- Constantine Sapuntzakis posted bug #2891595

(http://curl.haxx.se/bug/view.cgi?id=2891595) which identified how an entry
  in the DNS cache would linger too long if the request that added it was in
  use that long. He also provided the patch that now makes libcurl capable of
  still doing a request while the DNS hash entry may get timed out.

lib/hostip.c

@@ -123,6 +123,10 @@
 static struct curl_hash hostname_cache;
 static int host_cache_initialized;
 
+#ifdef CURLDEBUG
+static int ndns = 0;
+#endif
+
 static void freednsentry(void *freethis);
 
 /*
@@ -232,14 +236,7 @@ hostcache_timestamp_remove(void *datap, void *hc)
     (struct hostcache_prune_data *) datap;
   struct Curl_dns_entry *c = (struct Curl_dns_entry *) hc;
 
-  if((data->now - c->timestamp < data->cache_timeout) ||
-      c->inuse) {
-    /* please don't remove */
-    return 0;
-  }
-
-  /* fine, remove */
-  return 1;
+  return (data->now - c->timestamp >= data->cache_timeout);
 }
 
 /*
@@ -339,7 +336,6 @@ Curl_cache_addr(struct SessionHandle *data,
   size_t entry_len;
   struct Curl_dns_entry *dns;
   struct Curl_dns_entry *dns2;
-  time_t now;
 
   /* Create an entry id, based upon the hostname and port */
   entry_id = create_hostcache_id(hostname, port);
@@ -357,23 +353,22 @@ Curl_cache_addr(struct SessionHandle *data,
 
   dns->inuse = 0;   /* init to not used */
   dns->addr = addr; /* this is the address(es) */
+  time(&dns->timestamp);
+  if(dns->timestamp == 0)
+    dns->timestamp = 1;   /* zero indicates that entry isn't in hash table */
 
-  /* Store the resolved data in our DNS cache. This function may return a
-     pointer to an existing struct already present in the hash, and it may
-     return the same argument we pass in. Make no assumptions. */
+  /* Store the resolved data in our DNS cache. */
   dns2 = Curl_hash_add(data->dns.hostcache, entry_id, entry_len+1,
                        (void *)dns);
   if(!dns2) {
-    /* Major badness, run away. */
     free(dns);
     free(entry_id);
     return NULL;
   }
-  time(&now);
-  dns = dns2;
 
-  dns->timestamp = now; /* used now */
+  dns = dns2;
   dns->inuse++;         /* mark entry as in-use */
+  DEBUGF(ndns++);
 
   /* free the allocated entry_id again */
   free(entry_id);
@@ -436,6 +431,7 @@ int Curl_resolv(struct connectdata *conn,
 
   if(dns) {
     dns->inuse++; /* we use it! */
+    DEBUGF(ndns++);
     rc = CURLRESOLV_RESOLVED;
   }
 
@@ -688,6 +684,13 @@ void Curl_resolv_unlock(struct SessionHandle *data, struct Curl_dns_entry *dns)
     Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);
 
   dns->inuse--;
+  DEBUGF(ndns--);
+  /* only free if nobody is using AND it is not in hostcache (timestamp ==
+     0) */
+  if (dns->inuse == 0 && dns->timestamp == 0) {
+    Curl_freeaddrinfo(dns->addr);
+    free(dns);
+  }
 
   if(data->share)
     Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
@@ -700,12 +703,21 @@ static void freednsentry(void *freethis)
 {
   struct Curl_dns_entry *p = (struct Curl_dns_entry *) freethis;
 
-  if(p) {
+  /* mark the entry as not in hostcache */
+  p->timestamp = 0;
+  if (p->inuse == 0) {
     Curl_freeaddrinfo(p->addr);
     free(p);
   }
 }
 
+#ifdef CURLDEBUG
+int curl_get_ndns(void)
+{
+  return ndns;
+}
+#endif
+
 /*
  * Curl_mk_dnscache() creates a new DNS cache and returns the handle for it.
  */