Add a new TORNADO_EXTENSION environment variable for testing purposes.

This variable can be set to 0 to suppress or 1 to require the use of the
extension (instead of the default to use it when it is available but silently
fall back).  Require the extension to be present on travis-ci.

.travis.yml

@@ -19,10 +19,13 @@ install:
     - if [[ $TRAVIS_PYTHON_VERSION == 'pypy' && $DEPS == true ]]; then pip install --use-mirrors futures mock; fi
     - if [[ $TRAVIS_PYTHON_VERSION == '3.2' && $DEPS == true ]]; then pip install --use-mirrors pycares mock; fi
     - if [[ $TRAVIS_PYTHON_VERSION == '3.3' && $DEPS == true ]]; then pip install --use-mirrors pycares; fi
+    # On travis the extension should always be built
+    - if [[ $TRAVIS_PYTHON_VERSION != 'pypy' ]]; then export TORNADO_EXTENSION=1; fi
     - python setup.py install
     - pip install --use-mirrors coveralls
 
 script:
+    - if [[ $TRAVIS_PYTHON_VERSION != 'pypy' ]]; then export TORNADO_EXTENSION=1; fi
     - export TARGET="-m tornado.test.runtests"
     # We use "python -m coverage" instead of the "bin/coverage" script
     # so we can pass additional arguments to python.  However, this doesn't

setup.py

@@ -14,6 +14,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import os
 import platform
 import sys
 import warnings
@@ -118,14 +119,20 @@ def build_extension(self, ext):
 with open('README.rst') as f:
     kwargs['long_description'] = f.read()
 
-if platform.python_implementation() == 'CPython':
+if (platform.python_implementation() == 'CPython' and
+    os.environ.get('TORNADO_EXTENSION') != '0'):
     # This extension builds and works on pypy as well, although pypy's jit
     # produces equivalent performance.
     kwargs['ext_modules'] = [
         Extension('tornado.speedups',
                   sources=['tornado/speedups.c']),
     ]
 
+    if os.environ.get('TORNADO_EXTENSION') != '1':
+        # Unless the user has specified that the extension is mandatory,
+        # fall back to the pure-python implementation on any build failure.
+        kwargs['cmdclass'] = {'build_ext': custom_build_ext}
+
 if setuptools is not None:
     # If setuptools is not available, you're on your own for dependencies.
     if sys.version_info < (3, 2):
@@ -170,6 +177,5 @@ def build_extension(self, ext):
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy',
         ],
-    cmdclass={"build_ext": custom_build_ext},
     **kwargs
 )

tornado/websocket.py

@@ -913,12 +913,15 @@ def _websocket_mask_python(mask, data):
     else:
         return unmasked.tostring()
 
-if os.environ.get('TORNADO_NO_EXTENSION'):
-    # This environment variable exists to make it easier to do performance comparisons;
-    # it's not guaranteed to remain supported in the future.
+if (os.environ.get('TORNADO_NO_EXTENSION') or
+    os.environ.get('TORNADO_EXTENSION') == '0'):
+    # These environment variables exist to make it easier to do performance
+    # comparisons; they are not guaranteed to remain supported in the future.
     _websocket_mask = _websocket_mask_python
 else:
     try:
         from tornado.speedups import websocket_mask as _websocket_mask
     except ImportError:
+        if os.environ.get('TORNADO_EXTENSION') == '1':
+            raise
         _websocket_mask = _websocket_mask_python