Add optional C extension wrapper for Python JSON parsing

The pure Python in-tree JSON parser is *much* slower than the
in-tree C JSON parser. A local test parsing a 100Mb JSON file
showed the Python version taking 270 seconds. With the C wrapper,
it took under 4 seconds.

The C extension will be used automatically if it can be built. If
the extension fails to build, a warning is displayed and the build
is restarted without the extension.

The Serializer class is replaced with Python's built-in
JSON library since the ability to process chunked data is not
needed in that case.

The extension should work with both Python 2.7 and Python 3.3+.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

python/setup.py

@@ -13,6 +13,10 @@
 from __future__ import print_function
 import sys
 
+from distutils.command.build_ext import build_ext
+from distutils.errors import CCompilerError, DistutilsExecError, \
+    DistutilsPlatformError
+
 import setuptools
 
 VERSION = "unknown"
@@ -25,8 +29,33 @@ except IOError:
           file=sys.stderr)
     sys.exit(-1)
 
+ext_errors = (CCompilerError, DistutilsExecError, DistutilsPlatformError)
+if sys.platform == 'win32':
+    ext_errors += (IOError, ValueError)
 
-setuptools.setup(
+
+class BuildFailed(Exception):
+    pass
+
+
+class try_build_ext(build_ext):
+    # This class allows C extension building to fail
+    # NOTE: build_ext is not a new-style class
+
+    def run(self):
+        try:
+            build_ext.run(self)
+        except DistutilsPlatformError:
+            raise BuildFailed()
+
+    def build_extension(self, ext):
+        try:
+            build_ext.build_extension(self, ext)
+        except ext_errors:
+            raise BuildFailed()
+
+
+setup_args = dict(
     name='ovs',
     description='Open vSwitch library',
     version=VERSION,
@@ -46,5 +75,23 @@ setuptools.setup(
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.4',
-    ]
+    ],
+    ext_modules=[setuptools.Extension("ovs._json", sources=["ovs/_json.c"],
+                                      libraries=['openvswitch'])],
+    cmdclass={'build_ext': try_build_ext},
 )
+
+try:
+    setuptools.setup(**setup_args)
+except BuildFailed:
+    BUILD_EXT_WARNING = ("WARNING: The C extension could not be compiled, "
+                         "speedups are not enabled.")
+    print("*" * 75)
+    print(BUILD_EXT_WARNING)
+    print("Failure information, if any, is above.")
+    print("Retrying the build without the C extension.")
+    print("*" * 75)
+
+    del(setup_args['cmdclass'])
+    del(setup_args['ext_modules'])
+    setuptools.setup(**setup_args)