RunAndWaitForAllChildren.pl

#!/usr/bin/perl

# The following POD section contains placeholders, so it has to be preprocessed by this script first.
#
# HelpBeginMarker

=head1 OVERVIEW

SCRIPT_NAME version SCRIPT_VERSION

Run the given command and wait for all its child processes to terminate with PR_SET_CHILD_SUBREAPER (Linux only).

=head1 RATIONALE

You may be surprised to hear that, when a process terminates, it may have left other processes behind
that are still running, without your explicit permission.

Sometimes, that is exactly what you want, but you normally indicate your wish by appending an ampersand like this:

  this-process-should-stay-running-in-background &

Otherwise, you would normally expect that, when a command is finished, everything associated with it has really finished.

The main problem is that the standard Linux process management is problematic and leaky.
This article explains it in detail: L<< The Unix process API is unreliable and unsafe|https://catern.com/process.html >>
It is the kind of realisation that shakes your faith on long-standing, tried-and-true systems. And it is not the first one.

Most of the time, leaked processes don't stay for long and are no cause for concern,
but every now and then, they can cause problems. For example, say you interrupt a compilation
by signalling the top-level makefile (for example, by pressing Ctrl+C), and then restart it.
Some process still running from the old compilation may be compiling a file while the new compilation starts,
messing up the results. That shouldn't normally happen, because SIGINT from Ctrl+C should be sent to all processes,
but there is no real guarantee that the mechanism works.
And the reason why a compilation stops may be a bug in one tool, and not a signal sent to all processes.

Sometimes, you must make sure that, when a process terminates, all its children and grandchildren have terminated too.
Say you mount a filesystem, generate a backup, and unmount it at the end. If there are still processes reading
or writing to the filesystem, unmounting it will fail, and even if Linux could reliably force-unmount a filesystem,
your backup would be corrupt.

About reliably force-unmounting filesystems, another faith-shaking item, look for "umount -f"
in S<< L<< this article about FreeBSD|https://www.cons.org/cracauer/freebsd.html >> . >>

Basically, you have 2 options to deal with leaky processes: cgroups and PR_SET_CHILD_SUBREAPER.
This script implements PR_SET_CHILD_SUBREAPER.

Using cgroups is not always easy. You would normally resort to systemd-run, which has its own drawbacks.

=head1 USAGE

S<< perl SCRIPT_NAME [options] [--] command arguments... >>

=head1 OPTIONS

=over

=item *

B<-h, --help>

Print this help text.

=item *

B<--help-pod>

Preprocess and print the POD section. Useful to generate the README.pod file.

=item *

B<--version>

Print this tool's name and version number (SCRIPT_VERSION).

=item *

B<--license>

Print the license.

=item *

B<-->

Terminate options processing. Useful to avoid confusion between options for this script
and the user command and its arguments. Recommended when calling this script from another script,
where the command to run comes from a variable or from user input.

=item *

B<< --new-process-group >>

Creates a new process group for this script, the command child process and all of its descendants.

Creating a process group which does not include this script would have been desirable,
but it is harder to achieve.

With such a separate process group, you can run a Bash script which generates an advice for the user
along this line:

  In order to cancel the operation, send SIGTERM to process group $BASHPID like this: kill -SIGTERM -$BASHPID

Use a new process group only for background tasks (like services/daemons), because changing the process group
will break the usual way of cancelling commands with Ctrl+C (SIGINT) when using an interactive shell.

=back

=head1 EXIT CODE

If the user command can be executed, its exit code is returned.

If the user command dies from a signal, this wrapper will (attempt to) kill itself with the same signal.
Why that is the right thing to do is described here: L<< Proper handling of SIGINT/SIGQUIT|https://www.cons.org/cracauer/sigint.html >>

If something fails inside this script, the exit code will be nonzero.

=head1 SIGNALS

This script ignores SIGINT after it has started the given command. This is so that, when you press Ctrl+C
on the shell, this wrapper will still wait for all child processes to terminate.
Otherwise, pressing Ctrl+C would kill this script immediately
and you would not know whether other child processes are still running.

