protect CvGV weakref with backref

Each CV usually has a pointer, CvGV(cv), back to the GV that corresponds
to the CV's name (or to *foo::__ANON__ for anon CVs).  This pointer wasn't
reference counted, to avoid loops. This could leave it dangling if the GV
is deleted.

We fix this by:

For named subs, adding backref magic to the GV, so that when the GV is
freed, it can trigger processing the CV's CvGV field. This processing
consists of: if it looks like the freeing of the GV is about to trigger
freeing of the CV too, set it to NULL; otherwise make it point to
*foo::__ANON__ (and set CvAONON(cv)).

For anon subs, make CvGV a strong reference, i.e. increment the refcnt of
*foo::__ANON__. This doesn't cause a loop, since in this case the
__ANON__ glob doesn't point to the CV. This also avoids dangling pointers
if someone does an explicit 'delete $foo::{__ANON__}'.

Note that there was already some partial protection for CvGV with
commit f1c32fec87699aee2eeb638f44135f21217d2127. This worked by
anonymising any corresponding CV when freeing a stash or stash entry.
This had two drawbacks. First it didn't fix CVs that were anonmous or that
weren't currently pointed to by the GV (e.g. after local *foo), and
second, it caused *all* CVs to get anonymised during cleanup, even the
ones that would have been deleted shortly afterwards anyway. This commit
effectively removes that former commit, while reusing a bit of the
actual anonymising code.

1 files changed, 4 insertions, 1 deletions

diff --git a/cv.h b/cv.h
index 64eb02a9dd..fe96aa3e3a 100644
--- a/cv.h
+++ b/cv.h
@@ -70,7 +70,10 @@ Returns the stash of the CV.
 #define CVf_WEAKOUTSIDE	0x0010  /* CvOUTSIDE isn't ref counted */
 #define CVf_CLONE	0x0020	/* anon CV uses external lexicals */
 #define CVf_CLONED	0x0040	/* a clone of one of those */
-#define CVf_ANON	0x0080	/* CvGV() can't be trusted */
+#define CVf_ANON	0x0080	/* implies: CV is not pointed to by a GV,
+				            CvGV is refcounted, and
+					    points to an __ANON__ GV;
+				   at compile time only, also implies sub {} */
 #define CVf_UNIQUE	0x0100	/* sub is only called once (eg PL_main_cv,
 				 * require, eval). */
 #define CVf_NODEBUG	0x0200	/* no DB::sub indirection for this CV