aggregation plugin: Handle a start-up condition gracefully.

The resulting error message would confuse and annoy users.

diff --git a/src/aggregation.c b/src/aggregation.c
index 985fa7c..a924223 100644 (file)
--- a/src/aggregation.c
+++ b/src/aggregation.c
@@ -606,6 +606,18 @@ static int agg_read (void) /* {{{ */
 
   pthread_mutex_lock (&agg_instance_list_lock);
 
+  /* agg_instance_list_head only holds data, after the "write" callback has
+   * been called with a matching value list at least once. So on startup,
+   * there's a race between the aggregations read() and write() callback. If
+   * the read() callback is called first, agg_instance_list_head is NULL and
+   * "success" may be zero. This is expected and should not result in an error.
+   * Therefore we need to handle this case separately. */
+  if (agg_instance_list_head == NULL)
+  {
+    pthread_mutex_unlock (&agg_instance_list_lock);
+    return (0);
+  }
+
   for (this = agg_instance_list_head; this != NULL; this = this->next)
   {
     int status;