metadata: add comment about metadata functions use and threads safety

Signed-off-by: Florian Forster <octo@collectd.org>

diff --git a/src/meta_data.c b/src/meta_data.c
index 0fc58ff5c021e784a3bcc0feb902ab634db040f6..d2207843dd282e8f2297b95423f6f96fa748baf8 100644 (file)
--- a/src/meta_data.c
+++ b/src/meta_data.c
@@ -208,6 +208,17 @@ static meta_entry_t *md_entry_lookup (meta_data_t *md, /* {{{ */
   return (e);
 } /* }}} meta_entry_t *md_entry_lookup */
 
+/*
+ * Each value_list_t*, as it is going through the system, is handled by exactly
+ * one thread. Plugins which pass a value_list_t* to another thread, e.g. the
+ * rrdtool plugin, must create a copy first. The meta data within a
+ * value_list_t* is not thread safe and doesn't need to be.
+ *
+ * The meta data associated with cache entries are a different story. There, we
+ * need to ensure exclusive locking to prevent leaks and other funky business.
+ * This is ensured by the uc_meta_data_get_*() functions.
+ */
+
 /*
  * Public functions
  */