${"_<a.pl"}[$i] no longer numerical for pragmas

[ikegami]

See https://www.perlmonks.org/?node_id=11154956

perldebguts says:

Each array @{"_<$filename"} holds the lines of $filename for a file
compiled by Perl. [...] Values in this array are magical in numeric
context: they compare equal to zero only if the line is not breakable.

The last bit is not always true since 5.36.

Take the program

#!/usr/bin/perl

use strict;
use warnings;

use v5.10;

say 'Hello, world!';

In 5.36 and 5.38, we see:

  DB<1> use Devel::Peek;

  DB<2> Dump( ${"_<a.pl"}[8] );
SV = PVMG(0x55d8627958e0) at 0x55d86251ff80
  REFCNT = 1
  FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
  IV = 94387850872920
  NV = 0
  PV = 0x55d8628074b0 "say 'Hello, world!';\n"\0
  CUR = 21
  LEN = 32
  COW_REFCNT = 1

  DB<3> Dump( ${"_<a.pl"}[6] );
SV = PVMG(0x55d862786d50) at 0x55d86251fec0
  REFCNT = 1
  FLAGS = (POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55d8627d7d50 "use v5.10;\n"\0
  CUR = 11
  LEN = 16

Note that lack of IOK for line 6? It should look like line 5.

  DB<4> use Devel::Peek;Dump( ${"_<a.pl"}[5] );
SV = PVMG(0x55856a4198d0) at 0x55856a4e9630
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55856a486840 "\n"\0
  CUR = 1
  LEN = 16

This also means that only the lines with pragmas are broken.

This is a deviation from 5.34. In 5.34, we see

      DB<3> Dump( ${"_<a.pl"}[6] );
    SV = PVMG(0x55874ddaf680) at 0x55874db508a8
      REFCNT = 1
      FLAGS = (IOK,POK,pIOK,pPOK)
      IV = 94039612799016
      NV = 0
      PV = 0x55874db739a0 "use v5.10;\n"\0
      CUR = 11
      LEN = 16

(Same goes at least as far back as 5.10.)

So they were breakable in 5.34, but not in 5.36. Sounds like a valid bug fix. But it sounds like the person that fixed this forgot to set IOK when making that change.

[jkeenan]

I'm unclear as to how to reproduce this output. Even with a -DDEBUGGING perl, I get fewer lines of output in the debugger than @ikegami.

$ $THISPERL -d gh-21564-debug-pragma.pl

Loading DB routines from perl5db.pl version 1.77
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(gh-21564-debug-pragma.pl:8):
8:	say 'Hello, world!';
  DB<1> use Devel::Peek;

  DB<2> Dump( ${"_<a.pl"}[8] );
SV = NULL(0x0) at 0x55b2c651d3e0
  REFCNT = 908
  FLAGS = (READONLY,PROTECT)

  DB<3> 

What am I missing?

[mauke]

@jkeenan The name of the array needs to match the name of the source file. Your source file is named gh-21564-debug-pragma.pl, so you need to Dump( ${"_<gh-21564-debug-pragma.pl"}[8] );. Alternatively, rename the file to a.pl.

What you're seeing is the dump of an unset variable, basically.

[jkeenan]

@jkeenan The name of the array needs to match the name of the source file. Your source file is named gh-21564-debug-pragma.pl, so you need to Dump( ${"_<gh-21564-debug-pragma.pl"}[8] );. Alternatively, rename the file to a.pl.

What you're seeing is the dump of an unset variable, basically.

@mauke, thanks for that clarification. But now I'm seeing even fewer elements in the FLAGS() list on the line with the pragma call than, did @ikegami.

$ $THISPERL -d gh-21564-debug-pragma.pl

Loading DB routines from perl5db.pl version 1.77
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(gh-21564-debug-pragma.pl:8):
8:	say 'Hello, world!';
  DB<1> use Devel::Peek;

  DB<2> Dump( ${"_<gh-21564-debug-pragma.pl"}[8] );
SV = PVMG(0x55e304ea1a98) at 0x55e304c54660
  REFCNT = 1
  FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
  IV = 94433525995512
  NV = 0
  PV = 0x55e30507f208 "say 'Hello, world!';\n"\0
  CUR = 21
  LEN = 32
  COW_REFCNT = 1

  DB<3> Dump( ${"_<gh-21564-debug-pragma.pl"}[6]); 
SV = PVMG(0x55e3052f7208) at 0x55e304c545a0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55e304f03d98 "use v5.10;\n"\0
  CUR = 11
  LEN = 16

  DB<4> Dump( ${"_<gh-21564-debug-pragma.pl"}[5]); 
SV = PVMG(0x55e3052f71a8) at 0x55e3052c9978
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55e304c57b88 "\n"\0
  CUR = 1
  LEN = 16

I only get 2 flags where he got 4.

In any case, we'll have to figure out a way to bisect this.

[ikegami]

I only get 2 flags where he got 4.

The problem was originally written in terms of ${"_<a.pl"}[6] == 0 warning.
(Don't forget to use -w.)
So I must have run that earlier in the session.
That's what added pIOK and pNOK.

That's not relevant to the issue.

[ikegami]

In any case, we'll have to figure out a way to bisect this.

There shouldn't be many changes to the debugger between 5.34 and 5.36, I say with the confidence of the ignorant.

[tonycoz]

It's not (directly) a change to the debugger that caused it.

As to bisecting:

tony@venus:.../git/perl6$ cat ../21564/Devel/D21564.pm 
package DB;

sub DB {
}

1;
tony@venus:.../git/perl6$ cat ../21564b.pl
#!/usr/bin/perl

#use strict;
use warnings;
use B qw(svref_2object SVf_IOK);

use v5.10;

my $b = svref_2object(\(${"_<../21564b.pl"}[4]));
unless ($b->FLAGS & SVf_IOK) {
  die "Fail!";
}
say "Ok";

tony@venus:.../git/perl6$ perl ../bisect.pl --start=v5.34.0 -- ./perl -Ilib -I../21564 -d:D21564 ../21564b.pl
...
bad - non-zero exit from ./perl -Ilib -I../21564 -d:D21564 ../21564b.pl
0cdbe2162863542f45a5faa22e46c1aa76c029cb is the first bad commit
commit 0cdbe2162863542f45a5faa22e46c1aa76c029cb
Author: Tony Cook <tony@develop-help.com>
Date:   Wed Oct 20 15:36:44 2021 +1100

    remove cop addressed from saved lines when the cop is freed
    
    This could cause a bad read and write when the debugger tried to
    set a breakpoint on the line.
    
    Fixed #19198

 lib/perl5db.t | 43 +++++++++++++++++++++++++++++++++++++++++++
 op.c          | 14 ++++++++++++++
 2 files changed, 57 insertions(+)
bisect found first bad commitThat took 1013 seconds.

I don't remember why I turned off IOK.

Edit: add (trivial) Devel::D21564 source