Re: plperl on windows

Hi,

On 2021-10-04 14:38:16 -0700, Andres Freund wrote:
> 3) When building against strawberry perl 5.32.1.1 I see errors when loading
> plperl

The error is:
loadable library and perl binaries are mismatched (got handshake key 0000000012800080, needed 0000000012900080)

A bunch of research led me to believe this is because the struct sizes of
PerlInterpreter differ between perl being compiled and us embedding
perl.

After a lot of headscratching [1], I got a struct layout of both a gcc compiled
(just a test.c including the relevant headers) and and the msvc compiled
plperl.dll. And indeed they differ:

msvc:

+0x42d Iin_utf8_COLLATE_locale : Bool
+0x42e Iin_utf8_turkic_locale : Bool
+0x42f Ilocale_utf8ness : [256] Char
+0x530 Iwarn_locale : Ptr64 sv
+0x538 Icolors : [6] Ptr64 Char
+0x568 Ipeepp : Ptr64 void
..
+0x1278 IPrivate_Use : Ptr64 sv

gcc:
/* 0x042d | 0x0001 */ _Bool Iin_utf8_COLLATE_locale;
/* 0x042e | 0x0001 */ _Bool Iin_utf8_turkic_locale;
/* 0x0430 | 0x0004 */ int Ilc_numeric_mutex_depth;
/* 0x0434 | 0x0100 */ char Ilocale_utf8ness[256];
/* 0x0538 | 0x0008 */ SV *Iwarn_locale;
/* 0x0540 | 0x0030 */ char *Icolors[6];
/* 0x0570 | 0x0008 */ peep_t Ipeepp;
...
/* 0x1280 | 0x0008 */ SV *IPrivate_Use;

The gcc version has a Ilc_numeric_mutex_depth that the msvc version
doesn't. The relevant part of intrpvar.h:

PERLVAR(I, in_utf8_turkic_locale, bool)
#if defined(USE_ITHREADS) && ! defined(USE_THREAD_SAFE_LOCALE)
PERLVARI(I, lc_numeric_mutex_depth, int, 0) /* Emulate general semaphore */
#endif
PERLVARA(I, locale_utf8ness, 256, char)

This conditional piece didn't yet exist in 5.26.n. Which is why that's the
last version that actually works.

USE_ITHREADS is defined in perls' config.h, but USE_THREAD_SAFE_LOCALE is
derived from some other stuff. So that's the culprit.

I gotta do something else for a bit, so I'll stop here for now.

The error message about mismatched lib / perl binary could really use a bit
more detail. It's pretty darn annoying to figure out right now what it could
mean.

Greetings,

Andres Freund

[1] On linux I'd just use pahole to display struct layouts, but on
windows... Neither of the windows perl installations comes with debug symbols,
afaict.
For the gcc definition:
I compiled a test.c with msys ucrt64, including -g3, set a breakpoint on main,
dumped the struct with "ptype /ox my_interp"
For the msvc definition:
connected cdb.exe to a live backend, did a CREATE EXTENSION plperl, cdb
stopped at exit, then I could dump the type with "dt plperl!PerlInterpreter"

I'm sure there's a better way. And of course I'm mainly including this for a
future self that might remember needing to something like this before...