DNSResolver cancellation continued (followup to #29581) (#29631)

src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc

@@ -500,6 +500,9 @@ class AresDNSResolver : public DNSResolver {
     MutexLock lock(&mu_);
     if (!open_requests_.contains(handle)) {
       // Unknown request, possibly completed already, or an invalid handle.
+      GRPC_CARES_TRACE_LOG(
+          "AresDNSResolver:%p attempt to cancel unknown TaskHandle:%s", this,
+          HandleToString(handle).c_str());
       return false;
     }
     auto* request = reinterpret_cast<AresRequest*>(handle.keys[0]);

src/core/lib/http/httpcli.cc

@@ -219,6 +219,12 @@ void HttpRequest::Orphan() {
           "cancelled during DNS resolution"));
       Unref();
     } else if (connecting_) {
+      // gRPC's TCP connection establishment API doesn't currently have
+      // a mechanism for cancellation. So invoke the user callback now. The TCP
+      // connection will eventually complete (at least within its deadline), and
+      // we'll simply unref ourselves at that point.
+      // TODO(apolcyn): fix this to cancel the TCP connection attempt when
+      // an API to do so exists.
       Finish(GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
           "HTTP request cancelled during TCP connection establishment",
           &overall_error_, 1));

test/core/iomgr/resolve_address_test.cc

@@ -370,6 +370,17 @@ TEST_F(ResolveAddressTest, ImmediateCancel) {
   PollPollsetUntilRequestDone();
 }
 
+// Attempt to cancel a request after it has completed.
+TEST_F(ResolveAddressTest, CancelDoesNotSucceed) {
+  grpc_core::ExecCtx exec_ctx;
+  auto request_handle = grpc_core::GetDNSResolver()->ResolveName(
+      "localhost:1", "1", pollset_set(),
+      absl::bind_front(&ResolveAddressTest::MustSucceed, this));
+  grpc_core::ExecCtx::Get()->Flush();
+  PollPollsetUntilRequestDone();
+  ASSERT_FALSE(grpc_core::GetDNSResolver()->Cancel(request_handle));
+}
+
 namespace {
 
 int g_fake_non_responsive_dns_server_port;