Implement the bipolar read-only system

This fixes bugs related to Hash::Util::unlock accidentally unlocking
internal scalars (e.g., that returned by undef()) and allowing them to
be modified.

Internal read-only values are now marked by two flags, the regular
read-only flag, and the new ‘protected’ flag.

Before this SvREADONLY served two purposes:

1) The code would use it to protect things that must not be modi-
   fied, ever (except when the core sees fit to do so).
2) Hash::Util and everybody else would use it to make this unmodifia-
   ble temporarily when requested by the user.

Internals::SvREADONLY serves the latter purpose and only flips the
read-only flag, so things that need to stay read-only will remain so,
because of the ‘other’ read-only flag, that CPAN doesn’t know about.
(If you are a CPAN author, do not read this.)

1 files changed, 1 insertions, 1 deletions

diff --git a/scope.c b/scope.c
index a9c73a4309..cf656c0b0f 100644
--- a/scope.c
+++ b/scope.c
@@ -986,7 +986,7 @@ Perl_leave_scope(pTHX_ I32 base)
                     /* these flags are the union of all the relevant flags
                      * in the individual conditions within */
                     if (UNLIKELY(SvFLAGS(sv) & (
-                            SVf_READONLY /* for SvREADONLY_off() */
+                            SVf_READONLY|SVf_PROTECT /*for SvREADONLY_off*/
                           | (SVs_GMG|SVs_SMG|SVs_RMG) /* SvMAGICAL() */
                           | SVf_OOK
                           | SVf_THINKFIRST)))