Do not deadlock on empty chunks

Refs: nodejs/node#29758
Refs: nodejs/node@f58e8eb103ff3087

Author: lpinca

index.js

@@ -20,8 +20,12 @@ class DuplexSocket extends Duplex {
   }
 
   _write(chunk, encoding, callback) {
-    this[kOtherSide][kCallback] = callback;
-    this[kOtherSide].push(chunk);
+    if (chunk.length) {
+      this[kOtherSide][kCallback] = callback;
+      this[kOtherSide].push(chunk);
+    } else {
+      callback();
+    }
   }
 
   _final(callback) {

test/index.js

@@ -4,7 +4,7 @@ const DuplexPair = require('../');
 const assert = require('assert');
 
 describe('DuplexPair', function() {
-  it('passed data through', function() {
+  it('passes data through', function() {
     const pair = new DuplexPair({ encoding: 'utf8' });
     pair.socket1.write('Hello');
     assert.strictEqual(pair.socket1.read(), null);
@@ -18,4 +18,24 @@ describe('DuplexPair', function() {
     pair.socket2.end();
     assert.strictEqual(pair.socket1.read(), null);
   });
+
+  it('does not deadlock when writing empty chunks', function(done) {
+    const pair = new DuplexPair({ encoding: 'utf8' });
+
+    pair.socket2.resume();
+    pair.socket2.on('end', function() {
+      pair.socket2.write('Hello');
+      pair.socket2.write('');
+      pair.socket2.end();
+    });
+
+    pair.socket1.on('data', function(chunk) {
+      assert.strictEqual(chunk, 'Hello');
+    });
+    pair.socket1.on('end', function() {
+      done();
+    });
+
+    pair.socket1.end();
+  });
 });