ares: remove fd from multi fd set when ares is about to close the fd

When using c-ares for asyn dns, the dns socket fd was silently closed
by c-ares without curl being aware. curl would then 'realize' the fd
has been removed at next call of Curl_resolver_getsock, and only then
notify the CURLMOPT_SOCKETFUNCTION to remove fd from its poll set with
CURL_POLL_REMOVE. At this point the fd is already closed.

By using ares socket state callback (ARES_OPT_SOCK_STATE_CB), this
patch allows curl to be notified that the fd is not longer needed
for neither for write nor read. At this point by calling
Curl_multi_closed we are able to notify multi with CURL_POLL_REMOVE
before the fd is actually closed by ares.

In asyn-ares.c Curl_resolver_duphandle we can't use ares_dup anymore
since it does not allow passing a different sock_state_cb_data

Closes #3238

1 files changed, 26 insertions, 7 deletions

diff --git a/lib/asyn-ares.c b/lib/asyn-ares.c
index 5cfb2602d..6a49566c8 100644
--- a/lib/asyn-ares.c
+++ b/lib/asyn-ares.c
@@ -119,6 +119,17 @@ void Curl_resolver_global_cleanup(void)
 #endif
 }
 
+
+static void Curl_ares_sock_state_cb(void *data, ares_socket_t socket_fd,
+                                    int readable, int writable)
+{
+  struct Curl_easy *easy = data;
+  if(!readable && !writable) {
+    DEBUGASSERT(easy);
+    Curl_multi_closed(easy, socket_fd);
+  }
+}
+
 /*
  * Curl_resolver_init()
  *
@@ -126,9 +137,14 @@ void Curl_resolver_global_cleanup(void)
  * URL-state specific environment ('resolver' member of the UrlState
  * structure).  Fills the passed pointer by the initialized ares_channel.
  */
-CURLcode Curl_resolver_init(void **resolver)
+CURLcode Curl_resolver_init(struct Curl_easy *easy, void **resolver)
 {
-  int status = ares_init((ares_channel*)resolver);
+  int status;
+  struct ares_options options;
+  int optmask = ARES_OPT_SOCK_STATE_CB;
+  options.sock_state_cb = Curl_ares_sock_state_cb;
+  options.sock_state_cb_data = easy;
+  status = ares_init_options((ares_channel*)resolver, &options, optmask);
   if(status != ARES_SUCCESS) {
     if(status == ARES_ENOMEM)
       return CURLE_OUT_OF_MEMORY;
@@ -159,12 +175,15 @@ void Curl_resolver_cleanup(void *resolver)
  * environment ('resolver' member of the UrlState structure).  Duplicates the
  * 'from' ares channel and passes the resulting channel to the 'to' pointer.
  */
-int Curl_resolver_duphandle(void **to, void *from)
+CURLcode Curl_resolver_duphandle(struct Curl_easy *easy, void **to, void *from)
 {
-  /* Clone the ares channel for the new handle */
-  if(ARES_SUCCESS != ares_dup((ares_channel*)to, (ares_channel)from))
-    return CURLE_FAILED_INIT;
-  return CURLE_OK;
+  (void)from;
+  /*
+   * it would be better to call ares_dup instead, but right now
+   * it is not possible to set 'sock_state_cb_data' outside of
+   * ares_init_options
+   */
+  return Curl_resolver_init(easy, to);
 }
 
 static void destroy_async_data(struct Curl_async *async);