move file descriptor redirecting to Cython.Utils package and use it as context...

move file descriptor redirecting to Cython.Utils package and use it as context manager in test runner

Cython/Utils.py

@@ -8,6 +8,7 @@ import sys
 import re
 import io
 import codecs
+from contextlib import contextmanager
 
 modification_time = os.path.getmtime
 
@@ -344,3 +345,38 @@ def get_cython_cache_dir():
 
     # last fallback: ~/.cython
     return os.path.expanduser(os.path.join('~', '.cython'))
+
+
+@contextmanager
+def captured_fd(stream=2):
+    pipe_in = t = None
+    orig_stream = os.dup(stream)  # keep copy of original stream
+    try:
+        pipe_in, pipe_out = os.pipe()
+        os.dup2(pipe_out, stream)  # replace stream by copy of pipe
+        try:
+            os.close(pipe_out)  # close original pipe-out stream
+            data = []
+
+            def copy():
+                try:
+                    while True:
+                        d = os.read(pipe_in, 1000)
+                        if d:
+                            data.append(d)
+                        else:
+                            break
+                finally:
+                    os.close(pipe_in)
+
+            from threading import Thread
+            t = Thread(target=copy)
+            t.daemon = True  # just in case
+            t.start()
+            yield data
+        finally:
+            os.dup2(orig_stream, stream)  # restore original stream
+            if t is not None:
+                t.join()
+    finally:
+        os.close(orig_stream)

runtests.py

@@ -867,33 +867,24 @@ class CythonCompileTestCase(unittest.TestCase):
 
         so_path = None
         if not self.cython_only:
-            c_compiler_output = None
-            orig_stderr = os.dup(2)  # remember original stderr
-            try:
-                with tempfile.TemporaryFile() as temp_stderr:
-                    os.dup2(temp_stderr.fileno(), 2)  # replace stderr by temp file
-                    try:
+            from Cython.Utils import captured_fd
+            c_compiler_stderr = None
             try:
+                with captured_fd(2) as c_compiler_stderr:
                     so_path = self.run_distutils(test_directory, module, workdir, incdir)
-                        finally:
-                            temp_stderr.seek(0)
-                            c_compiler_output = temp_stderr.read().strip()
             except Exception:
                 if expected_errors == '_FAIL_C_COMPILE':
-                            assert c_compiler_output
+                    assert c_compiler_stderr
                 else:
                     raise
             else:
-                        if c_compiler_output:
+                c_compiler_stderr = ''.join(c_compiler_stderr).strip()
+                if c_compiler_stderr:
                     print("\n=== C/C++ compiler output: ===")
-                            print(c_compiler_output)
+                    print(c_compiler_stderr)
                 if expected_errors == '_FAIL_C_COMPILE':
                     # must raise this outside the try block
                     raise RuntimeError('should have failed C compile')
-                    finally:
-                        os.dup2(orig_stderr, 2)  # restore stderr
-            finally:
-                os.close(orig_stderr)
         return so_path
 
 class CythonRunTestCase(CythonCompileTestCase):