curl_multi_remove_handle() don't block terminating c-ares requests

Added Curl_resolver_kill() for all three resolver modes, which only
blocks when necessary, along with test 1592 to confirm
curl_multi_remove_handle() doesn't block unless it must.

Closes #3428
Fixes #3371

lib/asyn-ares.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -198,6 +198,17 @@ void Curl_resolver_cancel(struct connectdata *conn)
   destroy_async_data(&conn->async);
 }
 
+/*
+ * We're equivalent to Curl_resolver_cancel() for the c-ares resolver.  We
+ * never block.
+ */
+void Curl_resolver_kill(struct connectdata *conn)
+{
+  /* We don't need to check the resolver state because we can be called safely
+     at any time and we always do the same thing. */
+  Curl_resolver_cancel(conn);
+}
+
 /*
  * destroy_async_data() cleans up async resolver data.
  */
@@ -361,13 +372,13 @@ CURLcode Curl_resolver_is_resolved(struct connectdata *conn,
 /*
  * Curl_resolver_wait_resolv()
  *
- * waits for a resolve to finish. This function should be avoided since using
+ * Waits for a resolve to finish. This function should be avoided since using
  * this risk getting the multi interface to "hang".
  *
  * If 'entry' is non-NULL, make it point to the resolved dns entry
  *
- * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved, and
- * CURLE_OPERATION_TIMEDOUT if a time-out occurred.
+ * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved,
+ * CURLE_OPERATION_TIMEDOUT if a time-out occurred, or other errors.
  */
 CURLcode Curl_resolver_wait_resolv(struct connectdata *conn,
                                    struct Curl_dns_entry **entry)

lib/asyn-thread.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -461,14 +461,34 @@ static CURLcode resolver_error(struct connectdata *conn)
   return result;
 }
 
+/*
+ * Until we gain a way to signal the resolver threads to stop early, we must
+ * simply wait for them and ignore their results.
+ */
+void Curl_resolver_kill(struct connectdata *conn)
+{
+  struct thread_data *td = (struct thread_data*) conn->async.os_specific;
+
+  /* If we're still resolving, we must wait for the threads to fully clean up,
+     unfortunately.  Otherwise, we can simply cancel to clean up any resolver
+     data. */
+  if(td && td->thread_hnd != curl_thread_t_null)
+    (void)Curl_resolver_wait_resolv(conn, NULL);
+  else
+    Curl_resolver_cancel(conn);
+}
+
 /*
  * Curl_resolver_wait_resolv()
  *
- * waits for a resolve to finish. This function should be avoided since using
+ * Waits for a resolve to finish. This function should be avoided since using
  * this risk getting the multi interface to "hang".
  *
  * If 'entry' is non-NULL, make it point to the resolved dns entry
  *
+ * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved,
+ * CURLE_OPERATION_TIMEDOUT if a time-out occurred, or other errors.
+ *
  * This is the version for resolves-in-a-thread.
  */
 CURLcode Curl_resolver_wait_resolv(struct connectdata *conn,
@@ -478,6 +498,7 @@ CURLcode Curl_resolver_wait_resolv(struct connectdata *conn,
   CURLcode result = CURLE_OK;
 
   DEBUGASSERT(conn && td);
+  DEBUGASSERT(td->thread_hnd != curl_thread_t_null);
 
   /* wait for the thread to resolve the name */
   if(Curl_thread_join(&td->thread_hnd)) {

lib/asyn.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -87,10 +87,25 @@ CURLcode Curl_resolver_duphandle(struct Curl_easy *easy, void **to,
  *
  * It is called from inside other functions to cancel currently performing
  * resolver request. Should also free any temporary resources allocated to
- * perform a request.
+ * perform a request.  This never waits for resolver threads to complete.
+ *
+ * It is safe to call this when conn is in any state.
  */
 void Curl_resolver_cancel(struct connectdata *conn);
 
+/*
+ * Curl_resolver_kill().
+ *
+ * This acts like Curl_resolver_cancel() except it will block until any threads
+ * associated with the resolver are complete.  This never blocks for resolvers
+ * that do not use threads.  This is intended to be the "last chance" function
+ * that cleans up an in-progress resolver completely (before its owner is about
+ * to die).
+ *
+ * It is safe to call this when conn is in any state.
+ */
+void Curl_resolver_kill(struct connectdata *conn);
+
 /* Curl_resolver_getsock()
  *
  * This function is called from the multi_getsock() function.  'sock' is a
@@ -117,14 +132,13 @@ CURLcode Curl_resolver_is_resolved(struct connectdata *conn,
 /*
  * Curl_resolver_wait_resolv()
  *
- * waits for a resolve to finish. This function should be avoided since using
+ * Waits for a resolve to finish. This function should be avoided since using
  * this risk getting the multi interface to "hang".
  *
  * If 'entry' is non-NULL, make it point to the resolved dns entry
  *
- * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved, and
- * CURLE_OPERATION_TIMEDOUT if a time-out occurred.
-
+ * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved,
+ * CURLE_OPERATION_TIMEDOUT if a time-out occurred, or other errors.
  */
 CURLcode Curl_resolver_wait_resolv(struct connectdata *conn,
                                    struct Curl_dns_entry **dnsentry);
@@ -148,6 +162,7 @@ Curl_addrinfo *Curl_resolver_getaddrinfo(struct connectdata *conn,
 #ifndef CURLRES_ASYNCH
 /* convert these functions if an asynch resolver isn't used */
 #define Curl_resolver_cancel(x) Curl_nop_stmt
+#define Curl_resolver_kill(x) Curl_nop_stmt
 #define Curl_resolver_is_resolved(x,y) CURLE_COULDNT_RESOLVE_HOST
 #define Curl_resolver_wait_resolv(x,y) CURLE_COULDNT_RESOLVE_HOST
 #define Curl_resolver_getsock(x,y,z) 0

lib/multi.c

@@ -537,10 +537,8 @@ static CURLcode multi_done(struct connectdata **connp,
     /* Stop if multi_done() has already been called */
     return CURLE_OK;
 
-  if(data->mstate == CURLM_STATE_WAITRESOLVE) {
-    /* still waiting for the resolve to complete */
-    (void)Curl_resolver_wait_resolv(conn, NULL);
-  }
+  /* Stop the resolver and free its own resources (but not dns_entry yet). */
+  Curl_resolver_kill(conn);
 
   Curl_getoff_all_pipelines(data, conn);
 
@@ -587,7 +585,6 @@ static CURLcode multi_done(struct connectdata **connp,
   }
 
   data->state.done = TRUE; /* called just now! */
-  Curl_resolver_cancel(conn);
 
   if(conn->dns_entry) {
     Curl_resolv_unlock(data, conn->dns_entry); /* done with this */