From: Nicholas Clark <nick@ccl4.org>
Date: Fri, 30 Dec 2005 16:53:39 +0000 (+0000)
Subject: Document why it's a bad plan to move the backreferences array from
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=5b285ea4a6ead2df7b4a0a11b0aff949429e0500;p=p5sagit%2Fp5-mst-13.2.git

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
---

diff --git a/hv.c b/hv.c
index e560ba7..27c4bec 100644
--- a/hv.c
+++ b/hv.c
@@ -1664,11 +1664,16 @@ S_hfreeentries(pTHX_ HV *hv)
     if (iter) {
 	if (iter->xhv_backreferences) {
 	    /* So donate them to regular backref magic to keep them safe. The
-	       sv_magic will increase the reference count of the AV, so we need
-	       to drop it first.  */
+	       sv_magic will increase the reference count of the AV, so we
+	       need to drop it first. */
 	    SvREFCNT_dec(iter->xhv_backreferences);
-	    sv_magic((SV*)hv, (SV*)iter->xhv_backreferences,
-		     PERL_MAGIC_backref, NULL, 0);
+	    if (AvFILLp(iter->xhv_backreferences) == -1) {
+		/* Turns out that the array is empty. Just free it.  */
+		SvREFCNT_dec(iter->xhv_backreferences);
+	    } else {
+		sv_magic((SV*)hv, (SV*)iter->xhv_backreferences,
+			 PERL_MAGIC_backref, NULL, 0);
+	    }
 	    iter->xhv_backreferences = 0;
 	}
     }
diff --git a/sv.c b/sv.c
index 3e3bb18..5372215 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