[perl #7508] Use NULL for nonexistent array elems

This commit fixes bug #7508 and provides the groundwork for fixing
several other bugs.

Elements of @_ are aliased to the arguments, so that \$_[0] within
sub foo will reference the same scalar as \$x if the sub is called
as foo($x).

&PL_sv_undef (the global read-only undef scalar returned by the
‘undef’ operator itself) was being used to represent nonexistent
array elements.  So the pattern would be broken for foo(undef), where
\$_[0] would vivify a new $_[0] element, treating it as having been
nonexistent.

This also causes other problems with constants under ithreads
(#105906) and causes a pending fix for another bug (#118691) to trig-
ger this bug.

This commit changes the internals to use a null pointer to represent a
nonexistent element.

This requires that Storable be changed to account for it.  Also,
IPC::Open3 was relying on the bug.  So this commit patches
both modules.

1 files changed, 8 insertions, 4 deletions

diff --git a/pp_hot.c b/pp_hot.c
index b08643fcc1..58a30831d8 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -339,7 +339,11 @@ S_pushav(pTHX_ AV* const av)
         }
     }
     else {
-        Copy(AvARRAY(av), SP+1, maxarg, SV*);
+        U32 i;
+        for (i=0; i < (U32)maxarg; i++) {
+            SV * const sv = AvARRAY(av)[i];
+            SP[i+1] = sv ? sv : &PL_sv_undef;
+        }
     }
     SP += maxarg;
     PUTBACK;
@@ -1055,8 +1059,8 @@ PP(pp_aassign)
 	    i = 0;
 	    while (relem <= lastrelem) {	/* gobble up all the rest */
 		SV **didstore;
-		assert(*relem);
-		SvGETMAGIC(*relem); /* before newSV, in case it dies */
+		if (*relem)
+		    SvGETMAGIC(*relem); /* before newSV, in case it dies */
 		sv = newSV(0);
 		sv_setsv_nomg(sv, *relem);
 		*(relem++) = sv;
@@ -2830,7 +2834,7 @@ PP(pp_aelem)
 	      MEM_WRAP_CHECK_1(elem,SV*,oom_array_extend);
 	 }
 #endif
-	if (!svp || *svp == &PL_sv_undef) {
+	if (!svp || !*svp) {
 	    SV* lv;
 	    if (!defer)
 		DIE(aTHX_ PL_no_aelem, elem);