dns: handle implicit bind DNS errors

When send() triggers an implicit bind, the send operation is
added to an internal queue. If a DNS error occurs during the bind,
there is currently no mechanism for clearing the queue other than
sending more data. If DNS errors keep occurring, the queue will
continue to grow with no upper bound. This commit reports errors
with implicit binds, and clears the queue. This should be fine,
given the nature of UDP.

Refs: https://github.com/nodejs/node-v0.x-archive/pull/8705
Refs: https://github.com/nodejs/node/pull/10902
PR-URL: https://github.com/nodejs/node/pull/11036
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/dgram.js

@@ -282,13 +282,27 @@ function enqueue(self, toEnqueue) {
   // event handler that flushes the send queue after binding is done.
   if (!self._queue) {
     self._queue = [];
-    self.once('listening', clearQueue);
+    self.once('error', onListenError);
+    self.once('listening', onListenSuccess);
   }
   self._queue.push(toEnqueue);
   return;
 }
 
 
+function onListenSuccess() {
+  this.removeListener('error', onListenError);
+  clearQueue.call(this);
+}
+
+
+function onListenError(err) {
+  this.removeListener('listening', onListenSuccess);
+  this._queue = undefined;
+  this.emit('error', new Error('Unable to send data'));
+}
+
+
 function clearQueue() {
   const queue = this._queue;
   this._queue = undefined;

test/parallel/test-dgram-implicit-bind-failure.js

@@ -0,0 +1,55 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const dns = require('dns');
+
+// Monkey patch dns.lookup() so that it always fails.
+dns.lookup = function(address, family, callback) {
+  process.nextTick(() => { callback(new Error('fake DNS')); });
+};
+
+const socket = dgram.createSocket('udp4');
+let dnsFailures = 0;
+let sendFailures = 0;
+
+process.on('exit', () => {
+  assert.strictEqual(dnsFailures, 3);
+  assert.strictEqual(sendFailures, 3);
+});
+
+socket.on('error', (err) => {
+  if (/^Error: fake DNS$/.test(err)) {
+    // The DNS lookup should fail since it is monkey patched. At that point in
+    // time, the send queue should be populated with the send() operation. There
+    // should also be two listeners - this function and the dgram internal one
+    // time error handler.
+    dnsFailures++;
+    assert(Array.isArray(socket._queue));
+    assert.strictEqual(socket._queue.length, 1);
+    assert.strictEqual(socket.listenerCount('error'), 2);
+    return;
+  }
+
+  if (/^Error: Unable to send data$/.test(err)) {
+    // On error, the queue should be destroyed and this function should be
+    // the only listener.
+    sendFailures++;
+    assert.strictEqual(socket._queue, undefined);
+    assert.strictEqual(socket.listenerCount('error'), 1);
+    return;
+  }
+
+  common.fail(`Unexpected error: ${err}`);
+});
+
+// Initiate a few send() operations, which will fail.
+socket.send('foobar', common.PORT, 'localhost');
+
+process.nextTick(() => {
+  socket.send('foobar', common.PORT, 'localhost');
+});
+
+setImmediate(() => {
+  socket.send('foobar', common.PORT, 'localhost');
+});