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, 4 insertions, 3 deletions

diff --git a/lib/asyn-thread.c b/lib/asyn-thread.c
index 2a59294e5..4c8f88b14 100644
--- a/lib/asyn-thread.c
+++ b/lib/asyn-thread.c
@@ -108,8 +108,9 @@ void Curl_resolver_global_cleanup(void)
  * URL-state specific environment ('resolver' member of the UrlState
  * structure).
  */
-CURLcode Curl_resolver_init(void **resolver)
+CURLcode Curl_resolver_init(struct Curl_easy *easy, void **resolver)
 {
+  (void)easy;
   *resolver = calloc(1, sizeof(struct resdata));
   if(!*resolver)
     return CURLE_OUT_OF_MEMORY;
@@ -132,10 +133,10 @@ void Curl_resolver_cleanup(void *resolver)
  * Called from curl_easy_duphandle() to duplicate resolver URL state-specific
  * environment ('resolver' member of the UrlState structure).
  */
-int Curl_resolver_duphandle(void **to, void *from)
+CURLcode Curl_resolver_duphandle(struct Curl_easy *easy, void **to, void *from)
 {
   (void)from;
-  return Curl_resolver_init(to);
+  return Curl_resolver_init(easy, to);
 }
 
 static void destroy_async_data(struct Curl_async *);