tag/TagPool: optimize _dup_item()

When a reference counter is at its limit, don't allocate a new
TagPoolSlot - that would result in many TagPoolSlot instances with
ref==1.  This in turn would make the linked list very very large,
which means quadratic runtime for many operations.

NEWS

@@ -4,6 +4,7 @@ ver 0.19.14 (not yet released)
   - opus: limit tag size to 64 kB
 * archive
   - iso9660: fix buffer overflow
+* fix quadratic runtime bug in the tag pool
 * fix build failures on non-glibc builds due to constexpr Mutex
 
 ver 0.19.13 (2016/02/23)

src/tag/TagPool.cxx

@@ -144,14 +144,10 @@ tag_pool_dup_item(TagItem *item)
 		return item;
 	} else {
 		/* the reference counter overflows above MAX_REF;
-		   duplicate the item, and start with 1 */
+		   obtain a reference to a different TagPoolSlot which
+		   isn't yet "full" */
 		size_t length = strlen(item->value);
-		auto slot_p = tag_value_slot_p(item->type,
-					       item->value, length);
-		slot = TagPoolSlot::Create(*slot_p, item->type,
-					   item->value, strlen(item->value));
-		*slot_p = slot;
-		return &slot->item;
+		return tag_pool_get_item(item->type, item->value, length);
 	}
 }