Document why it's a bad plan to move the backreferences array from

the magic structure to the hv_aux structure during backreference
deletion. Add an optimisation for an empty backreference array.

p4raw-id: //depot/perl@26542

1 files changed, 4 insertions, 0 deletions

diff --git a/sv.c b/sv.c
index 3e3bb18fa8..5372215d4d 100644
--- a/sv.c
+++ b/sv.c
@@ -4409,6 +4409,10 @@ S_sv_del_backref(pTHX_ SV *tsv, SV *sv)
 
     if (SvTYPE(tsv) == SVt_PVHV && SvOOK(tsv)) {
 	av = *Perl_hv_backreferences_p(aTHX_ (HV*)tsv);
+	/* We mustn't attempt to "fix up" the hash here by moving the
+	   backreference array back to the hv_aux structure, as that is stored
+	   in the main HvARRAY(), and hfreentries assumes that no-one
+	   reallocates HvARRAY() while it is running.  */
     }
     if (!av) {
 	const MAGIC *const mg