Fixes Python C extensions for Python 3 compatibility.

These are necessary, but not sufficient, changes to make the C
extensions work "polyglot".  These are believed to be complete as far
as the C code is concerned, and don't break Python 2 compatibility.

This puts all the stuff that needs to differ between Python 2 and
Python 3 into conditionally-defined macros in a new header file
python_compatibility.h.  The definitions assume Python 2.6 or later.

In addition to the things requiring conditionals, the Lexer object was
using the deprecated tp_getattr entry, with a function based on
Py_FindMethod, which is gone in Python 3.  However, the newer
tp_getattro entry can be pointed directly to PyObject_GenericGetAttr,
which works in Python 2 and 3.

Packet data returned by the Lexer is now 'bytes' in Python 3, which is
appropriate given that it may contain binary data.  However, it means
that packet data can't be passed directly to anything expecting a
'str'.  In Python 2, the data is just a 'str' as usual.

TESTED:
Ran "scons build-all check" and xgps (using Python 2.7).  Also ran the
daemon and maidenhead-locator tests with Python 2.6.

3 files changed, 66 insertions, 22 deletions

diff --git a/gpsclient.c b/gpsclient.c
index 276900e3..7b04ddc0 100644
--- a/gpsclient.c
+++ b/gpsclient.c
@@ -10,6 +10,7 @@
 #include "gps.h"
 #include "gpsdclient.h"
 #include "compiler.h"	/* for UNUSED */
+#include "python_compatibility.h"
 
 /*
  * Client utility functions
@@ -83,13 +84,16 @@ PyDoc_STRVAR(module_doc,
 /* banishes a pointless compiler warning */
 extern PyMODINIT_FUNC initclienthelpers(void);
 
-PyMODINIT_FUNC
 // cppcheck-suppress unusedFunction
-initclienthelpers(void)
+GPSD_PY_MODULE_INIT(clienthelpers)
 {
     PyObject *m;
 
-    m = Py_InitModule3("gps.clienthelpers", gpsclient_methods, module_doc);
+    /* Create the module and add the functions */
+    GPSD_PY_MODULE_DEF(m, "clienthelpers", module_doc, gpsclient_methods)
+
+    if (m == NULL)
+	return GPSD_PY_MODULE_ERROR_VAL;
 
     PyModule_AddIntConstant(m, "deg_dd", deg_dd);
     PyModule_AddIntConstant(m, "deg_ddmm", deg_ddmm);
@@ -99,5 +103,6 @@ initclienthelpers(void)
     PyModule_AddIntConstant(m, "imperial", imperial);
     PyModule_AddIntConstant(m, "nautical", nautical);
     PyModule_AddIntConstant(m, "metric", metric);
-}
 
+    return GPSD_PY_MODULE_SUCCESS_VAL(m);
+}
diff --git a/gpspacket.c b/gpspacket.c
index 06a41d36..0c9ade28 100644
--- a/gpspacket.c
+++ b/gpspacket.c
@@ -9,6 +9,7 @@
 
 #include <stdio.h>
 #include "gpsd.h"
+#include "python_compatibility.h"
 
 static PyObject *ErrorObject = NULL;
 
@@ -94,7 +95,7 @@ Lexer_get(LexerObject *self, PyObject *args)
     if (PyErr_Occurred())
 	return NULL;
 
-    return Py_BuildValue("(i, i, s#, i)",
+    return Py_BuildValue("(i, i, " GPSD_PY_BYTE_FORMAT ", i)",
 			 len,
 			 self->lexer.type,
 			 self->lexer.outbuffer,
@@ -125,12 +126,6 @@ static PyMethodDef Lexer_methods[] = {
     {NULL,		NULL}		/* sentinel */
 };
 
