fix stderr test output printing in Py3

Cython/Utils.py

@@ -348,7 +348,7 @@ def get_cython_cache_dir():
 
 
 @contextmanager
-def captured_fd(stream=2):
+def captured_fd(stream=2, encoding=None):
     pipe_in = t = None
     orig_stream = os.dup(stream)  # keep copy of original stream
     try:
@@ -369,11 +369,17 @@ def captured_fd(stream=2):
                 finally:
                     os.close(pipe_in)
 
+            def get_output():
+                output = b''.join(data)
+                if encoding:
+                    output = output.decode(encoding)
+                return output
+
             from threading import Thread
             t = Thread(target=copy)
             t.daemon = True  # just in case
             t.start()
-            yield data
+            yield get_output
         finally:
             os.dup2(orig_stream, stream)  # restore original stream
             if t is not None:

runtests.py

@@ -868,20 +868,25 @@ class CythonCompileTestCase(unittest.TestCase):
         so_path = None
         if not self.cython_only:
             from Cython.Utils import captured_fd
-            c_compiler_stderr = None
+            get_stderr = None
             try:
-                with captured_fd(2) as c_compiler_stderr:
+                with captured_fd(2) as get_stderr:
                     so_path = self.run_distutils(test_directory, module, workdir, incdir)
             except Exception:
-                if expected_errors == '_FAIL_C_COMPILE':
-                    assert c_compiler_stderr
+                if expected_errors == '_FAIL_C_COMPILE' and get_stderr and get_stderr():
+                    pass
                 else:
                     raise
             else:
-                c_compiler_stderr = ''.join(c_compiler_stderr).strip()
+                c_compiler_stderr = get_stderr()
                 if c_compiler_stderr:
-                    print("\n=== C/C++ compiler output: ===")
-                    print(c_compiler_stderr)
+                    print("\n=== C/C++ compiler error output: ===")
+                    sys.stdout.flush()
+                    try:
+                        out = sys.stdout.buffer  # Py3
+                    except AttributeError:
+                        out = sys.stdout         # Py2
+                    out.write(c_compiler_stderr)
                 if expected_errors == '_FAIL_C_COMPILE':
                     # must raise this outside the try block
                     raise RuntimeError('should have failed C compile')