We could also ignore SIGHUP, which is the signal sent when you close a terminal window. But the
terminal emulator should be the one waiting for all its child processes. Most terminal
emulators do not wait, so if this script waited, the user would not see it anyway.
You can always use this script together with run-in-new-console.sh in order
to wait for all children after the console closes.

This script ignores SIGTERM too, which is typically used to administratively and gracefully terminate a process.
SIGTERM tends to be sent to entire process groups, and this script would typically be at the root
of the process tree. Therefore, it makes sense for it to wait until all children have terminated.

=head1 FEEDBACK

Please send feedback to rdiezmail-tools at yahoo.de

=head1 LICENSE

Copyright (C) 2022 R. Diez

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License version 3 as published by
the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License version 3 for more details.

You should have received a copy of the GNU Affero General Public License version 3
along with this program.  If not, see L<http://www.gnu.org/licenses/>.

=cut

# HelpEndMarker

use strict;
use warnings;

use FindBin qw( $Bin $Script );
use Getopt::Long;
use Pod::Usage;
use IPC::Open3 qw( open3 );
use Config qw( %Config );

use constant SCRIPT_VERSION => "1.01";

use constant EXIT_CODE_SUCCESS => 0;
use constant EXIT_CODE_FAILURE => 1;  # Beware that other errors, like those from die(), can yield other exit codes.

use constant TRUE  => 1;
use constant FALSE => 0;


sub write_stdout ( $ )
{
  ( print STDOUT $_[0] ) or
     die "Error writing to standard output: $!\n";
}

sub write_stderr ( $ )
{
  ( print STDERR $_[0] ) or
     die "Error writing to standard error: $!\n";
}


my @signal_name_by_number;

@signal_name_by_number[ split( ' ', $Config{sig_num} ) ] = split( ' ', $Config{sig_name} );


#------------------------------------------------------------------------
#
# Reads a whole binary file, returns it as a scalar.
#
# Security warning: The error messages contain the file path.
#
# Alternative: use Perl module File::Slurp
#

sub read_whole_binary_file ( $ )
{
  my $file_path = shift;

  open( my $file, "<", $file_path )
    or die "Cannot open file \"$file_path\": $!\n";

  binmode( $file )  # Avoids CRLF conversion.
    or die "Cannot access file in binary mode: $!\n";

  my $file_content;
  my $file_size = -s $file;

  my $read_res = read( $file, $file_content, $file_size );

  if ( not defined($read_res) )
  {
    die qq<Error reading from file "$file_path": $!>;
  }

  if ( $read_res != $file_size )
  {
    die qq<Error reading from file "$file_path".>;
  }

  close( $file ) or die "Cannot close file descriptor: $!\n";

  return $file_content;
}


sub get_pod_from_this_script ()
{
  # POSSIBLE OPTIMISATION:
  #   We do not actually need to read the whole file. We could read line-by-line,
  #   discard everything before HelpBeginMarker and stop as soon as HelpEndMarker is found.

  my $sourceCodeOfThisScriptAsString = read_whole_binary_file( "$Bin/$Script" );

  # We do not actually need to isolate the POD section, but it is cleaner this way.

  my $regex = "# HelpBeginMarker[\\s]+(.*?)[\\s]+# HelpEndMarker";

  my @podParts = $sourceCodeOfThisScriptAsString =~ m/$regex/s;

  if ( scalar( @podParts ) != 1 )
  {
    die "Internal error isolating the POD documentation.\n";
  }

  my $podAsStr = $podParts[0];


  # Replace some known placeholders. This is the only practical way to make sure
  # that the script name and version number in the help text are always right.
  # If you duplicate name and version in the source code and in the help text,
  # they will inevitably get out of sync at some point in time.
  #
  # There are faster ways to replace multiple placeholders, but optimising this
  # is not worth the effort.

