peer: handle message encoding errors on write

In this commit, we modify the peer's writeMessage
method to properly handle errors returned from
encoding an lnwire message and from setting the
write deadline on the connection. Since an error
would likely result in an empty byte slice, the
worse case seems to be that we may have tried to
send an empty message on the wire.

Lastly, we correct the way we compute bytes sent
on the wire to properly count the number of bytes
*written*, and not just the length of the encoded
message.

peer.go

@@ -1367,13 +1367,25 @@ func (p *peer) writeMessage(msg lnwire.Message) error {
 
 	// With the temp buffer created and sliced properly (length zero, full
 	// capacity), we'll now encode the message directly into this buffer.
-	n, err := lnwire.WriteMessage(b, msg, 0)
+	_, err := lnwire.WriteMessage(b, msg, 0)
-	atomic.AddUint64(&p.bytesSent, uint64(n))
+	if err != nil {
+		return err
+	}
 
-	p.conn.SetWriteDeadline(time.Now().Add(writeMessageTimeout))
+	// Compute and set the write deadline we will impose on the remote peer.
+	writeDeadline := time.Now().Add(writeMessageTimeout)
+	err = p.conn.SetWriteDeadline(writeDeadline)
+	if err != nil {
+		return err
+	}
 
 	// Finally, write the message itself in a single swoop.
-	_, err = p.conn.Write(b.Bytes())
+	n, err := p.conn.Write(b.Bytes())
+
+	// Regardless of the error returned, record how many bytes were written
+	// to the wire.
+	atomic.AddUint64(&p.bytesSent, uint64(n))
+
 	return err
 }