Flatten vstrings modified in place

A substitution forces its target to a string upon successful substitu-
tion, even if the substitution did nothing:

$ ./perl -Ilib -le '$a = *f; $a =~ s/f/f/; print ref \$a'
SCALAR

Notice that $a is no longer a glob after s///.

But vstrings are different:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/f/; print ref \$a'
VSTRING

I fixed this in 5.16 (1e6bda93) for those cases where the vstring ends
up with a value that doesn’t correspond to the actual string:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/o/; print ref \$a'
SCALAR

It works through vstring set-magic, that does the check and removes
the magic if it doesn’t match.

I did it that way because I couldn’t think of any other way to fix
bug #29070, and I didn’t realise at the time that I hadn’t fixed
all the bugs.

By making SvTHINKFIRST true on a vstring, we force it through
sv_force_normal before any in-place string operations.  We can also
make sv_force_normal handle vstrings as well.  This fixes all the lin-
gering-vstring-magic bugs in just two lines, making the vstring set-
magic (which is also slow) redundant.  It also allows the special case
in sv_setsv_flags to be removed.

Or at least that was what I had hoped.

It turns out that pp_subst, twists and turns in tortuous ways, and
needs special treatment for things like this.

And do_trans functions wasn’t checking SvTHINKFIRST when arguably it
should have.

I tweaked sv_2pv{utf8,byte} to avoid copying magic variables that do
not need copying.

1 files changed, 2 insertions, 6 deletions

diff --git a/mg_vtable.h b/mg_vtable.h
index 3c73c2beff..2490394895 100644
--- a/mg_vtable.h
+++ b/mg_vtable.h
@@ -86,7 +86,6 @@ enum {		/* pass one of these to get_vtbl */
     want_vtbl_utf8,
     want_vtbl_uvar,
     want_vtbl_vec,
-    want_vtbl_vstring,
     magic_vtable_max
 };
 
@@ -120,8 +119,7 @@ EXTCONST char *PL_magic_vtable_names[magic_vtable_max] = {
     "taint",
     "utf8",
     "uvar",
-    "vec",
-    "vstring"
+    "vec"
 };
 #else
 EXTCONST char *PL_magic_vtable_names[magic_vtable_max];
@@ -182,8 +180,7 @@ EXT_MGVTBL PL_magic_vtables[magic_vtable_max] = {
   { Perl_magic_gettaint, Perl_magic_settaint, 0, 0, 0, 0, 0, 0 },
   { 0, Perl_magic_setutf8, 0, 0, 0, 0, 0, 0 },
   { Perl_magic_getuvar, Perl_magic_setuvar, 0, 0, 0, 0, 0, 0 },
-  { Perl_magic_getvec, Perl_magic_setvec, 0, 0, 0, 0, 0, 0 },
-  { 0, Perl_magic_setvstring, 0, 0, 0, 0, 0, 0 }
+  { Perl_magic_getvec, Perl_magic_setvec, 0, 0, 0, 0, 0, 0 }
 };
 #else
 EXT_MGVTBL PL_magic_vtables[magic_vtable_max];
@@ -223,6 +220,5 @@ EXT_MGVTBL PL_magic_vtables[magic_vtable_max];
 #define PL_vtbl_utf8 PL_magic_vtables[want_vtbl_utf8]
 #define PL_vtbl_uvar PL_magic_vtables[want_vtbl_uvar]
 #define PL_vtbl_vec PL_magic_vtables[want_vtbl_vec]
-#define PL_vtbl_vstring PL_magic_vtables[want_vtbl_vstring]
 
 /* ex: set ro: */