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, 2 insertions, 2 deletions

diff --git a/universal.c b/universal.c
index 906f74c554..94169a6a6e 100644
--- a/universal.c
+++ b/universal.c
@@ -565,12 +565,12 @@ XS(XS_Internals_SvREADONLY)	/* This is dangerous stuff. */
 #ifdef PERL_OLD_COPY_ON_WRITE
 	    if (SvIsCOW(sv)) sv_force_normal(sv);
 #endif
-	    SvREADONLY_on(sv);
+	    SvFLAGS(sv) |= SVf_READONLY;
 	    XSRETURN_YES;
 	}
 	else {
 	    /* I hope you really know what you are doing. */
-	    SvREADONLY_off(sv);
+	    SvFLAGS(sv) &=~ SVf_READONLY;
 	    XSRETURN_NO;
 	}
     }