  $podAsStr =~ s/SCRIPT_VERSION/@{[ SCRIPT_VERSION ]}/gs;
  $podAsStr =~ s/SCRIPT_NAME/$Script/gs;

  return $podAsStr;
}


sub print_help_text ()
{
  my $podAsStr = get_pod_from_this_script();


  # Prepare an in-memory file with the POD contents.

  my $memFileWithPodContents;

  open( my $memFileWithPod, '+>', \$memFileWithPodContents )
    or die "Cannot create in-memory file: $!\n";

  binmode( $memFileWithPod )  # Avoids CRLF conversion.
    or die "Cannot access in-memory file in binary mode: $!\n";

  ( print $memFileWithPod $podAsStr ) or
    die "Error writing to in-memory file: $!\n";

  seek $memFileWithPod, 0, 0
    or die "Cannot seek inside in-memory file: $!\n";


  write_stdout( "\n" );

  # Unfortunately, pod2usage does not return any error indication.
  # However, if the POD text has syntax errors, the user will see
  # error messages in a "POD ERRORS" section at the end of the output.

  pod2usage( -exitval    => "NOEXIT",
             -verbose    => 2,
             -noperldoc  => 1,  # Perl does not come with the perl-doc package as standard (at least on Debian 4.0).
             -input      => $memFileWithPod,
             -output     => \*STDOUT );

  $memFileWithPod->close()
    or die "Cannot close in-memory file: $!\n";
}


sub get_license_text ()
{
  return ( <<EOL

                    GNU AFFERO GENERAL PUBLIC LICENSE
                       Version 3, 19 November 2007

 Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
 Everyone is permitted to copy and distribute verbatim copies
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The GNU General Public License permits making a modified version and
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receives a license from the original licensors, to run, modify and
propagate that work, subject to this License.  You are not responsible
for enforcing compliance by third parties with this License.

  An "entity transaction" is a transaction transferring control of an
organization, or substantially all assets of one, or subdividing an
organization, or merging organizations.  If propagation of a covered
work results from an entity transaction, each party to that
transaction who receives a copy of the work also receives whatever
licenses to the work the party's predecessor in interest had or could
give under the previous paragraph, plus a right to possession of the
Corresponding Source of the work from the predecessor in interest, if
the predecessor has it or can get it with reasonable efforts.

  You may not impose any further restrictions on the exercise of the
rights granted or affirmed under this License.  For example, you may
not impose a license fee, royalty, or other charge for exercise of
rights granted under this License, and you may not initiate litigation
(including a cross-claim or counterclaim in a lawsuit) alleging that
any patent claim is infringed by making, using, selling, offering for
sale, or importing the Program or any portion of it.

  11. Patents.

  A "contributor" is a copyright holder who authorizes use under this
License of the Program or a work on which the Program is based.  The
work thus licensed is called the contributor's "contributor version".

  A contributor's "essential patent claims" are all patent claims
owned or controlled by the contributor, whether already acquired or
hereafter acquired, that would be infringed by some manner, permitted
by this License, of making, using, or selling its contributor version,
but do not include claims that would be infringed only as a
consequence of further modification of the contributor version.  For
purposes of this definition, "control" includes the right to grant
patent sublicenses in a manner consistent with the requirements of
this License.

  Each contributor grants you a non-exclusive, worldwide, royalty-free
patent license under the contributor's essential patent claims, to
make, use, sell, offer for sale, import and otherwise run, modify and
propagate the contents of its contributor version.

  In the following three paragraphs, a "patent license" is any express
agreement or commitment, however denominated, not to enforce a patent
(such as an express permission to practice a patent or covenant not to
sue for patent infringement).  To "grant" such a patent license to a
party means to make such an agreement or commitment not to enforce a
patent against the party.

  If you convey a covered work, knowingly relying on a patent license,
and the Corresponding Source of the work is not available for anyone
to copy, free of charge and under the terms of this License, through a
publicly available network server or other readily accessible means,
then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the
patent license for this particular work, or (3) arrange, in a manner
consistent with the requirements of this License, to extend the patent
license to downstream recipients.  "Knowingly relying" means you have
actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient's use of the covered work
in a country, would infringe one or more identifiable patents in that
country that you have reason to believe are valid.

