[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, 20 insertions, 10 deletions

diff --git a/pad.c b/pad.c
index d8d9322c60..92765f0f79 100644
--- a/pad.c
+++ b/pad.c
@@ -285,6 +285,7 @@ Perl_pad_new(pTHX_ int flags)
 	av_store(pad, 0, NULL);
 	padname = newAV();
 	AvPAD_NAMELIST_on(padname);
+	av_store(padname, 0, &PL_sv_undef);
     }
 
     /* Most subroutines never recurse, hence only need 2 entries in the padlist
@@ -1651,11 +1652,13 @@ Perl_pad_swipe(pTHX_ PADOFFSET po, bool refadjust)
     PL_curpad[po] = newSV(0);
     SvPADTMP_on(PL_curpad[po]);
 #else
-    PL_curpad[po] = &PL_sv_undef;
+    PL_curpad[po] = NULL;
 #endif
     if (PadnamelistMAX(PL_comppad_name) != -1
      && (PADOFFSET)PadnamelistMAX(PL_comppad_name) >= po) {
-	assert(!PadnameLEN(PadnamelistARRAY(PL_comppad_name)[po]));
+	if (PadnamelistARRAY(PL_comppad_name)[po]) {
+	    assert(!PadnameLEN(PadnamelistARRAY(PL_comppad_name)[po]));
+	}
 	PadnamelistARRAY(PL_comppad_name)[po] = &PL_sv_undef;
     }
     if ((I32)po < PL_padix)
@@ -1767,21 +1770,23 @@ Perl_pad_tidy(pTHX_ padtidy_type type)
 	av_store(PL_comppad_name, AvFILLp(PL_comppad), NULL);
 
     if (type == padtidy_SUBCLONE) {
-	SV * const * const namep = AvARRAY(PL_comppad_name);
+	SV ** const namep = AvARRAY(PL_comppad_name);
 	PADOFFSET ix;
 
 	for (ix = AvFILLp(PL_comppad); ix > 0; ix--) {
 	    SV *namesv;
+	    if (!namep[ix]) namep[ix] = &PL_sv_undef;
 
 	    /*
 	     * The only things that a clonable function needs in its
 	     * pad are anonymous subs, constants and GVs.
 	     * The rest are created anew during cloning.
 	     */
-	    if (SvIMMORTAL(PL_curpad[ix]) || IS_PADGV(PL_curpad[ix]))
+	    if (!PL_curpad[ix] || SvIMMORTAL(PL_curpad[ix])
+		 || IS_PADGV(PL_curpad[ix]))
 		continue;
-	    if (!((namesv = namep[ix]) != NULL &&
-		  PadnamePV(namesv) &&
+	    namesv = namep[ix];
+	    if (!(PadnamePV(namesv) &&
 		   (!PadnameLEN(namesv) || *SvPVX_const(namesv) == '&')))
 	    {
 		SvREFCNT_dec(PL_curpad[ix]);
@@ -1797,10 +1802,12 @@ Perl_pad_tidy(pTHX_ padtidy_type type)
     }
 
     if (type == padtidy_SUB || type == padtidy_FORMAT) {
-	SV * const * const namep = AvARRAY(PL_comppad_name);
+	SV ** const namep = AvARRAY(PL_comppad_name);
 	PADOFFSET ix;
 	for (ix = AvFILLp(PL_comppad); ix > 0; ix--) {
-	    if (SvIMMORTAL(PL_curpad[ix]) || IS_PADGV(PL_curpad[ix]) || IS_PADCONST(PL_curpad[ix]))
+	    if (!namep[ix]) namep[ix] = &PL_sv_undef;
+	    if (!PL_curpad[ix] || SvIMMORTAL(PL_curpad[ix])
+		 || IS_PADGV(PL_curpad[ix]) || IS_PADCONST(PL_curpad[ix]))
 		continue;
 	    if (!SvPADMY(PL_curpad[ix])) {
 		SvPADTMP_on(PL_curpad[ix]);
@@ -2417,7 +2424,8 @@ Perl_padlist_dup(pTHX_ PADLIST *srcpad, CLONE_PARAMS *param)
 	    for ( ;ix > 0; ix--) {
 		if (!oldpad[ix]) {
 		    pad1a[ix] = NULL;
-		} else if (names_fill >= ix && PadnameLEN(names[ix])) {
+		} else if (names_fill >= ix && names[ix] &&
+			   PadnameLEN(names[ix])) {
 		    const char sigil = SvPVX_const(names[ix])[0];
 		    if ((SvFLAGS(names[ix]) & SVf_FAKE)
 			|| (SvFLAGS(names[ix]) & SVpad_STATE)
@@ -2446,7 +2454,9 @@ Perl_padlist_dup(pTHX_ PADLIST *srcpad, CLONE_PARAMS *param)
 			}
 		    }
 		}
-		else if (IS_PADGV(oldpad[ix]) || PadnamePV(names[ix])) {
+		else if (IS_PADGV(oldpad[ix])
+		      || (  names_fill >= ix && names[ix]
+			 && PadnamePV(names[ix])  )) {
 		    pad1a[ix] = sv_dup_inc(oldpad[ix], param);
 		}
 		else {