, the UNIX exec() call immediately replaces the copy of
       the parent with a completely new program.

       When executing internal coderefs, however, we must preserve the code and any memory it
       might reference.  This leads to some potential side effects.

       DESTROY and END Blocks Run Twice

       Objects that were created in the parent process are copied into the child.  When the child
       exits normally, any DESTROY and END blocks are executed there.  Later, when the parent
       exits, they may run again.

       POE::Wheel::Run takes steps to avoid running DESTROY and END blocks in the child process.
       It uses POSIX::_exit() to bypass them.  If that fails, it may even kill() itself.

       If an application needs to exit explicitly, for example to return an error code to the
       parent process, then please use POSIX::_exit() rather than Perl's core exit().

       POE::Kernel's run() method was never called

       This warning is displayed from POE::Kernel's DESTROY method.  It's a side effect of
       calling exit() in a child process that was started before "POE::Kernel->run()" could be
       called.  The child process receives a copy of POE::Kernel where run() wasn't called, even
       if it was called later in the parent process.

       The most direct solution is to call POSIX::_exit() rather than exit().  This will bypass
       POE::Kernel's DESTROY, and the message it emits.

       Running POE::Kernel in the Child

       Calling "POE::Kernel->run()" in the child process effectively resumes the copy of the
       parent process.  This is rarely (if ever) desired.

       More commonly, an application wants to run an entirely new POE::Kernel instance in the
       child process.  This is supported by first stop()ping the copied instance, starting one or
       more new sessions, and calling run() again.  For example:

         Program => sub {
           # Wipe the existing POE::Kernel clean.
           $poe_kernel->stop();

           # Start a new session, or more.
           POE::Session->create(
             ...
           );

           # Run the new sessions.
           POE::Kernel->run();
         }

       Strange things are bound to happen if the program does not call "stop" in POE::Kernel
       before "run" in POE::Kernel.  However this is vaguely supported in case it's the right
       thing to do at the time.

SEE ALSO
       POE::Wheel describes wheels in general.

       The SEE ALSO section in POE contains a table of contents covering the entire POE
       distribution.

CAVEATS & TODOS
       POE::Wheel::Run's constructor should emit proper events when it fails.  Instead, it just
       dies, carps or croaks.  This isn't necessarily bad; a program can trap the death in new()
       and move on.

       Priority is a delta, not an absolute niceness value.

       It might be nice to specify User by name rather than just UID.

       It might be nice to specify Group by name rather than just GID.

       POE::Pipe::OneWay and Two::Way don't require the rest of POE.  They should be spun off
       into a separate distribution for everyone to enjoy.

       If StdinFilter and StdoutFilter seem backwards, remember that it's the filters for the
       child process.  StdinFilter is the one that dictates what the child receives on STDIN.
       StdoutFilter tells the parent how to parse the child's STDOUT.

AUTHORS & COPYRIGHTS
       Please see POE for more information about authors and contributors.

perl v5.30.0                                2020-02-07                       POE::Wheel::Run(3pm)

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