[Perl/perl5] 37f34a: perlipc.pod: fix the "exec from signal handler" ex...

[stepnem via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 37f34a4f18f0b069e5274462e3cd5d5c19a78086
      
https://github.com/Perl/perl5/commit/37f34a4f18f0b069e5274462e3cd5d5c19a78086
  Author: Štěpán Němec <step...@smrk.net>
  Date:   2024-08-01 (Thu, 01 Aug 2024)

  Changed paths:
    M pod/perlipc.pod

  Log Message:
  -----------
  perlipc.pod: fix the "exec from signal handler" example

This is a partial revert of de7ba5179657 ("Doc patch to
perlipc", 2011-09-11).

I tested both the pre-de7ba5179657 and current version of
the example program (section "Handling the SIGHUP Signal in
Daemons") on Arch Linux (perl v5.38.2 built for x86_64-linux-thread-multi)
and OpenBSD -current (perl v5.36.3 built for amd64-openbsd).
On both systems, the handler is called only once with the
current (broken) version of the example program.
With the pre-de7ba5179657 version the handler is called on
every delivery of the signal, as expected.

POSIX shell test script (perlipc-sighandler.sh):
================================================
-->8--
#!/bin/sh
check() {
  printf '\nTesting %s\n' "$*"
  "$@" &
  sleep 1
  kill -HUP $!
  sleep 1
  kill -HUP $!
  sleep 1
  kill $!
}

bad=./perlipc-sighandler-bad.perl
good=./perlipc-sighandler-good.perl

# from current perlipc.pod (perl5 commit e78caca8144e v5.39.9-35-ge78caca8144e)
cat <<\EOF >"$bad"
#!/usr/bin/perl

use v5.36;

use POSIX ();
use FindBin ();
use File::Basename ();
use File::Spec::Functions qw(catfile);

$| = 1;

# make the daemon cross-platform, so exec always calls the script
# itself with the right path, no matter how the script was invoked.
my $script = File::Basename::basename($0);
my $SELF   = catfile( $FindBin::Bin, $script );

# POSIX unmasks the sigprocmask properly
$SIG{HUP} = sub {
  print "got SIGHUP\n";
  exec( $SELF, @ARGV ) || die "$0: couldn't restart: $!";
};

code();

sub code {
  print "PID: $$\n";
  print "ARGV: @ARGV\n";
  my $count = 0;
  while (1) {
    sleep 2;
    print ++$count, "\n";
  }
}
EOF

# from perlipc.pod before perl5 commit de7ba5179657 v5.15.2-343-gde7ba5179657
cat <<\EOF >"$good"
#!/usr/bin/perl

use v5.36;

use POSIX ();
use FindBin ();
use File::Basename ();
use File::Spec::Functions qw(catfile);

$| = 1;

# make the daemon cross-platform, so exec always calls the script
# itself with the right path, no matter how the script was invoked.
my $script = File::Basename::basename($0);
my $SELF   = catfile( $FindBin::Bin, $script );

# POSIX unmasks the sigprocmask properly
my $sigset = POSIX::SigSet->new();
my $action = POSIX::SigAction->new("sigHUP_handler",
                                   $sigset,
                                   &POSIX::SA_NODEFER);
POSIX::sigaction(&POSIX::SIGHUP, $action);

sub sigHUP_handler {
  print "got SIGHUP\n";
  exec( $SELF, @ARGV ) || die "$0: couldn't restart: $!";
}

code();

sub code {
  print "PID: $$\n";
  print "ARGV: @ARGV\n";
  my $count = 0;
  while (1) {
    sleep 2;
    print ++$count, "\n";
  }
}
EOF

chmod +x "$bad" "$good"

check "$bad"
check "$good"
-->8--

Example observed output:
========================
; ./perlipc-sighandler.sh

Testing ./perlipc-sighandler-bad.perl
PID: 19120
ARGV:
got SIGHUP
PID: 19120
ARGV:

Testing ./perlipc-sighandler-good.perl
PID: 19980
ARGV:
got SIGHUP
PID: 19980
ARGV:
got SIGHUP
PID: 19980
ARGV:

Cc: Leon Timmermans <faw...@gmail.com>
Fixes: de7ba5179657 ("Doc patch to perlipc")


  Commit: 37d9b9c0bb48b00900b977532e86fdd99e577df0
      
https://github.com/Perl/perl5/commit/37d9b9c0bb48b00900b977532e86fdd99e577df0
  Author: Štěpán Němec <step...@smrk.net>
  Date:   2024-08-01 (Thu, 01 Aug 2024)

  Changed paths:
    M pod/perlipc.pod

  Log Message:
  -----------
  perlipc.pod: improve "exec from signal handler" signal mask comment

"POSIX unmasks the sigprocmask properly" seems rather opaque.
Instead, explain the SA_NODEFER motivation directly.


  Commit: 6cc5fbb9234d1f71fc32008ff15b82b4c7e80822
      
https://github.com/Perl/perl5/commit/6cc5fbb9234d1f71fc32008ff15b82b4c7e80822
  Author: Štěpán Němec <step...@smrk.net>
  Date:   2024-08-01 (Thu, 01 Aug 2024)

  Changed paths:
    M AUTHORS

  Log Message:
  -----------
  Update AUTHORS (add stepnem)


  Commit: 9b9c0391a41918597cf73b1318bbec469b46aaa5
      
https://github.com/Perl/perl5/commit/9b9c0391a41918597cf73b1318bbec469b46aaa5
  Author: Štěpán Němec <step...@smrk.net>
  Date:   2024-08-01 (Thu, 01 Aug 2024)

  Changed paths:
    M pod/perlipc.pod

  Log Message:
  -----------
  perlipc.pod: avoid race in "exec from signal handler" example

Instead of using SA_NODEFER, which makes the handler
function itself interruptible by another SIGHUP, use
a plain $SIG{HUP} handler and unblock SIGHUP using
sigprocmask from the main program.


Compare: https://github.com/Perl/perl5/compare/9cca022465b6...9b9c0391a419

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