  If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate, modify
or convey a specific copy of the covered work, then the patent license
you grant is automatically extended to all recipients of the covered
work and works based on it.

  A patent license is "discriminatory" if it does not include within
the scope of its coverage, prohibits the exercise of, or is
conditioned on the non-exercise of one or more of the rights that are
specifically granted under this License.  You may not convey a covered
work if you are a party to an arrangement with a third party that is
in the business of distributing software, under which you make payment
to the third party based on the extent of your activity of conveying
the work, and under which the third party grants, to any of the
parties who would receive the covered work from you, a discriminatory
patent license (a) in connection with copies of the covered work
conveyed by you (or copies made from those copies), or (b) primarily
for and in connection with specific products or compilations that
contain the covered work, unless you entered into that arrangement,
or that patent license was granted, prior to 28 March 2007.

  Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may
otherwise be available to you under applicable patent law.

  12. No Surrender of Others' Freedom.

  If conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot convey a
covered work so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you may
not convey it at all.  For example, if you agree to terms that obligate you
to collect a royalty for further conveying from those to whom you convey
the Program, the only way you could satisfy both those terms and this
License would be to refrain entirely from conveying the Program.

  13. Remote Network Interaction; Use with the GNU General Public License.

  Notwithstanding any other provision of this License, if you modify the
Program, your modified version must prominently offer all users
interacting with it remotely through a computer network (if your version
supports such interaction) an opportunity to receive the Corresponding
Source of your version by providing access to the Corresponding Source
from a network server at no charge, through some standard or customary
means of facilitating copying of software.  This Corresponding Source
shall include the Corresponding Source for any work covered by version 3
of the GNU General Public License that is incorporated pursuant to the
following paragraph.

  Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU General Public License into a single
combined work, and to convey the resulting work.  The terms of this
License will continue to apply to the part which is the covered work,
but the work with which it is combined will remain governed by version
3 of the GNU General Public License.

  14. Revised Versions of this License.

  The Free Software Foundation may publish revised and/or new versions of
the GNU Affero General Public License from time to time.  Such new versions
will be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

  Each version is given a distinguishing version number.  If the
Program specifies that a certain numbered version of the GNU Affero General
Public License "or any later version" applies to it, you have the
option of following the terms and conditions either of that numbered
version or of any later version published by the Free Software
Foundation.  If the Program does not specify a version number of the
GNU Affero General Public License, you may choose any version ever published
by the Free Software Foundation.

  If the Program specifies that a proxy can decide which future
versions of the GNU Affero General Public License can be used, that proxy's
public statement of acceptance of a version permanently authorizes you
to choose that version for the Program.

  Later license versions may give you additional or different
permissions.  However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.

  15. Disclaimer of Warranty.

  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

  16. Limitation of Liability.

  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGES.

  17. Interpretation of Sections 15 and 16.

  If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates
an absolute waiver of all civil liability in connection with the
Program, unless a warranty or assumption of liability accompanies a
copy of the Program in return for a fee.

                     END OF TERMS AND CONDITIONS

            How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.

    <one line to give the program's name and a brief idea of what it does.>
    Copyright (C) <year>  <name of author>

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.

Also add information on how to contact you by electronic and paper mail.

  If your software can interact with users remotely through a computer
network, you should also make sure that it provides a way for users to
get its source.  For example, if your program is a web application, its
interface could display a "Source" link that leads users to an archive
of the code.  There are many ways you could offer source, and different
solutions will be better for different programs; see section 13 for the
specific requirements.

  You should also get your employer (if you work as a programmer) or school,
if any, to sign a "copyright disclaimer" for the program, if necessary.
For more information on this, and how to apply and follow the GNU AGPL, see
<http://www.gnu.org/licenses/>.

