tests: add tests for when stdout or stderr is connected to `os.devnull`

The original motivation was that creating PTYs on Windows is not possible, but
`NUL` is recognized as a TTY, so we can have at least some test coverage for
the TTY case. I think it doesn’t hurt to run the test cases on all systems.

tests/test-stdio.py

@@ -1,219 +1,245 b''
 #!/usr/bin/env python
 """
 Tests the buffering behavior of stdio streams in `mercurial.utils.procutil`.
 """
 from __future__ import absolute_import
 
 import contextlib
 import errno
 import os
 import signal
 import subprocess
 import sys
 import unittest
 
 from mercurial import pycompat
 
 
 TEST_BUFFERING_CHILD_SCRIPT = r'''
 import os
 
 from mercurial import dispatch
 from mercurial.utils import procutil
 
 dispatch.initstdio()
 procutil.{stream}.write(b'aaa')
 os.write(procutil.{stream}.fileno(), b'[written aaa]')
 procutil.{stream}.write(b'bbb\n')
 os.write(procutil.{stream}.fileno(), b'[written bbb\\n]')
 '''
 UNBUFFERED = b'aaa[written aaa]bbb\n[written bbb\\n]'
 LINE_BUFFERED = b'[written aaa]aaabbb\n[written bbb\\n]'
 FULLY_BUFFERED = b'[written aaa][written bbb\\n]aaabbb\n'
 
 
 TEST_LARGE_WRITE_CHILD_SCRIPT = r'''
 import signal
 import sys
 
 from mercurial import dispatch
 from mercurial.utils import procutil
 
 signal.signal(signal.SIGINT, lambda *x: None)
 dispatch.initstdio()
 procutil.{stream}.write(b'x' * 1048576)
 '''
 
 
 @contextlib.contextmanager
 def _closing(fds):
     try:
         yield
     finally:
         for fd in fds:
             try:
                 os.close(fd)
             except EnvironmentError:
                 pass
 
 
 @contextlib.contextmanager
+def _devnull():
+    devnull = os.open(os.devnull, os.O_WRONLY)
+    with _closing([devnull]):
+        yield (None, devnull)
+
+
[e-mail]
 def _pipes():
     rwpair = os.pipe()
     with _closing(rwpair):
         yield rwpair
 
 
 @contextlib.contextmanager
 def _ptys():
     if pycompat.iswindows:
         raise unittest.SkipTest("PTYs are not supported on Windows")
     import pty
     import tty
 
     rwpair = pty.openpty()
     with _closing(rwpair):
         tty.setraw(rwpair[0])
         yield rwpair
 
 
 def _readall(fd, buffer_size, initial_buf=None):
     buf = initial_buf or []
     while True:
         try:
             s = os.read(fd, buffer_size)
         except OSError as e:
             if e.errno == errno.EIO:
                 # If the child-facing PTY got closed, reading from the
                 # parent-facing PTY raises EIO.
                 break
             raise
         if not s:
             break
         buf.append(s)
     return b''.join(buf)
 
 
 class TestStdio(unittest.TestCase):
     def _test(
         self,
         child_script,
         stream,
         rwpair_generator,
         check_output,
         python_args=[],
     ):
         assert stream in ('stdout', 'stderr')
         with rwpair_generator() as (stream_receiver, child_stream), open(
             os.devnull, 'rb'
         ) as child_stdin:
             proc = subprocess.Popen(
                 [sys.executable] + python_args + ['-c', child_script],
                 stdin=child_stdin,
                 stdout=child_stream if stream == 'stdout' else None,
                 stderr=child_stream if stream == 'stderr' else None,
             )
             try:
                 os.close(child_stream)
-                check_output(stream_receiver, proc)
+                if stream_receiver is not None:
+                    check_output(stream_receiver, proc)
             except:  # re-raises
                 proc.terminate()
                 raise
             finally:
                 retcode = proc.wait()
             self.assertEqual(retcode, 0)
 
     def _test_buffering(
         self, stream, rwpair_generator, expected_output, python_args=[]
     ):
         def check_output(stream_receiver, proc):
             self.assertEqual(_readall(stream_receiver, 1024), expected_output)
 
         self._test(
             TEST_BUFFERING_CHILD_SCRIPT.format(stream=stream),
             stream,
             rwpair_generator,
             check_output,
             python_args,
         )
 
