express: make Datagram.get_message() return bytes in Python 3

This is done using a Python extension function, which also happens to make the call more efficient as this avoids an extra copy.  The C++ version still returns std::string as there is still a lot of C++ code that relies on that.

Fixes #297

panda/src/express/datagram.I

@@ -316,18 +316,6 @@ get_message() const {
   }
 }
 
-/**
- * Returns the datagram's data as a bytes object.
- */
-INLINE vector_uchar Datagram::
-__bytes__() const {
-  if (!_data.empty()) {
-    return vector_uchar(_data.v());
-  } else {
-    return vector_uchar();
-  }
-}
-
 /**
  * Returns a pointer to the beginning of the datagram's data.
  */

panda/src/express/datagram.h

@@ -85,11 +85,16 @@ PUBLISHED:
   void append_data(const void *data, size_t size);
   INLINE void append_data(const vector_uchar &data);
 
+public:
   void assign(const void *data, size_t size);
 
   INLINE std::string get_message() const;
-  INLINE vector_uchar __bytes__() const;
   INLINE const void *get_data() const;
+
+PUBLISHED:
+  EXTENSION(INLINE PyObject *get_message() const);
+  EXTENSION(INLINE PyObject *__bytes__() const);
+
   INLINE size_t get_length() const;
 
   INLINE void set_array(PTA_uchar data);

panda/src/express/datagram_ext.I

@@ -0,0 +1,35 @@
+/**
+ * PANDA 3D SOFTWARE
+ * Copyright (c) Carnegie Mellon University.  All rights reserved.
+ *
+ * All use of this software is subject to the terms of the revised BSD
+ * license.  You should have received a copy of this license along
+ * with this source code in a file named "LICENSE."
+ *
+ * @file datagram_ext.I
+ * @author rdb
+ * @date 2018-08-19
+ */
+
+/**
+ * Returns the datagram's data as a bytes object.
+ */
+INLINE PyObject *Extension<Datagram>::
+get_message() const {
+  const char *data = (const char *)_this->get_data();
+  size_t size = _this->get_length();
+
+#if PY_MAJOR_VERSION >= 3
+  return PyBytes_FromStringAndSize((char *)data, size);
+#else
+  return PyString_FromStringAndSize((char *)data, size);
+#endif
+}
+
+/**
+ * Returns the datagram's data as a bytes object.
+ */
+PyObject *Extension<Datagram>::
+__bytes__() const {
+  return get_message();
+}

panda/src/express/datagram_ext.h

@@ -0,0 +1,40 @@
+/**
+ * PANDA 3D SOFTWARE
+ * Copyright (c) Carnegie Mellon University.  All rights reserved.
+ *
+ * All use of this software is subject to the terms of the revised BSD
+ * license.  You should have received a copy of this license along
+ * with this source code in a file named "LICENSE."
+ *
+ * @file datagram_ext.h
+ * @author rdb
+ * @date 2018-08-19
+ */
+
+#ifndef DATAGRAM_EXT_H
+#define DATAGRAM_EXT_H
+
+#include "dtoolbase.h"
+
+#ifdef HAVE_PYTHON
+
+#include "extension.h"
+#include "datagram.h"
+#include "py_panda.h"
+
+/**
+ * This class defines the extension methods for Datagram, which are called
+ * instead of any C++ methods with the same prototype.
+ */
+template<>
+class Extension<Datagram> : public ExtensionBase<Datagram> {
+public:
+  INLINE PyObject *get_message() const;
+  INLINE PyObject *__bytes__() const;
+};
+
+#include "datagram_ext.I"
+
+#endif  // HAVE_PYTHON
+
+#endif  // DATAGRAM_EXT_H