#!/usr/bin/env perl
# Run a single instance of a process
@ARGV or die "Usage: single <command>\n";

use strict;
use warnings;
use Digest::MD5 'md5_hex';
use File::Spec;

{
  local %ENV = %ENV;
  $ENV{PATH} ||= '/usr/local/bin:/usr/bin:/bin';
  $ENV{ALREADY_RUNNING_STATUS} ||= 0;
  $ENV{SINGLE_HUP_SIGNAL} ||= 'TERM';
  $ENV{SINGLE_ID} = md5_hex "@ARGV";
  $ENV{SINGLE_INFO_FILE} ||= File::Spec->catfile(File::Spec->tmpdir, "single-$ENV{SINGLE_ID}");

  return {%ENV} if defined wantarray;
  exec perldoc => 'App::single' if @ARGV == 1 and $ARGV[0] =~ /^--?(?:h|help)$/;
  exit $ENV{ALREADY_RUNNING_STATUS} if App::single::script::running();
  exit App::single::script::run(App::single::script::start());
}

package App::single::script;

sub run {
  my $pid = shift;
  my $exit_status = 255;
  my $restart;

  local $SIG{HUP} = sub {
    kill $ENV{SINGLE_HUP_SIGNAL}, $pid;
    $restart = 1;
  };

  while ($pid) {
    waitpid $pid, 0;
    $exit_status = $? >> 8;
    unlink $ENV{SINGLE_INFO_FILE};
    $pid = $restart ? App::single::script::start() : 0;
    $restart = 0;
  }

  return $exit_status;
}

sub running {
  my ($info, $pid) = ('', 0);
  open my $INFO, '<', $ENV{SINGLE_INFO_FILE} or return 0;

  while (<$INFO>) {
    $pid = $1 if /pid:\s*(\d+)/;
    $info .= $_;
  }

  if ($pid and kill 0, $pid) {
    print $info, "status: running\n" unless $ENV{SINGLE_SILENT};
    return 1;
  }

  return 0;
}

sub start {
  my $pid = fork // die "Could not fork @ARGV: $!\n";

  if ($pid == 0) { # child
    exec @ARGV;
    die "Could not exec @ARGV: $!\n";
  }

  open my $INFO, '>', $ENV{SINGLE_INFO_FILE} or die "Write $ENV{SINGLE_INFO_FILE}: $!";
  print $INFO "pid: $pid\n";
  print $INFO "command: @ARGV\n";
  print $INFO "id: $ENV{SINGLE_ID}\n";
  print $INFO "started: @{[~~localtime]}\n";
  close $INFO;

  return $pid;
}