Reset method caches when GPs are shared

The new MRO stuff in 5.10 made PL_sub_generation++ mostly unnecessary,
and almost all uses of it were replaced with mro_method_changed_in.

There is only one problem: That doesn’t actually work properly.  After
glob-to-glob assignment (*foo = *bar), both globs share the same GP
(glob pointer, or list of glob slots).  But there is no list of GVs
associated with any GP.  So there is no way, given a GV whose GP
is shared, to find out what other classes might need their method
caches reset.

sub B::b { "b" }
*A::b = *B::b;
@C::ISA = "A";
print C->b, "\n";  # should print "b"
eval 'sub B::b { "c" }';
print C->b, "\n";  # should print "c"
__END__

$ perl5.8.9 foo
b
c
$ perl5.10.0 foo
b
b

And it continues up to 5.16.x.

If a GP is shared, then those places where mro_method_changed_in is
called after the GP has been modified must do PL_sub_generation++
instead if the GP is shared, which can be detected through its refer-
ence count.

1 files changed, 7 insertions, 0 deletions

diff --git a/gv.h b/gv.h
index 8e09340053..1e17f35d46 100644
--- a/gv.h
+++ b/gv.h
@@ -264,6 +264,13 @@ Return the CV from the GV.
 #define gv_autoload4(stash, name, len, method) \
 	gv_autoload_pvn(stash, name, len, !!(method))
 #define newGVgen(pack)  newGVgen_flags(pack, 0)
+#define gv_method_changed(gv)		    \
+    (					     \
+    	assert_(isGV_with_GP(gv))	      \
+	GvREFCNT(gv) > 1		       \
+	    ? (void)++PL_sub_generation		\
+	    : mro_method_changed_in(GvSTASH(gv)) \
+    )
 
 #define gv_AVadd(gv) gv_add_by_type((gv), SVt_PVAV)
 #define gv_HVadd(gv) gv_add_by_type((gv), SVt_PVHV)