make pregexec() handle zero-length strings again

GH #23903

In embed.fnc, commit v5.43.3-167-g45ea12db26 added SPTR, EPTR parameter
modifiers to (amongst other API functions), Perl_pregexec().

These cause assert constraints to be added to the effect that SPTR <
EPTR (since the latter is supposed to be a pointer to the byte after the
last character in the string).

This falls down for an empty string since in this case pregexec() is
called with strbeg == strend.

This was causing an assert failure in the test suite for
Package-Stash-XS.

The reason it wasn't noticed before is because:

1) pregexec() is a thin wrapper over regexec_flags();

2) The perl core (e.g. pp_match()) calls regexec_flags() rather than
   pregexec();

3) Package::Stash::XS has XS code which calls pregexec() directly rather
   than using CALLREGEXEC() (which would call regexec_flags());

4) In embed.fnc, regexec_flags()'s strend parameter is declared as
   NN rather than EPTR, so it doesn't get the assert added.

So very little code was actually using pregexec().

This commit, for now, changes pregexec()'s strend parameter from EPTR to
EPTRQ, which has the net effect of allowing zero-length strings to be
passed, and thus fixes the CPAN issue.

But longer term, we need to decide: is the general logic for EPTR wrong?
Should the assert be SPTR <= EPTR? And should EPTR be applied to
regexec_flags()'s strend parameter too?

Author: iabyn

embed.fnc

@@ -2713,7 +2713,7 @@ Adhp	|REGEXP *|pregcomp	|NN SV * const pattern			\
 				|const U32 flags
 Adhp	|I32	|pregexec	|NN REGEXP * const prog 		\
 				|MPTR char *stringarg			\
-				|EPTR char *strend			\
+				|EPTRQ char *strend			\
 				|SPTR char *strbeg			\
 				|SSize_t minend 			\
 				|NN SV *screamer			\

ext/XS-APItest/APItest.pm

@@ -4,7 +4,7 @@ use strict;
 use warnings;
 use Carp;
 
-our $VERSION = '1.47';
+our $VERSION = '1.48';
 
 require XSLoader;
 

ext/XS-APItest/APItest.xs

@@ -4780,6 +4780,29 @@ CODE:
 OUTPUT:
     RETVAL
 
+ # provide access to pregexec, except replace pointers within the
+ # string with offsets from the start of the string
+
+I32
+callpregexec(SV *prog, STRLEN stringarg, STRLEN strend, I32 minend, SV *sv, U32 nosave)
+CODE:
+    {
+        STRLEN len;
+        char *strbeg;
+        if (SvROK(prog))
+            prog = SvRV(prog);
+        strbeg = SvPV_force(sv, len);
+        RETVAL = pregexec((REGEXP *)prog,
+                            strbeg + stringarg,
+                            strbeg + strend,
+                            strbeg,
+                            minend,
+                            sv,
+                            nosave);
+    }
+OUTPUT:
+    RETVAL
+
 void
 lexical_import(SV *name, CV *cv)
     CODE:

ext/XS-APItest/t/callregexec.t

@@ -1,6 +1,6 @@
 #!perl
 
-# test CALLREGEXEC()
+# test CALLREGEXEC() and pregexec()
 # (currently it just checks that it handles non-\0 terminated strings;
 # full tests haven't been added yet)
 
@@ -10,7 +10,7 @@ use strict;
 use XS::APItest;
 *callregexec = *XS::APItest::callregexec;
 
-use Test::More tests => 48;
+use Test::More tests => 75;
 
 # Test that the regex engine can handle strings without terminating \0
 # XXX This is by no means comprehensive; it doesn't test all ops, nor all
@@ -34,6 +34,8 @@ sub try {
     my $bytes = do { use bytes; length $str1 };
     ok  !!$exp == !!callregexec($re, 0, $bytes, 0, $str, 0),
 	    "$desc callregexec";
+    ok  !!$exp == !!callpregexec($re, 0, $bytes, 0, $str, 0),
+	    "$desc callpregexec";
 }
 
 
@@ -62,4 +64,5 @@ sub try {
     try "ab\t",        qr/^.+\h/,         0, 'HORIZWS';
     try "abx",         qr/^.+\H/,         1, 'NHORIZWS';
     try "abx",         qr/a.*x/,          0, 'CURLY';
+    try "",            qr/x?/,            1, 'empty';
 }