-static PyObject *
-Lexer_getattr(LexerObject *self, char *name)
-{
-    return Py_FindMethod(Lexer_methods, (PyObject *)self, name);
-}
-
 PyDoc_STRVAR(Lexer__doc__,
 "GPS packet lexer object\n\
 \n\
@@ -139,15 +134,14 @@ Fetch a single packet from file descriptor");
 static PyTypeObject Lexer_Type = {
 	/* The ob_type field must be initialized in the module init function
 	 * to be portable to Windows without using C++. */
-	PyObject_HEAD_INIT(NULL)
-	0,			/*ob_size*/
+	PyVarObject_HEAD_INIT(NULL, 0)
 	"gps.packet.lexer",	/*tp_name*/
 	sizeof(LexerObject),	/*tp_basicsize*/
 	0,			/*tp_itemsize*/
 	/* methods */
 	(destructor)Lexer_dealloc, /*tp_dealloc*/
 	0,			/*tp_print*/
-	(getattrfunc)Lexer_getattr,			/*tp_getattr*/
+	0,			/*tp_getattr*/
 	0,			/*tp_setattr*/
 	0,			/*tp_compare*/
 	0,			/*tp_repr*/
@@ -157,11 +151,11 @@ static PyTypeObject Lexer_Type = {
 	0,			/*tp_hash*/
         0,                      /*tp_call*/
         0,                      /*tp_str*/
-        0,                      /*tp_getattro*/
+        PyObject_GenericGetAttr,  /*tp_getattro*/
         0,                      /*tp_setattro*/
         0,                      /*tp_as_buffer*/
         Py_TPFLAGS_DEFAULT,     /*tp_flags*/
-        Lexer__doc__,          /*tp_doc*/
+        Lexer__doc__,           /*tp_doc*/
         0,                      /*tp_traverse*/
         0,                      /*tp_clear*/
         0,                      /*tp_richcompare*/
@@ -255,17 +249,16 @@ level of the message and the message itself.\n\
 /* banishes a pointless compiler warning */
 extern PyMODINIT_FUNC initpacket(void);
 
-PyMODINIT_FUNC
 // cppcheck-suppress unusedFunction
-initpacket(void)
+GPSD_PY_MODULE_INIT(packet)
 {
     PyObject *m;
 
-    if (PyType_Ready(&Lexer_Type) < 0)
-	return;
-
     /* Create the module and add the functions */
-    m = Py_InitModule3("packet", packet_methods, module_doc);
+    GPSD_PY_MODULE_DEF(m, "packet", module_doc, packet_methods)
+
+    if (m == NULL || PyType_Ready(&Lexer_Type) < 0)
+	return GPSD_PY_MODULE_ERROR_VAL;
 
     PyModule_AddIntConstant(m, "BAD_PACKET", BAD_PACKET);
     PyModule_AddIntConstant(m, "COMMENT_PACKET", COMMENT_PACKET);
@@ -298,4 +291,6 @@ initpacket(void)
     PyModule_AddIntConstant(m, "LOG_DATA", LOG_DATA);
     PyModule_AddIntConstant(m, "LOG_SPIN", LOG_SPIN);
     PyModule_AddIntConstant(m, "LOG_RAW", LOG_RAW);
+
+    return GPSD_PY_MODULE_SUCCESS_VAL(m);
 }
diff --git a/python_compatibility.h b/python_compatibility.h
new file mode 100644
index 00000000..635cd06c
--- /dev/null
+++ b/python_compatibility.h
@@ -0,0 +1,44 @@
+/*
+ * python_compatibility.h -- macros for Python 2/3 compatibility
+ *
+ * This file is Copyright (c) 2016 by the GPSD project
+ * BSD terms apply: see the file COPYING in the distribution root for details.
+ *
+ * Definitions based on examples in "Supporting Python 3 - The Book Site"
+ *     http://python3porting.com/cextensions.html
+ */
+
+#ifndef _PYTHON_COMPATIBILITY_H_
+#define _PYTHON_COMPATIBILITY_H_
+
+#include <Python.h>
+
+#if PY_MAJOR_VERSION >= 3
+
+#define GPSD_PY_MODULE_INIT(name) PyMODINIT_FUNC PyInit_##name(void)
+
+#define GPSD_PY_MODULE_DEF(mod, name, doc, methods) \
+    static struct PyModuleDef moduledef = { \
+        PyModuleDef_HEAD_INIT, name, doc, -1, methods, }; \
+    mod = PyModule_Create(&moduledef);
+
+#define GPSD_PY_MODULE_ERROR_VAL NULL
+#define GPSD_PY_MODULE_SUCCESS_VAL(val) val
+
+#define GPSD_PY_BYTE_FORMAT "y#"
+
+#else /* !Python 3 */
+
+#define GPSD_PY_MODULE_INIT(name) PyMODINIT_FUNC init##name(void)
+
+#define GPSD_PY_MODULE_DEF(mod, name, doc, methods) \
+    mod = Py_InitModule3(name, methods, doc);
+
+#define GPSD_PY_MODULE_ERROR_VAL
+#define GPSD_PY_MODULE_SUCCESS_VAL(val)
+
+#define GPSD_PY_BYTE_FORMAT "s#"
+
+#endif /* !Python 3 */
+
+#endif /* _PYTHON_COMPATIBILITY_H_ */