net: fix bytesWritten during writev

When a writev is caused on a socket (sometimes through corking and
uncorking), previously net would call Buffer.byteLength on the array of
buffers and chunks, giving a completely erroneous byte length for the
bulk of them (this is because byteLength is weird and coerces to
strings).

This commit fixes this bug by iterating over each chunk in the pending
stack and calculating the length individually. Also adds a regression
test.

Author: brendanashworth

lib/net.js

@@ -742,7 +742,18 @@ Socket.prototype.__defineGetter__('bytesWritten', function() {
   });
 
   var buffer = this._pendingWrite;
-  if (buffer) {
+  if (Array.isArray(buffer)) {
+    // was a writev, iterate over chunks to get total length
+    for (let i = 0; i < buffer.length; i++) {
+      let chunk = buffer[i];
+
+      if (chunk instanceof Buffer) {
+        bytes += chunk.length;
+      } else {
+        bytes += Buffer.byteLength(chunk.data, chunk.encoding);
+      }
+    }
+  } else if (buffer) {
     if (buffer.data instanceof Buffer) {
       bytes += buffer.data.length;
     } else {

test/parallel/test-net-socket-byteswritten.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const Buffer = require('buffer').Buffer;
+
+var server = net.createServer(function(socket) {
+  socket.end();
+});
+
+server.listen(common.PORT, common.mustCall(function() {
+  var socket = net.connect(common.PORT);
+
+  // Cork the socket, then write twice; this should cause a writev, which
+  // previously caused an err in the bytesWritten count.
+  socket.cork();
+
+  socket.write('one');
+  socket.write(new Buffer('twø', 'utf8'));
+
+  socket.uncork();
+
+  // one = 3 bytes, twø = 4 bytes
+  assert.equal(socket.bytesWritten, 3 + 4);
+
+  socket.on('connect', common.mustCall(function() {
+    assert.equal(socket.bytesWritten, 3 + 4);
+  }));
+
+  socket.on('end', common.mustCall(function() {
+    assert.equal(socket.bytesWritten, 3 + 4);
+
+    server.close();
+  }));
+}));