panic: invalid pad in intro_my with perl -MO=... and use v5.39*

[wolfsage]

Description

With -DDEBUGGING:

$ ./perl -Ilib -MO=Concise -e 'use v5.39'
panic: invalid pad in intro_my: 0x55ef8a17ca20[0x55ef8a1945b0] at -e line 1

Also

$ ./perl -Ilib -MO=Deparse -e 'use v5.39'
panic: invalid pad in intro_my: 0x55cd471baa20[0x55cd471d2630] at -e line 1.
sub BEGIN {
    require v5.39;
    ()
}

Note this doesn't happen with anything use v5.38 or less, only 5.39 and higher.

According to a bisect this appears to have been introduced here:

HEAD is now at 6d65c70ff0 Import from `builtin` version bundles should be idempotent on existing symbols
good - zero exit from ./perl -Ilib -MO=Concise -e use v5.39;
48d382667948f6a16c732c47791ca4be891a9bdb is the first bad commit
commit 48d382667948f6a16c732c47791ca4be891a9bdb
Author: Paul "LeoNerd" Evans <[email protected]>
Date:   Fri Jan 19 15:00:35 2024 +0000

    Have `use VERSION` and `perl -E` import a corresponding `use builtin` bundle

[leonerd]

Hmm. Specifically it's about Concise:

$ ./perl -Ilib -MO=Concise -e 'use v5.39'
panic: invalid pad in intro_my: 0x565259cf7920[0x565259d179f8] at -e line 1.

$ ./perl -Ilib -e 'use v5.39'
(ok)

The panic appears in pad.h:

$ findsrc | grepin "invalid pad in "
./pad.h:172:        Perl_croak(aTHX_ "panic: invalid pad in %s: 0x%" UVxf "[0x%" UVxf "]",\

169 #  define ASSERT_CURPAD_ACTIVE(label) \  
170     pad_peg(label); \  
171     if (!PL_comppad || (AvARRAY(PL_comppad) != PL_curpad))                \  
172         Perl_croak(aTHX_ "panic: invalid pad in %s: 0x%" UVxf "[0x%" UVxf "]",\  
173             label, PTR2UV(PL_comppad), PTR2UV(PL_curpad));  

Invoked in pad.c:

./pad.c:1478:    ASSERT_CURPAD_ACTIVE("intro_my");

I'll have to read more into it to work out why.

[leonerd]

Hrm; gdb doesn't show much more of any real excitement:

$ gdb --args ./perl -Ilib -MO=Concise -e 'use v5.39; no v5.39'
...
(gdb) break Perl_croak
Breakpoint 1 at 0x45c745: file util.c, line 1942.
(gdb) run
Starting program: /home/leo/src/bleadperl/perl/perl -Ilib -MO=Concise -e use\ v5.39\;\ no\ v5.39
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, Perl_croak (my_perl=0x555555c942a0, 
    pat=0x555555a19db8 "panic: illegal pad in %s: 0x%lx[0x%lx]") at util.c:1942
1942    {
(gdb) bt
#0  Perl_croak (my_perl=0x555555c942a0, 
    pat=0x555555a19db8 "panic: illegal pad in %s: 0x%lx[0x%lx]") at util.c:1942
#1  0x00005555556b7704 in Perl_pad_new (my_perl=0x555555c942a0, flags=6) at pad.c:199
#2  0x0000555555997e35 in Perl_start_subparse (my_perl=0x555555c942a0, is_format=0, 
    flags=0) at toke.c:12824
#3  0x00005555556e05c6 in Perl_yyparse (my_perl=0x555555c942a0, gramtype=258)
    at perly.y:766
#4  0x00005555555ec7b1 in S_parse_body (my_perl=0x555555c942a0, env=0x0, 
    xsinit=0x55555559b396 <xs_init>) at perl.c:2672
#5  0x00005555555e9eee in perl_parse (my_perl=0x555555c942a0, 
    xsinit=0x55555559b396 <xs_init>, argc=5, argv=0x7fffffffe218, env=0x0) at perl.c:1912
#6  0x000055555559b286 in main (argc=5, argv=0x7fffffffe218, env=0x7fffffffe248)
    at perlmain.c:106

[leonerd]

More excitement: order matters:

leo@hevva:~/src/bleadperl/perl [git]
$ ./perl -Ilib -e 'use v5.39; use O q(Concise);'
3  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter v ->2
2     <0> stub v ->3
-     <;> ex-nextstate(main 1 -e:1) v:us,*,&,{,$,fea=7 ->3
-e syntax OK

$ ./perl -Ilib -e 'use O q(Concise); use v5.39'
panic: invalid pad in intro_my: 0x5622c15488f0[0x5622c15689a8] at -e line 1.

[tonycoz]

It looks like AvARRAY() of the saved pad is being modified after the saves:

tony@venus:.../git/perl6$ gdb --args ./perl -Ilib -MO=Concise -e 'use v5.39'
...
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./perl...
(gdb) b Perl_finish_export_lexical 
Breakpoint 1 at 0xb35be: file builtin.c, line 66.
(gdb) r
Starting program: /home/tony/dev/perl/git/perl6/perl -Ilib -MO=Concise -e use\ v5.39
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, Perl_finish_export_lexical () at builtin.c:66
66          intro_my();
(gdb) n
68          LEAVE;
(gdb) 
69      }
(gdb) p AvARRAY(PL_comppad)
$1 = (SV **) 0x555555ccb9c0
(gdb) p PL_curpad
$2 = (SV **) 0x555555c49f90
(gdb)

[tonycoz]

Changing Perl_prepare_export_lexical to use SAVECOMPPAD() fixes it:

void
Perl_prepare_export_lexical(pTHX)
{
    assert(PL_compcv);

    /* We need to have PL_comppad / PL_curpad set correctly for lexical importing */
    ENTER;
    SAVESPTR(PL_comppad_name); PL_comppad_name = PadlistNAMES(CvPADLIST(PL_compcv));
    SAVECOMPPAD();
    PL_comppad      = PadlistARRAY(CvPADLIST(PL_compcv))[1];
    PL_curpad       = PadARRAY(PL_comppad);
}

Trying to see why -MO=... cause this, it's not just Concise, I see it for Deparse, Showlex and Xref too.

[tonycoz]

tony@venus:.../git/perl6$ ./perl -Ilib -MB -e 'BEGIN { B::save_BEGINs; } use v5.39'
panic: invalid pad in intro_my: 0x55c071135a20[0x55c07114cf00] at -e line 1.

though I still don't know why.