Fallback to generating files if not generated

Even if GRPC_PYTHON_BUILD_WITH_CYTHON is not specified, if the files are
not present then we will fall back to generating with Cython. This
relegates GRPC_PYTHON_BUILD_WITH_CYTHON to providing a regeneration
option rather than being a necessary build environment variable.

setup.py

@@ -64,7 +64,9 @@ LICENSE = '3-clause BSD'
 
 # Environment variable to determine whether or not the Cython extension should
 # *use* Cython or use the generated C files. Note that this requires the C files
-# to have been generated by building first *with* Cython support.
+# to have been generated by building first *with* Cython support. Even if this
+# is set to false, if the script detects that the generated `.c` file isn't
+# present, then it will still attempt to use Cython.
 BUILD_WITH_CYTHON = os.environ.get('GRPC_PYTHON_BUILD_WITH_CYTHON', False)
 
 # Environment variable to determine whether or not to enable coverage analysis
@@ -129,10 +131,20 @@ def cython_extensions(module_names, extra_sources, include_dirs,
   if ENABLE_CYTHON_TRACING:
     define_macros = define_macros + [('CYTHON_TRACE_NOGIL', 1)]
     cython_compiler_directives['linetrace'] = True
-  file_extension = 'pyx' if build_with_cython else 'c'
-  module_files = [os.path.join(PYTHON_STEM,
-                               name.replace('.', '/') + '.' + file_extension)
-                  for name in module_names]
+  pyx_module_files = [os.path.join(PYTHON_STEM,
+                                   name.replace('.', '/') + '.pyx')
+                      for name in module_names]
+  c_module_files = [os.path.join(PYTHON_STEM,
+                                 name.replace('.', '/') + '.c')
+                    for name in module_names]
+  if not build_with_cython:
+    for module_file in c_module_files:
+      if not os.path.isfile(module_file):
+        sys.stderr.write('Cython-generated files are missing; '
+                         'forcing Cython build...\n')
+        build_with_cython = True
+        break
+  module_files = pyx_module_files if build_with_cython else c_module_files
   extensions = [
       _extension.Extension(
           name=module_name,