varnishevent: encourage re-use of space from the free list

bin/varnishevent/data.c

@@ -39,6 +39,17 @@
 #include "vqueue.h"
 #include "vsb.h"
 
+/*
+ * Place the contents of head1 before head2, then empty head2
+ */
+#define VSTAILQ_PREPEND(head1, head2) do {                      \
+        if (!VSTAILQ_EMPTY((head2))) {                          \
+                *(head2)->vstqh_last = (head1)->vstqh_first;    \
+                (head1)->vstqh_first = (head2)->vstqh_first;    \
+                VSTAILQ_INIT((head2));                          \
+        }                                                       \
+} while (0)
+
 static const char *statename[3] = { "EMPTY", "OPEN", "DONE" };
 
 static pthread_mutex_t freelist_lock;
@@ -176,12 +187,14 @@ DATA_Take_Freelist(struct freehead_s *dst)
 /*
  * return to global freelist
  * returned must be locked by caller, if required
+ * Space is returned to the front of the free list, so that it is likely
+ * to be re-used
  */
 void
 DATA_Return_Freelist(struct freehead_s *returned, unsigned nreturned)
 {
     AZ(pthread_mutex_lock(&freelist_lock));
-    VSTAILQ_CONCAT(&freehead, returned);
+    VSTAILQ_PREPEND(&freehead, returned);
     global_nfree += nreturned;
     AZ(pthread_mutex_unlock(&freelist_lock));
 }