improve presentation of C compiler stderr/stdout output in test runner

Cython/Utils.py

@@ -388,12 +388,14 @@ def captured_fd(stream=2, encoding=None):
         os.close(orig_stream)
 
 
-def print_bytes(s, stream=sys.stdout):
-    stream.flush()
+def print_bytes(s, end=b'\n', file=sys.stdout, flush=True):
+    file.flush()
     try:
-        out = stream.buffer  # Py3
+        out = file.buffer  # Py3
     except AttributeError:
-        out = stream         # Py2
+        out = file         # Py2
     out.write(s)
-    out.write(b'\n')
-    out.flush()
+    if end:
+        out.write(end)
+    if flush:
+        out.flush()

runtests.py

@@ -865,25 +865,35 @@ class CythonCompileTestCase(unittest.TestCase):
         so_path = None
         if not self.cython_only:
             from Cython.Utils import captured_fd, print_bytes
-            get_stderr = None
+            show_output = True
+            get_stderr = get_stdout = None
             try:
-                with captured_fd(2) as get_stderr:
-                    so_path = self.run_distutils(test_directory, module, workdir, incdir)
+                with captured_fd(1) as get_stdout:
+                    with captured_fd(2) as get_stderr:
+                        so_path = self.run_distutils(test_directory, module, workdir, incdir)
             except Exception:
                 if 'cerror' in self.tags['tag'] and get_stderr and get_stderr():
-                    pass
+                    show_output = False  # expected C compiler failure
                 else:
                     raise
             else:
-                c_compiler_stderr = get_stderr().strip()
-                if c_compiler_stderr:
-                    print("\n=== C/C++ compiler error output: ===")
-                    print_bytes(c_compiler_stderr)
                 if 'cerror' in self.tags['tag']:
-                    # must raise this outside the try block
                     raise RuntimeError('should have failed C compile')
+            finally:
+                if show_output:
+                    stdout = get_stdout and get_stdout().strip()
+                    if stdout:
+                        print("\n=== C/C++ compiler output: ===")
+                        print_bytes(stdout, end=None)
+                    stderr = get_stderr and get_stderr().strip()
+                    if stderr:
+                        print("\n=== C/C++ compiler error output: ===")
+                        print_bytes(stderr, end=None)
+                    if stdout or stderr:
+                        print("\n==============================")
         return so_path
 
+
 class CythonRunTestCase(CythonCompileTestCase):
     def setUp(self):
         CythonCompileTestCase.setUp(self)