av.c, pad.c, pp.c, pp_hot.c, scope.c: resolve fencepost errors related to av_extend()

apparently over the years there has been confusion about what the
count argument to av_extend() is for, and at least some of our
code has been incorrectly assuming it means the *index* in the
array that one wants to write to, and not the number of elements
that one expects to be available to write to. This means that
part of the code was calling av_extend with arguments one less
than they should have been.

One such example is the code in pp_aassign() which passes in 1 less
than it should, and the code in av_extend() for tied hashes would
then add 1 back, thus making the tied interface seem to behave
properly. However this is wrong, and it shows up in

   @tied_array = sort @tied_array;

which correctly calls av_extend with the number of elements in
the array, which in turn would cause tied hashes to EXTEND one
more element than they should, which for something like Tie::File
causes the creation of an empty element at the end of the file.
It is a little debatable whether Tie::File::EXTEND should be a
no-op or not (I lean towards not once the internals are fixed)
but it is certainly the case that what we were doing before this
was not correct.

This fixes [rt.cpan.org #39196] Issue #17496, although does not
add tests, which will come in a follow up patch once I have time
and I have seen the smoke tests from this patch.

Author: demerphq

av.c

@@ -72,7 +72,7 @@ Perl_av_extend(pTHX_ AV *av, SSize_t key)
     mg = SvTIED_mg((const SV *)av, PERL_MAGIC_tied);
     if (mg) {
 	SV *arg1 = sv_newmortal();
-	sv_setiv(arg1, (IV)(key + 1));
+	sv_setiv(arg1, (IV)(key));
 	Perl_magic_methcall(aTHX_ MUTABLE_SV(av), mg, SV_CONST(EXTEND), G_DISCARD, 1,
 			    arg1);
 	return;

pad.c

@@ -2493,7 +2493,7 @@ Perl_padlist_dup(pTHX_ PADLIST *srcpad, CLONE_PARAMS *param)
 
 	pad1 = newAV();
 
-	av_extend(pad1, ix);
+	av_extend(pad1, ix+1);
 	PadlistARRAY(dstpad)[1] = pad1;
 	pad1a = AvARRAY(pad1);
 

pp.c

@@ -4895,7 +4895,7 @@ PP(pp_aslice)
 		    max = elem;
 	    }
 	    if (max > AvMAX(av))
-		av_extend(av, max);
+		av_extend(av, max+1);
 	}
 
 	while (++MARK <= SP) {

pp_hot.c

@@ -2416,7 +2416,7 @@ PP(pp_aassign)
 
             if (SvMAGICAL(ary) || SvREADONLY(ary) || !AvREAL(ary)) {
                 /* for arrays we can't cheat with, use the official API */
-                av_extend(ary, nelems - 1);
+                av_extend(ary, nelems);
                 for (i = 0; i < nelems; i++) {
                     SV **svp = &(PL_tmps_stack[tmps_base + i]);
                     SV *rsv = *svp;
@@ -4926,7 +4926,7 @@ Perl_clear_defarray(pTHX_ AV* av, bool abandon)
     }
     else {
         AV *newav = newAV();
-        av_extend(newav, fill);
+        av_extend(newav, fill+1);
         AvREIFY_only(newav);
         PAD_SVl(0) = MUTABLE_SV(newav);
         SvREFCNT_dec_NN(av);

scope.c

@@ -76,7 +76,7 @@ Perl_new_stackinfo(pTHX_ I32 stitems, I32 cxitems)
     Newx(si, 1, PERL_SI);
     si->si_stack = newAV();
     AvREAL_off(si->si_stack);
-    av_extend(si->si_stack, stitems > 0 ? stitems-1 : 0);
+    av_extend(si->si_stack, stitems > 0 ? stitems : 0);
     AvALLOC(si->si_stack)[0] = &PL_sv_undef;
     AvFILLp(si->si_stack) = 0;
     si->si_prev = 0;