dns: add channel.cancel()

This can be used to implement custom timeouts.

Fixes: #7231
PR-URL: #14518
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api/dns.md

@@ -95,6 +95,14 @@ The following methods from the `dns` module are available:
 * [`resolver.resolveTxt()`][`dns.resolveTxt()`]
 * [`resolver.reverse()`][`dns.reverse()`]
 
+### resolver.cancel()
+<!-- YAML
+added: REPLACEME
+-->
+
+Cancel all outstanding DNS queries made by this resolver. The corresponding
+callbacks will be called with an error with code `ECANCELLED`.
+
 ## dns.getServers()
 <!-- YAML
 added: v0.11.3

lib/dns.js

@@ -247,6 +247,10 @@ class Resolver {
   constructor() {
     this._handle = new ChannelWrap();
   }
+
+  cancel() {
+    this._handle.cancel();
+  }
 }
 
 function resolver(bindingName) {

src/cares_wrap.cc

@@ -2131,6 +2131,13 @@ void SetServers(const FunctionCallbackInfo<Value>& args) {
   args.GetReturnValue().Set(err);
 }
 
+void Cancel(const FunctionCallbackInfo<Value>& args) {
+  ChannelWrap* channel;
+  ASSIGN_OR_RETURN_UNWRAP(&channel, args.Holder());
+
+  ares_cancel(channel->cares_channel());
+}
+
 
 void StrError(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
@@ -2215,6 +2222,7 @@ void Initialize(Local<Object> target,
 
   env->SetProtoMethod(channel_wrap, "getServers", GetServers);
   env->SetProtoMethod(channel_wrap, "setServers", SetServers);
+  env->SetProtoMethod(channel_wrap, "cancel", Cancel);
 
   channel_wrap->SetClassName(
       FIXED_ONE_BYTE_STRING(env->isolate(), "ChannelWrap"));

test/parallel/test-dns-channel-cancel.js

@@ -0,0 +1,29 @@
+'use strict';
+const common = require('../common');
+const dnstools = require('../common/dns');
+const { Resolver } = require('dns');
+const assert = require('assert');
+const dgram = require('dgram');
+
+const server = dgram.createSocket('udp4');
+const resolver = new Resolver();
+
+server.bind(0, common.mustCall(() => {
+  resolver.setServers([`127.0.0.1:${server.address().port}`]);
+  resolver.resolve4('example.org', common.mustCall((err, res) => {
+    assert.strictEqual(err.code, 'ECANCELLED');
+    assert.strictEqual(err.errno, 'ECANCELLED');
+    assert.strictEqual(err.syscall, 'queryA');
+    assert.strictEqual(err.hostname, 'example.org');
+    server.close();
+  }));
+}));
+
+server.on('message', common.mustCall((msg, { address, port }) => {
+  const parsed = dnstools.parseDNSPacket(msg);
+  const domain = parsed.questions[0].domain;
+  assert.strictEqual(domain, 'example.org');
+
+  // Do not send a reply.
+  resolver.cancel();
+}));