EOL
  );
}


sub format_str_for_message ( $ )
{
  my $str = shift;

  # Here we should escape and truncate if necessary.
  return '"' . $str . '"';
}


sub is_windows ()
{
  return $^O eq 'MSWin32';
}


use constant DEV_NULL_FILENAME => is_windows ? "NUL" : "/dev/null";

sub start_process ( $ $ $ $ )
{
  my $cmdArgs             = shift;
  my $childStdinFilename  = shift;  # 'undef' means "use stdin".
  my $childStdoutFilename = shift;  # 'undef' means "use stdout".
  my $childStderrFilename = shift;  # 'undef' means "use stderr".


  if ( FALSE )
  {
    my @cmdArgs2 = @$cmdArgs;
    write_stdout( "Running process: " . $cmdArgs2[0] . " " . join( ' ', @cmdArgs2[ 1 .. $#cmdArgs2 ] ) . "\n" );
  }


  # The only way I managed to get stdin redirection to work is by using 'local' and barewords.
  # Using filehandles in any other way did not work.
  # I suspect that this is another bug in IPC::Open3.
  # I have tested with Perl v5.30.0 built for x86_64-linux-gnu-thread-multi.
  local *CHILD_STDIN;

  if ( defined( $childStdinFilename ) )
  {
    # Possible optimisation: If the caller uses DEV_NULL_FILENAME often,
    # we could open a filehandle for it just once on start-up.

    open( CHILD_STDIN, '<', $childStdinFilename )
      or die "Cannot open file " . format_str_for_message( $childStdinFilename ) . " for CHILD_STDIN: $!\n";
  }
  else
  {
    # We need to duplicate the STDIN file handle because open3() closes it automatically in the parent process.
    # If we pass STDIN, and it is closed in this parent process, that will trigger problems all over the place.
    # After all, all software assumes that stdin is available at file descriptor 0.
    # After closing it, that slot 0 will may be reused for other files.
    # I noticed because Perl starting printing warnings like
    # "Filehandle STDIN reopened as STDERR only for output" afterwards.
    # If you pass a file descriptor (a number) to open3() with fileno(), then it will not be automatically closed,
    # but I did not manage to get stdin redirection to work that way.

    open( CHILD_STDIN, "<&", *STDIN )
      or die "Cannot duplicate stdin to CHILD_STDIN: $!\n";
  }


  # I could not get open3() to work with normal file handles of the type "my $filehandle" for stdout.
  # This is apparently a bug in IPC::Open3:
  #   IPC::Open3 directs output incorrectly when STDOUT not connected to fd#1
  #   https://github.com/perl/perl5/issues/9759
  # I have tested with Perl v5.30.0 built for x86_64-linux-gnu-thread-multi.
  # Using barewords and local does work. Another way which does work is to save STDOUT and STDERR
  # by duplicating them, reopen them before calling open3(), and restoring them afterwards.
  # But that is tricky and probably slower.

  local *CHILD_STDOUT;
  local *CHILD_STDERR;

  if ( defined( $childStdoutFilename ) )
  {
    open( CHILD_STDOUT, ">", $childStdoutFilename )
      or die "Cannot create or overwrite file " . format_str_for_message( $childStdoutFilename ) . " for CHILD_STDOUT: $!\n";
  }
  else
  {
    # Possible optimisation: In this case, we could pass STDOUT directly to open3().
    open( CHILD_STDOUT, ">&", *STDOUT )
      or die "Cannot duplicate STDOUT to CHILD_STDOUT: $!\n";
  }

  if ( defined( $childStderrFilename ) )
  {
    if ( defined( $childStdoutFilename ) &&
         $childStdoutFilename eq $childStderrFilename )
    {
      # Possible optimisation: In this case, we could probably pass the same STDOUT as stderr directly to open3().
      open( CHILD_STDERR, ">&", *CHILD_STDOUT )
        or die "Cannot duplicate CHILD_STDOUT to CHILD_STDERR: $!\n";
    }
    else
    {
      open( CHILD_STDERR, ">", $childStderrFilename )
        or die "Cannot create or overwrite file " . format_str_for_message( $childStderrFilename ) . " for CHILD_STDERR: $!\n";
    }
  }
  else
  {
    open( CHILD_STDERR, ">&", *STDERR )
      or die "Cannot duplicate STDERR to CHILD_STDERR: $!\n";
  }


