hostip: cache negative name resolves

Hold them for half the normal lifetime. Helps when told to transfer N
URLs in quick succession that all use the same non-resolving hostname.

Done by storing a DNS entry with a NULL pointer for 'addr'.

Previously an attempt was made in #12406 by Björn Stenberg that was
ultimately never merged.

Closes #18157

lib/asyn-thrdd.c

@@ -368,6 +368,14 @@ static CURLcode async_rr_start(struct Curl_easy *data)
     thrdd->rr.channel = NULL;
     return CURLE_FAILED_INIT;
   }
+#ifdef CURLDEBUG
+  if(getenv("CURL_DNS_SERVER")) {
+    const char *servers = getenv("CURL_DNS_SERVER");
+    status = ares_set_servers_ports_csv(thrdd->rr.channel, servers);
+    if(status)
+      return CURLE_FAILED_INIT;
+  }
+#endif
 
   memset(&thrdd->rr.hinfo, 0, sizeof(thrdd->rr.hinfo));
   thrdd->rr.hinfo.port = -1;
@@ -375,6 +383,7 @@ static CURLcode async_rr_start(struct Curl_easy *data)
                     data->conn->host.name, ARES_CLASS_IN,
                     ARES_REC_TYPE_HTTPS,
                     async_thrdd_rr_done, data, NULL);
+  CURL_TRC_DNS(data, "Issued HTTPS-RR request for %s", data->conn->host.name);
   return CURLE_OK;
 }
 #endif

lib/hostip.c

@@ -202,6 +202,8 @@ dnscache_entry_is_stale(void *datap, void *hc)
   if(dns->timestamp.tv_sec || dns->timestamp.tv_usec) {
     /* get age in milliseconds */
     timediff_t age = curlx_timediff(prune->now, dns->timestamp);
+    if(!dns->addr)
+      age *= 2; /* negative entries age twice as fast */
     if(age >= prune->max_age_ms)
       return TRUE;
     if(age > prune->oldest_ms)
@@ -798,6 +800,28 @@ static bool can_resolve_ip_version(struct Curl_easy *data, int ip_version)
   return TRUE;
 }
 
+static CURLcode store_negative_resolve(struct Curl_easy *data,
+                                       const char *host,
+                                       int port)
+{
+  struct Curl_dnscache *dnscache = dnscache_get(data);
+  struct Curl_dns_entry *dns;
+  DEBUGASSERT(dnscache);
+  if(!dnscache)
+    return CURLE_FAILED_INIT;
+
+  /* put this new host in the cache */
+  dns = dnscache_add_addr(data, dnscache, NULL, host, 0, port, FALSE);
+  if(dns) {
+    /* release the returned reference; the cache itself will keep the
+     * entry alive: */
+    dns->refcount--;
+    infof(data, "Store negative name resolve for %s:%d", host, port);
+    return CURLE_OK;
+  }
+  return CURLE_OUT_OF_MEMORY;
+}
+
 /*
  * Curl_resolv() is the main name resolve function within libcurl. It resolves
  * a name and returns a pointer to the entry in the 'entry' argument (if one
@@ -917,6 +941,11 @@ out:
    * or `respwait` is set for an async operation.
    * Everything else is a failure to resolve. */
   if(dns) {
+    if(!dns->addr) {
+      infof(data, "Negative DNS entry");
+      dns->refcount--;
+      return CURLE_COULDNT_RESOLVE_HOST;
+    }
     *entry = dns;
     return CURLE_OK;
   }
@@ -942,6 +971,7 @@ error:
     Curl_resolv_unlink(data, &dns);
   *entry = NULL;
   Curl_async_shutdown(data);
+  store_negative_resolve(data, hostname, port);
   return CURLE_COULDNT_RESOLVE_HOST;
 }
 
@@ -1523,6 +1553,9 @@ CURLcode Curl_resolv_check(struct Curl_easy *data,
   result = Curl_async_is_resolved(data, dns);
   if(*dns)
     show_resolve_info(data, *dns);
+  if(result)
+    store_negative_resolve(data, data->state.async.hostname,
+                           data->state.async.port);
   return result;
 }
 #endif