+    def test_buffering_stdout_devnull(self):
+        self._test_buffering('stdout', _devnull, None)
+
     def test_buffering_stdout_pipes(self):
         self._test_buffering('stdout', _pipes, FULLY_BUFFERED)
 
     def test_buffering_stdout_ptys(self):
         self._test_buffering('stdout', _ptys, LINE_BUFFERED)
 
+    def test_buffering_stdout_devnull_unbuffered(self):
+        self._test_buffering('stdout', _devnull, None, python_args=['-u'])
+
     def test_buffering_stdout_pipes_unbuffered(self):
         self._test_buffering('stdout', _pipes, UNBUFFERED, python_args=['-u'])
 
     def test_buffering_stdout_ptys_unbuffered(self):
         self._test_buffering('stdout', _ptys, UNBUFFERED, python_args=['-u'])
 
     if not pycompat.ispy3 and not pycompat.iswindows:
         # On Python 2 on non-Windows, we manually open stdout in line-buffered
         # mode if connected to a TTY. We should check if Python was configured
         # to use unbuffered stdout, but it's hard to do that.
         test_buffering_stdout_ptys_unbuffered = unittest.expectedFailure(
             test_buffering_stdout_ptys_unbuffered
         )
 
     def _test_large_write(self, stream, rwpair_generator, python_args=[]):
         if not pycompat.ispy3 and pycompat.isdarwin:
             # Python 2 doesn't always retry on EINTR, but the libc might retry.
             # So far, it was observed only on macOS that EINTR is raised at the
             # Python level. As Python 2 support will be dropped soon-ish, we
             # won't attempt to fix it.
             raise unittest.SkipTest("raises EINTR on macOS")
 
         def check_output(stream_receiver, proc):
             if not pycompat.iswindows:
                 # On Unix, we can provoke a partial write() by interrupting it
                 # by a signal handler as soon as a bit of data was written.
                 # We test that write() is called until all data is written.
                 buf = [os.read(stream_receiver, 1)]
                 proc.send_signal(signal.SIGINT)
             else:
                 # On Windows, there doesn't seem to be a way to cause partial
                 # writes.
                 buf = []
             self.assertEqual(
                 _readall(stream_receiver, 131072, buf), b'x' * 1048576
             )
 
         self._test(
             TEST_LARGE_WRITE_CHILD_SCRIPT.format(stream=stream),
             stream,
             rwpair_generator,
             check_output,
             python_args,
         )
 
+    def test_large_write_stdout_devnull(self):
+        self._test_large_write('stdout', _devnull)
+
     def test_large_write_stdout_pipes(self):
         self._test_large_write('stdout', _pipes)
 
     def test_large_write_stdout_ptys(self):
         self._test_large_write('stdout', _ptys)
 
+    def test_large_write_stdout_devnull_unbuffered(self):
+        self._test_large_write('stdout', _devnull, python_args=['-u'])
+
     def test_large_write_stdout_pipes_unbuffered(self):
         self._test_large_write('stdout', _pipes, python_args=['-u'])
 
     def test_large_write_stdout_ptys_unbuffered(self):
         self._test_large_write('stdout', _ptys, python_args=['-u'])
 
+    def test_large_write_stderr_devnull(self):
+        self._test_large_write('stderr', _devnull)
+
     def test_large_write_stderr_pipes(self):
         self._test_large_write('stderr', _pipes)
 
     def test_large_write_stderr_ptys(self):
         self._test_large_write('stderr', _ptys)
 
+    def test_large_write_stderr_devnull_unbuffered(self):
+        self._test_large_write('stderr', _devnull, python_args=['-u'])
+
     def test_large_write_stderr_pipes_unbuffered(self):
         self._test_large_write('stderr', _pipes, python_args=['-u'])
 
     def test_large_write_stderr_ptys_unbuffered(self):
         self._test_large_write('stderr', _ptys, python_args=['-u'])
 
 
 if __name__ == '__main__':
     import silenttestrunner
 
     silenttestrunner.main(__name__)