test: add common.dns.errorLookupMock

PR-URL: https://github.com/nodejs/node/pull/17296
Refs: https://github.com/nodejs/help/issues/687
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common/README.md

@@ -419,7 +419,26 @@ called before the callback is invoked.
 
 ## DNS Module
 
-The `DNS` module provides a naïve DNS parser/serializer.
+The `DNS` module provides utilities related to the `dns` built-in module.
+
+### errorLookupMock(code, syscall)
+
+* `code` [<String>] Defaults to `dns.mockedErrorCode`.
+* `syscall` [<String>] Defaults to `dns.mockedSysCall`.
+* return [<Function>]
+
+
+A mock for the `lookup` option of `net.connect()` that would result in an error
+with the `code` and the `syscall` specified. Returns a function that has the
+same signature as `dns.lookup()`.
+
+### mockedErrorCode
+
+The default `code` of errors generated by `errorLookupMock`.
+
+### mockedSysCall
+
+The default `syscall` of errors generated by `errorLookupMock`.
 
 ### readDomainFromPacket(buffer, offset)
 

test/common/dns.js

@@ -1,8 +1,6 @@
 /* eslint-disable required-modules */
 'use strict';
 
-// Naïve DNS parser/serializer.
-
 const assert = require('assert');
 const os = require('os');
 
@@ -22,6 +20,8 @@ const classes = {
   IN: 1
 };
 
+// Naïve DNS parser/serializer.
+
 function readDomainFromPacket(buffer, offset) {
   assert.ok(offset < buffer.length);
   const length = buffer[offset];
@@ -287,6 +287,26 @@ function writeDNSPacket(parsed) {
   }));
 }
 
+const mockedErrorCode = 'ENOTFOUND';
+const mockedSysCall = 'getaddrinfo';
+
+function errorLookupMock(code = mockedErrorCode, syscall = mockedSysCall) {
+  return function lookupWithError(host, dnsopts, cb) {
+    const err = new Error(`${syscall} ${code} ${host}`);
+    err.code = code;
+    err.errno = code;
+    err.syscall = syscall;
+    err.hostname = host;
+    cb(err);
+  };
+}
+
 module.exports = {
-  types, classes, writeDNSPacket, parseDNSPacket
+  types,
+  classes,
+  writeDNSPacket,
+  parseDNSPacket,
+  errorLookupMock,
+  mockedErrorCode,
+  mockedSysCall
 };