sv_setsv_cow() will SEGV if dstsv != perfectsv (design question)

[bulk88]

Description

Design question.

I have 4-6 choices in my head how to fix this.
My opinion is all 4-6 choices are equally "the correct and best fix".
Looking for input before I write a patch making a permanent internal API change.
Or 6 years from now this could be a permanent CPAN XS pubic API

1 caller + PE export table inside perl543.dll @ 5.43.2, for a total of 2 consumers.

S_reg_set_capture_string+182	call    Perl_sv_setsv_cow
rdata:off_18030FE58	dd rva PL_EXACTFish_bitmask, rva PL_EXACT_REQ8_bitmask;

#ifdef PERL_ANY_COW
#  define SVt_COW SVt_PV
SV *
Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
{
    STRLEN cur = SvCUR(ssv);
    STRLEN len = SvLEN(ssv);
    char *new_pv;
    U32 new_flags = (SVt_COW|SVf_POK|SVp_POK|SVf_IsCOW);
#if defined(PERL_DEBUG_READONLY_COW) && defined(PERL_COPY_ON_WRITE)
    const bool already = cBOOL(SvIsCOW(ssv));
#endif

    PERL_ARGS_ASSERT_SV_SETSV_COW;
#ifdef DEBUGGING
    if (DEBUG_C_TEST) {
        PerlIO_printf(Perl_debug_log, "Fast copy on write: %p -> %p\n",
                      (void*)ssv, (void*)dsv);
        sv_dump(ssv);
        if (dsv)
                    sv_dump(dsv);
    }
#endif
    if (dsv) {
        if (SvTHINKFIRST(dsv))
            sv_force_normal_flags(dsv, SV_COW_DROP_PV);
        else if (SvPVX_const(dsv))      <<<<< ---- WHAT ??? !!!!!!!!!!!!!!!!!!!!!!
            Safefree(SvPVX_mutable(dsv));
        SvUPGRADE(dsv, SVt_COW); <<<<< ---- WHAT ??? !!!!!!!!!!!!!!!!!!!!!!
    }
    else
        dsv = newSV_type(SVt_COW);

Steps to Reproduce

use Devel::Peek;
use Data::Dumper;
#use Inline  ('force', 'noclean');

use Inline C => Config =>
 PRE_HEAD => '

 #define PERL_NO_GET_CONTEXT 1

 ';
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => Config => PREFIX => 'MXS';

use Inline C => <<'END_OF_C_CODE';

static void S_setsv_cow(pTHX_ CV* cv) {
    dXSARGS;
    if(items != 2)
		croak_xs_usage(cv, "dsv, ssv");
    SV* ssv = POPs;
	SV* dsv = POPs;
	SV* retsv = Perl_sv_setsv_cow(aTHX_ dsv, ssv);
	PUSHs(retsv);
	PUTBACK;
    return;
}

void BOOT1() {
    dTHX;
	newXS( "setsv_cow", S_setsv_cow, __FILE__);
	return;
}

END_OF_C_CODE

$, = "\n";

#use v5.30;
#use strict;
#use warnings;
#$DB::single = 1;

use Benchmark qw(:all);
use Devel::Peek 'Dump';

BOOT1();

my $spath = 'txt.t';
my $dpath = (length(pack('p',undef)) * 3) + 2;
Dump($spath);
Dump($dpath);

# Unhandled exception thrown: read access violation.
# **ptr** was 0x2.
my $rpath = setsv_cow($dpath,$spath);

# unreached
Dump($rpath); 

__END__

>	perl541.dll!VMem::Free(void * pMem) Line 215	C++
 	[Inline Frame] perl541.dll!Perl_safesysfree(void * where) Line 433	C++
 	perl541.dll!Perl_sv_setsv_cow(interpreter * my_perl, sv * dsv, sv * ssv) Line 4922	C++
 	inlinec_pl_ae81.dll!S_setsv_cow(interpreter * my_perl, cv * cv) Line 40	C
 	[Inline Frame] perl541.dll!Perl_rpp_invoke_xs(interpreter *) Line 1177	C++
 	perl541.dll!Perl_pp_entersub(interpreter * my_perl) Line 6529	C++
 	perl541.dll!Perl_runops_standard(interpreter * my_perl) Line 41	C++
 	perl541.dll!S_run_body(interpreter * my_perl, long oldscope) Line 2885	C++
 	perl541.dll!perl_run(interpreter * my_perl) Line 2798	C++
 	perl541.dll!RunPerl(int argc, char * * argv, char * * env) Line 206	C++
 	[Inline Frame] perl.exe!invoke_main() Line 78	C++
 	perl.exe!__scrt_common_main_seh() Line 288	C++
 	kernel32.dll!BaseThreadInitThunk�()	Unknown
 	ntdll.dll!RtlUserThreadStart�()	Unknown

Expected behavior

Not

# Unhandled exception thrown: read access violation (0xC0000005)
# **ptr** was 0x2.

Fix is some permutation of P5P to P5P doc change (C comments), P5P to perlintern.pod doc change, P5P to perlapi.pod doc change, and C code change.

Its a matrix of choices, not pick 1 of 4.

doc change and/or code change

Perl configuration

5.43.2 and going back many years