setup.py: use pure-Python implementation as fallback

setup.py

@@ -18,11 +18,48 @@
 
 This works with Python 2 and 3, and PyPy. The license is Apache 2.0.
 """
+import sys
 
 try:
-  from setuptools import setup
+  from setuptools import setup, Extension
 except ImportError:
   from distutils.core import setup
+  from distutils.extension import Extension
+
+from distutils.command.build_ext import build_ext
+from distutils.errors import CCompilerError, DistutilsExecError, \
+  DistutilsPlatformError
+
+
+class BuildFailed(Exception):
+  pass
+
+class ve_build_ext(build_ext):
+  """This class allows C extension building to fail."""
+  ext_errors = (CCompilerError, DistutilsExecError, DistutilsPlatformError)
+  if sys.platform == 'win32' and sys.version_info < (2, 7):
+    # 2.6's distutils.msvc9compiler can raise an IOError when failing to
+    # find the compiler
+    # It can also raise ValueError http://bugs.python.org/issue7511
+    ext_errors += (IOError, ValueError)
+
+  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 self.ext_errors:
+      raise BuildFailed()
+    except ValueError:
+      # this can happen on Windows 64 bit, see Python issue 7511
+      if "'path'" in str(sys.exc_info()[1]): # works with Python 2 and 3
+        raise BuildFailed()
+      raise
+
 
 common = dict(
     name = 'streql',
@@ -49,12 +86,41 @@
     ],
 )
 
-try:
-  import __pypy__
-  setup(py_modules=['streql'], package_dir={'':'pypy'}, **common)
-except ImportError:
+def setup_c_extension():
+  setup(ext_modules=[Extension("streql", ["streql.c"])],
+        cmdclass={'build_ext': ve_build_ext},
+        **common)
+
+def setup_pure_python():
+  setup(py_modules=['streql'], package_dir={'': 'pypy'}, **common)
+
+
+is_pypy = hasattr(sys, 'pypy_version_info')
+
+if is_pypy:
+  setup_pure_python()
+else:
   try:
-    from setuptools import Extension
-  except ImportError:
-    from distutils.extension import Extension
-  setup(ext_modules = [Extension("streql", ["streql.c"])], **common)
+    setup_c_extension()
+  except BuildFailed:
+    # install the pure-Python version,
+    # while printing useful output so the user knows what happened
+    def echo(msg=''):
+      sys.stdout.write(msg + '\n')
+
+    line = '=' * 74
+    build_ext_warning = 'WARNING: The C extensions could not be ' \
+                        'compiled; speedups are not enabled.'
+
+    echo(line)
+    echo(build_ext_warning)
+    echo('Failure information, if any, is above.')
+    echo('Retrying the build without the C extension now.')
+    echo()
+
+    setup_pure_python()
+
+    echo(line)
+    echo(build_ext_warning)
+    echo('Plain-Python installation succeeded.')
+    echo(line)