dns: validate default c-ares servers on first query

Author: mcollina

src/cares_wrap.h

@@ -186,7 +186,9 @@ class ChannelWrap final : public AsyncWrap {
  private:
   uv_timer_t* timer_handle_ = nullptr;
   ares_channel channel_ = nullptr;
-  bool query_last_ok_ = true;
+  // Start pessimistically so the first query validates whether c-ares fell
+  // back to a loopback resolver while using the system DNS configuration.
+  bool query_last_ok_ = false;
   bool is_servers_default_ = true;
   bool library_inited_ = false;
   int timeout_;

test/cctest/test_cares_wrap.cc

@@ -0,0 +1,50 @@
+#include "cares_wrap.h"
+
+#include "gtest/gtest.h"
+#include "node.h"
+#include "node_test_fixture.h"
+#include "v8.h"
+
+#include <winsock2.h>
+
+class CaresWrapTest : public EnvironmentTestFixture {};
+
+TEST_F(CaresWrapTest, FreshChannelChecksServersBeforeFirstQuery) {
+  v8::HandleScope handle_scope(isolate_);
+  const Argv argv;
+  Env env{handle_scope, argv};
+
+  v8::Local<v8::FunctionTemplate> templ =
+      node::BaseObject::MakeLazilyInitializedJSTemplate(*env);
+  v8::Local<v8::Object> object = templ->GetFunction(env.context())
+                                     .ToLocalChecked()
+                                     ->NewInstance(env.context())
+                                     .ToLocalChecked();
+
+  node::cares_wrap::ChannelWrap channel(*env, object, -1, 4, 0);
+
+  ares_addr_port_node server{};
+  server.family = AF_INET;
+  server.addr.addr4.s_addr = htonl(INADDR_LOOPBACK);
+  server.tcp_port = 12345;
+  server.udp_port = 12345;
+
+  ASSERT_EQ(ares_set_servers_ports(channel.cares_channel(), &server),
+            ARES_SUCCESS);
+  channel.set_is_servers_default(true);
+
+  channel.EnsureServers();
+
+  ares_addr_port_node* current = nullptr;
+  ares_get_servers_ports(channel.cares_channel(), &current);
+  ASSERT_NE(current, nullptr);
+
+  const bool still_using_test_loopback =
+      current->next == nullptr && current->family == AF_INET &&
+      current->addr.addr4.s_addr == htonl(INADDR_LOOPBACK) &&
+      current->tcp_port == 12345 && current->udp_port == 12345;
+
+  ares_free_data(current);
+
+  EXPECT_FALSE(still_using_test_loopback);
+}