  my $childPid;

  eval
  {
    $childPid = open3( '<&CHILD_STDIN',  # This file descriptor will be closed automatically
                                         # on this process, at least if open3() succeeds.
                                         # If the process filename does not exist, open3() does not
                                         # close it, but I am not sure what would happen for other errors.
                       '>&CHILD_STDOUT',
                       '>&CHILD_STDERR',
                        @$cmdArgs );
  };

  my $open3ErrorMessage = $@;

  # In order to find out whether open3() has closed CHILD_STDIN or not,
  # we could use openhandle() from Scalar::Util.
  # But Perl should automatically close it anyway.
  if ( FALSE )
  {
    close( CHILD_STDIN )
      or die "Cannot close file handle CHILD_STDIN: $!\n";
  }

  close( CHILD_STDOUT )
    or die "Cannot close file handle CHILD_STDOUT: $!\n";

  close( CHILD_STDERR )
    or die "Cannot close file handle CHILD_STDERR: $!\n";


  if ( $open3ErrorMessage )
  {
    # Unfortunately, open3 generates rather user-unfriendly error messages.
    #
    # The first ugly component, the "open3:" prefix, is documented, and we can delete it.
    # After all, it offers no relevant information to the user.

    $open3ErrorMessage =~ s/^open3:\s*//;

    # The second ugly component is the suffix like "at ./MyScript.pl line 1234".
    # That only only helps confuse the user. Unfortunately, it is not so easy to reliably detect
    # and remove such a suffix. The source is a call to croak that is hard-coded in IPC::Open3::open3,
    # and I could find no way to disable such a suffix from here.

    die "Error running the child process: $open3ErrorMessage\n";
  }

  return $childPid;
}


# ----------- Main routine -----------

sub main ()
{
  my $arg_help      = 0;
  my $arg_h         = 0;
  my $arg_help_pod  = 0;
  my $arg_version   = 0;
  my $arg_license   = 0;
  my $arg_new_process_group = 0;

  # We need 'require_order' because otherwise a user command like "bash -c 'echo a'" would not work,
  # for this script would then attempt to process '-c' as one of its own options.
  Getopt::Long::Configure( "no_auto_abbrev", "prefix_pattern=(--|-)", "no_ignore_case", "require_order" );

  my $result = GetOptions(
                 'help'      => \$arg_help,
                 'h'         => \$arg_h,
                 'help-pod'  => \$arg_help_pod,
                 'version'   => \$arg_version,
                 'license'   => \$arg_license,
                 'new-process-group' => \$arg_new_process_group,
               );

  if ( not $result )
  {
    # GetOptions has already printed an error message.
    return EXIT_CODE_FAILURE;
  }

  if ( $arg_help || $arg_h )
  {
    print_help_text();
    return EXIT_CODE_SUCCESS;
  }

  if ( $arg_help_pod )
  {
    write_stdout( "This file is written in Perl's Plain Old Documentation (POD) format\n" );
    write_stdout( "and has been generated with option --help-pod .\n" );
    write_stdout( "Run the following Perl commands to convert it to HTML or to plain text for easy reading:\n" );
    write_stdout( "\n" );
    write_stdout( "  pod2html README.pod >README.html\n" );
    write_stdout( "  pod2text README.pod >README.txt\n" );
    write_stdout( "\n\n" );
    write_stdout( get_pod_from_this_script() );
    write_stdout( "\n" );
    return EXIT_CODE_SUCCESS;
  }

  if ( $arg_version )
  {
    write_stdout( "$Script version " . SCRIPT_VERSION . "\n" );
    return EXIT_CODE_SUCCESS;
  }

  if ( $arg_license )
  {
    write_stdout( get_license_text() );
    return EXIT_CODE_SUCCESS;
  }

  if ( scalar( @ARGV ) == 0 )
  {
    die "Invalid number of command-line arguments. Run this tool with the --help option for usage information.\n";
  }


