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

8 files changed, 225 insertions, 21 deletions

diff --git a/lib/asyn-ares.c b/lib/asyn-ares.c
index 6a49566c8..04a25b321 100644
--- a/lib/asyn-ares.c
+++ b/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
@@ -199,6 +199,17 @@ void Curl_resolver_cancel(struct connectdata *conn)
 }
 
 /*
+ * 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.
  */
 static void destroy_async_data(struct Curl_async *async)
@@ -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)
diff --git a/lib/asyn-thread.c b/lib/asyn-thread.c
index 74208d7ec..a9679d062 100644
--- a/lib/asyn-thread.c
+++ b/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
@@ -462,13 +462,33 @@ static CURLcode resolver_error(struct connectdata *conn)
 }
 
 /*
+ * 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)) {
diff --git a/lib/asyn.h b/lib/asyn.h
index 43625bc3b..ccd4b1f7e 100644
--- a/lib/asyn.h
+++ b/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
diff --git a/lib/multi.c b/lib/multi.c
index 54d954e65..249280f2b 100644
--- a/lib/multi.c
+++ b/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 */
diff --git a/tests/data/Makefile.inc b/tests/data/Makefile.inc
index 59635be04..23ee19b36 100644
--- a/tests/data/Makefile.inc
+++ b/tests/data/Makefile.inc
@@ -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
@@ -179,8 +179,8 @@ test1550 test1551 test1552 test1553 test1554 test1555 test1556 test1557 \
 \
 test1560 test1561 \
 \
-test1590 \
-test1591 \
+test1590 test1591 test1592 \
+\
 test1600 test1601 test1602 test1603 test1604 test1605 test1606 test1607 \
 test1608 test1609 test1620 \
 \
diff --git a/tests/data/test1592 b/tests/data/test1592
new file mode 100644
index 000000000..d1346e1e3
--- /dev/null
+++ b/tests/data/test1592
@@ -0,0 +1,37 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+multi
+resolve
+speedcheck
+</keywords>
+</info>
+
+# Client-side
+<client>
+<server>
+none
+</server>
+<tool>
+lib1592
+</tool>
+ <name>
+HTTP request, remove handle while resolving, don't block
+ </name>
+
+ <command>
+http://a-site-never-accessed.example.org/1592
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<valgrind>
+disable
+</valgrind>
+<errorcode>
+0
+</errorcode>
+</verify>
+</testcase>
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;
+}