Cache a checkpoint when we iterate over busy objects, so we don't

have to walk the full list all the time.

This is a minimal fix for backporting to 4.1.x

Fixes:	#1798
Fixes:	#1788

bin/varnishd/cache/cache_obj.c

@@ -93,6 +93,8 @@ struct objiter {
 	struct storage			*st;
 	struct worker			*wrk;
 	ssize_t				len;
+	struct storage			*checkpoint;
+	ssize_t				checkpoint_len;
 };
 
 void *
@@ -126,6 +128,7 @@ ObjIter(struct objcore *oc, void *oix, void **p, ssize_t *l)
 	struct objiter *oi;
 	ssize_t ol;
 	ssize_t nl;
+	ssize_t sl;
 	const struct storeobj_methods *om = obj_getmethods(oc);
 
 	AN(oix);
@@ -167,7 +170,15 @@ ObjIter(struct objcore *oc, void *oix, void **p, ssize_t *l)
 		}
 		Lck_Lock(&oi->bo->mtx);
 		AZ(VTAILQ_EMPTY(&oi->obj->list));
-		VTAILQ_FOREACH(oi->st, &oi->obj->list, list) {
+		if (oi->checkpoint == NULL) {
+			oi->st = VTAILQ_FIRST(&oi->obj->list);
+			sl = 0;
+		} else {
+			oi->st = oi->checkpoint;
+			sl = oi->checkpoint_len;
+			ol -= oi->checkpoint_len;
+		}
+		while (oi->st != NULL) {
 			if (oi->st->len > ol) {
 				*p = oi->st->ptr + ol;
 				*l = oi->st->len - ol;
@@ -178,6 +189,12 @@ ObjIter(struct objcore *oc, void *oix, void **p, ssize_t *l)
 			assert(ol >= 0);
 			nl -= oi->st->len;
 			assert(nl > 0);
+			sl += oi->st->len;
+			oi->st = VTAILQ_NEXT(oi->st, list);
+			if (VTAILQ_NEXT(oi->st, list) != NULL) {
+				oi->checkpoint = oi->st;
+				oi->checkpoint_len = sl;
+			}
 		}
 		CHECK_OBJ_NOTNULL(oi->obj, OBJECT_MAGIC);
 		CHECK_OBJ_NOTNULL(oi->st, STORAGE_MAGIC);