Previously, processing a large batch of timeouts was O(n^2) in the number of

outstanding queries, and processing a DNS response packet was O(n) in the
number of outstanding queries. To speed things up in Google, we added a few circular,
doubly-linked lists of queries that are hash-bucketed based on
the attributes we care about, so most important operations are now O(1).

It might be that the number of buckets are higher than most people would need,
but on a quick calculation it should only be 100kB or so even on a 64-bit
system, so I've let it stay as-is.

ares_destroy.c

@@ -16,6 +16,7 @@
  */
 
 #include "setup.h"
+#include <assert.h>
 #include <stdlib.h>
 #include "ares.h"
 #include "ares_private.h"
@@ -37,13 +38,42 @@ void ares_destroy(ares_channel channel)
 {
   int i;
   struct query *query;
+  struct list_node* list_head;
+  struct list_node* list_node;
+
+  list_head = &(channel->all_queries);
+  for (list_node = list_head->next; list_node != list_head; )
+    {
+      query = list_node->data;
+      list_node = list_node->next;  /* since we're deleting the query */
+      query->callback(query->arg, ARES_EDESTRUCTION, 0, NULL, 0);
+      ares__free_query(query);
+    }
+#ifndef NDEBUG
+  /* Freeing the query should remove it from all the lists in which it sits,
+   * so all query lists should be empty now.
+   */
+  assert(ares__is_list_empty(&(channel->all_queries)));
+  for (i = 0; i < ARES_QID_TABLE_SIZE; i++)
+    {
+      assert(ares__is_list_empty(&(channel->queries_by_qid[i])));
+    }
+  for (i = 0; i < ARES_TIMEOUT_TABLE_SIZE; i++)
+    {
+      assert(ares__is_list_empty(&(channel->queries_by_timeout[i])));
+    }
+#endif
 
   if (!channel)
     return;
 
   if (channel->servers) {
     for (i = 0; i < channel->nservers; i++)
-      ares__close_sockets(channel, &channel->servers[i]);
+      {
+        struct server_state *server = &channel->servers[i];
+        ares__close_sockets(channel, server);
+        assert(ares__is_list_empty(&(server->queries_to_server)));
+      }
     free(channel->servers);
   }
 
@@ -59,16 +89,5 @@ void ares_destroy(ares_channel channel)
   if (channel->lookups)
     free(channel->lookups);
 
-  while (channel->queries) {
-    query = channel->queries;
-    channel->queries = query->next;
-    query->callback(query->arg, ARES_EDESTRUCTION, 0, NULL, 0);
-    if (query->tcpbuf)
-      free(query->tcpbuf);
-    if (query->server_info)
-      free(query->server_info);
-    free(query);
-  }
-
   free(channel);
 }