brontide/conn: expose WriteMessage and Flush on brontide.Conn

This commit exposes the WriteMessage and Flush interfaces of the
underlying brontide.Machine, such that callers can have greater
flexibility in when blocking network operations take place.

brontide/conn.go

@@ -203,6 +203,29 @@ func (c *Conn) Write(b []byte) (n int, err error) {
 	return bytesWritten, nil
 }
 
+// WriteMessage encrypts and buffers the next message p for the connection. The
+// ciphertext of the message is prepended with an encrypt+auth'd length which
+// must be used as the AD to the AEAD construction when being decrypted by the
+// other side.
+//
+// NOTE: This DOES NOT write the message to the wire, it should be followed by a
+// call to Flush to ensure the message is written.
+func (c *Conn) WriteMessage(b []byte) error {
+	return c.noise.WriteMessage(b)
+}
+
+// Flush attempts to write a message buffered using WriteMessage to the
+// underlying connection. If no buffered message exists, this will result in a
+// NOP. Otherwise, it will continue to write the remaining bytes, picking up
+// where the byte stream left off in the event of a partial write. The number of
+// bytes returned reflects the number of plaintext bytes in the payload, and
+// does not account for the overhead of the header or MACs.
+//
+// NOTE: It is safe to call this method again iff a timeout error is returned.
+func (c *Conn) Flush() (int, error) {
+	return c.noise.Flush(c.conn)
+}
+
 // Close closes the connection.  Any blocked Read or Write operations will be
 // unblocked and return errors.
 //