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

2 files changed, 124 insertions, 1 deletions

diff --git a/tests/libtest/Makefile.inc b/tests/libtest/Makefile.inc
index 4c41fe7a1..497c4832a 100644
--- a/tests/libtest/Makefile.inc
+++ b/tests/libtest/Makefile.inc
@@ -31,7 +31,7 @@ noinst_PROGRAMS = chkhostname libauthretry libntlmconnect                \
  lib1540 \
  lib1550 lib1551 lib1552 lib1553 lib1554 lib1555 lib1556 lib1557 \
  lib1560 \
- lib1591 \
+ lib1591 lib1592 \
  lib1900 \
  lib2033
 
@@ -523,6 +523,10 @@ lib1591_SOURCES = lib1591.c $(SUPPORTFILES) $(TESTUTIL) $(WARNLESS)
 lib1591_LDADD = $(TESTUTIL_LIBS)
 lib1591_CPPFLAGS = $(AM_CPPFLAGS) -DLIB1591
 
+lib1592_SOURCES = lib1592.c $(SUPPORTFILES) $(TESTUTIL) $(WARNLESS)
+lib1592_LDADD = $(TESTUTIL_LIBS)
+lib1592_CPPFLAGS = $(AM_CPPFLAGS) -DLIB1592
+
 lib1900_SOURCES = lib1900.c $(SUPPORTFILES) $(TESTUTIL) $(WARNLESS)
 lib1900_LDADD = $(TESTUTIL_LIBS)
 lib1900_CPPFLAGS = $(AM_CPPFLAGS)
diff --git a/tests/libtest/lib1592.c b/tests/libtest/lib1592.c
new file mode 100644
index 000000000..5e6bf04eb
--- /dev/null
+++ b/tests/libtest/lib1592.c
@@ -0,0 +1,119 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * 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
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+/*
+ * See https://github.com/curl/curl/issues/3371
+ *
+ * This test case checks whether curl_multi_remove_handle() cancels
+ * asynchronous DNS resolvers without blocking where possible.  Obviously, it
+ * only tests whichever resolver cURL is actually built with.
+ */
+
+/* We're willing to wait a very generous two seconds for the removal.  This is
+   as low as we can go while still easily supporting SIGALRM timing for the
+   non-threaded blocking resolver.  It doesn't matter that much because when
+   the test passes, we never wait this long. */
+#define TEST_HANG_TIMEOUT 2 * 1000
+
+#include "test.h"
+#include "testutil.h"
+
+#include <sys/stat.h>
+
+int test(char *URL)
+{
+  int stillRunning;
+  CURLM *multiHandle = NULL;
+  CURL *curl = NULL;
+  CURLMcode res = CURLM_OK;
+  int timeout;
+
+  global_init(CURL_GLOBAL_ALL);
+
+  multi_init(multiHandle);
+
+  easy_init(curl);
+
+  easy_setopt(curl, CURLOPT_VERBOSE, 1L);
+  easy_setopt(curl, CURLOPT_URL, URL);
+
+  /* Set a DNS server that hopefully will not respond when using c-ares. */
+  if(curl_easy_setopt(curl, CURLOPT_DNS_SERVERS, "0.0.0.0") == CURLE_OK)
+    /* Since we could set the DNS server, presume we are working with a
+       resolver that can be cancelled (i.e. c-ares).  Thus,
+       curl_multi_remove_handle() should not block even when the resolver
+       request is outstanding.  So, set a request timeout _longer_ than the
+       test hang timeout so we will fail if the handle removal call incorrectly
+       blocks. */
+    timeout = TEST_HANG_TIMEOUT * 2;
+  else {
+    /* If we can't set the DNS server, presume that we are configured to use a
+       resolver that can't be cancelled (i.e. the threaded resolver or the
+       non-threaded blocking resolver).  So, we just test that the
+       curl_multi_remove_handle() call does finish well within our test
+       timeout.
+
+       But, it is very unlikely that the resolver request will take any time at
+       all because we haven't been able to configure the resolver to use an
+       non-responsive DNS server.  At least we exercise the flow.
+       */
+    fprintf(stderr,
+            "CURLOPT_DNS_SERVERS not supported; "
+            "assuming curl_multi_remove_handle() will block\n");
+    timeout = TEST_HANG_TIMEOUT / 2;
+  }
+
+  /* Setting a timeout on the request should ensure that even if we have to
+     wait for the resolver during curl_multi_remove_handle(), it won't take
+     longer than this, because the resolver request inherits its timeout from
+     this. */
+  easy_setopt(curl, CURLOPT_TIMEOUT_MS, timeout);
+
+  multi_add_handle(multiHandle, curl);
+
+  /* This should move the handle from INIT => CONNECT => WAITRESOLVE. */
+  fprintf(stderr, "curl_multi_perform()...\n");
+  multi_perform(multiHandle, &stillRunning);
+  fprintf(stderr, "curl_multi_perform() succeeded\n");
+
+  /* Start measuring how long it takes to remove the handle. */
+  fprintf(stderr, "curl_multi_remove_handle()...\n");
+  start_test_timing();
+  res = curl_multi_remove_handle(multiHandle, curl);
+  if(res) {
+    fprintf(stderr, "curl_multi_remove_handle() failed, "
+            "with code %d\n", (int)res);
+    goto test_cleanup;
+  }
+  fprintf(stderr, "curl_multi_remove_handle() succeeded\n");
+
+  /* Fail the test if it took too long to remove.  This happens after the fact,
+     and says "it seems that it would have run forever", which isn't true, but
+     it's close enough, and simple to do. */
+  abort_on_test_timeout();
+
+test_cleanup:
+  curl_easy_cleanup(curl);
+  curl_multi_cleanup(multiHandle);
+  curl_global_cleanup();
+
+  return (int)res;
+}