  # We could hard-code here the indexes for syscalls prctl and setrlimit for the most popular platforms,
  # in order to skip loading syscall.ph, which is probably slow and may not be available or reliable everywhere.
  require "syscall.ph";

  use constant PR_SET_CHILD_SUBREAPER => 36;

  # We do not really need to pass to prctl() arg3, arg4 and arg5, because they are actually ignored.
  # But I find it cleaner to pass all the arguments that the syscall declares.

  my $prctl_res = syscall( &SYS_prctl, PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0 );

  if ( $prctl_res != 0 )
  {
    die "Syscall prctl( PR_SET_CHILD_SUBREAPER ) failed: $!\n";
  }


  if ( $arg_new_process_group )
  {
    setpgrp( 0, 0 );
  }


  my $cmd_pid = start_process( \@ARGV, undef, undef, undef );


  $SIG{ INT }   = 'IGNORE';
  $SIG{ TERM }  = 'IGNORE';


  my $cmd_status = undef;

  use constant SHOULD_TRACE => 0;

  for ( ; ; )
  {
    my $pid = wait();

    last if ( $pid == -1 );

    my $status = $?;

    if ( $pid == $cmd_pid )
    {
      $cmd_status = $status;

      if ( SHOULD_TRACE )
      {
        write_stderr( "Reaped command process with PID $pid.\n" );
      }
    }
    else
    {
      if ( SHOULD_TRACE )
      {
        write_stderr( "Reaped other process with PID $pid.\n" );
      }
    }
  }

  if ( ! defined( $cmd_status ) )
  {
    die "Could not retrieve the exit status of the command's child process.\n";
  }

  # We would normally use WIFEXITED etc. below from Perl module POSIX,
  # but such definitions are not available under Microsoft Windows.
  my $signal_number = $cmd_status & 127;

  if ( $signal_number )
  {
    my $signal_name = $signal_name_by_number[ $signal_number ];

    if ( SHOULD_TRACE )
    {
      write_stderr( "The command terminated due to signal $signal_number (SIG$signal_name).\n" );
    }

    # This is necessary for at least SIGFPE under Linux x86_64,
    # and for all other signals that we ignore.
    $SIG{ $signal_name }  = 'DEFAULT';


    # Disable the creation of core dump files for this process.
    # Otherwise, killing itself with a signal like SIGABRT may generate a core file,
    # possibly overwriting the original core file from the crashing child process.
    # The inspiration for this code came from FreeBSD's time utility:
    #   https://svnweb.freebsd.org/base/head/usr.bin/time/time.c?view=co

    # Instead of manually doing a syscall, We could use Perl module BSD::Resource,
    # but it is not part of the core modules so it will not be installed everywhere.
    use constant RLIMIT_CORE => 4;
    use constant CORE_DUMP_FILE_SIZE_LIMIT => 0;  # A value of 0 disables creating a core file.

    my $struct_rlimit = pack "L!L!", CORE_DUMP_FILE_SIZE_LIMIT, CORE_DUMP_FILE_SIZE_LIMIT;
    my $setrlimit_res = syscall( &SYS_setrlimit, &RLIMIT_CORE, $struct_rlimit );

    if ( $setrlimit_res != 0 )
    {
      die "Syscall setrlimit( RLIMIT_CORE ) failed: $!\n";
    }


    kill $signal_number, $$;

    die "Could not kill myself with the same signal $signal_number (SIG$signal_name) that killed the command process.\n";
  }

  my $cmd_exit_code = $cmd_status >> 8;

  if ( SHOULD_TRACE )
  {
    write_stderr( "The command terminated normally with exit status " . $cmd_exit_code . ".\n" );
  }

  return $cmd_exit_code;
}


eval
{
  exit main();
};

my $errorMessage = $@;

# We want the error message to be the last thing on the screen,
# so we need to flush the standard output first.
STDOUT->flush();

print STDERR "\nError running '$Script': $errorMessage";

exit EXIT_CODE_FAILURE;