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. This throws a TypeError, because Buffer.byteLength
throws when passed a non-string.

In dbfe8c4e, behavior changed to throw when passed a non-string. This is
correct behavior. Previously, it would cast the argument to a string, so
before this commit, bytesWritten would give an erroneous value. This
commit corrects the behavior equally both before and after dbfe8c4e.

This commit fixes this bug by iterating over each chunk in the pending
stack and calculating the length individually. Also adds a regression
test. This additionally changes an `instanceof Buffer` check to `typeof
!== 'string'`, which should be equivalent.

PR-URL: https://github.com/nodejs/node/pull/14236
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Refs: https://github.com/nodejs/node/pull/2960

lib/net.js

@@ -827,8 +827,19 @@ protoGetter('bytesWritten', function bytesWritten() {
       bytes += Buffer.byteLength(el.chunk, el.encoding);
   });
 
-  if (data) {
-    if (data instanceof Buffer)
+  if (Array.isArray(data)) {
+    // was a writev, iterate over chunks to get total length
+    for (var i = 0; i < data.length; i++) {
+      const chunk = data[i];
+
+      if (data.allBuffers || chunk instanceof Buffer)
+        bytes += chunk.length;
+      else
+        bytes += Buffer.byteLength(chunk.chunk, chunk.encoding);
+    }
+  } else if (data) {
+    // Writes are either a string or a Buffer.
+    if (typeof data !== 'string')
       bytes += data.length;
     else
       bytes += Buffer.byteLength(data, encoding);

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

@@ -0,0 +1,35 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+const server = net.createServer(function(socket) {
+  socket.end();
+});
+
+server.listen(0, common.mustCall(function() {
+  const socket = net.connect(server.address().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.strictEqual(socket.bytesWritten, 3 + 4);
+
+  socket.on('connect', common.mustCall(function() {
+    assert.strictEqual(socket.bytesWritten, 3 + 4);
+  }));
+
+  socket.on('end', common.mustCall(function() {
+    assert.strictEqual(socket.bytesWritten, 3 + 4);
+
+    server.close();
+  }));
+}));