gpython: Support unbuffered mode option -u

--------
kirr: Redo the test via checking buffering behaviour without relying on
timings. Original Boxiang's test can be seen here: https://lab.nexedi.com/Daetalus/pygolang/-/commit/307b9ce2

/reviewed-by @kirr
/reviewed-on https://lab.nexedi.com/nexedi/pygolang/-/merge_requests/33

Author: Boxiang Sun

gpython/__init__.py

@@ -41,7 +41,7 @@
 from __future__ import print_function, absolute_import
 
 
-_pyopt = "c:Eim:OvVW:X:"
+_pyopt = "c:Eim:OuvVW:X:"
 _pyopt_long = ('version',)
 
 # pymain mimics `python ...`
@@ -117,6 +117,7 @@ def pymain(argv, init=None):
                 '-O',   # optimize
                 '-v',   # trace import statements
                 '-X',   # set implementation-specific option
+                '-u',   # unbuffered mode
             ):
 
             # but keep `-X gpython.*` in user part of argv in case of reexec

gpython/gpython_test.py

@@ -353,6 +353,13 @@ def test_pymain_X():
     check_gpy_vs_py(['-X', 'faulthandler', 'testprog/print_faulthandler.py'], cwd=here)
 
 
+# pymain -u
+@gpython_only
+def test_pymain_u():
+    _check_gpy_vs_py([      'testprog/print_stdio_bufmode.py'], cwd=here)
+    _check_gpy_vs_py(['-u', 'testprog/print_stdio_bufmode.py'], cwd=here)
+
+
 # pymain -v
 @gpython_only
 def test_pymain_v():
@@ -478,3 +485,15 @@ def check_gpy_vs_py(argv, postprocessf=None, **kw):
             postprocessf(gpyoutv, stdpyoutv)
 
         assert gpyoutv == stdpyoutv
+
+# _check_gpy_vs_py verifies that gpython stdout/stderr match underlying python stdout/stderr.
+def _check_gpy_vs_py(argv, **kw):
+    kw = kw.copy()
+    kw['stdout'] = PIPE
+    kw['stderr'] = PIPE
+    gpyret, gpyout, gpyerr = _pyrun(argv, **kw)
+    stdret, stdout, stderr = _pyrun(argv, pyexe=sys._gpy_underlying_executable, **kw)
+
+    assert gpyout == stdout
+    assert gpyerr == stderr
+    assert (gpyret, stdret) == (0, 0)

gpython/testprog/print_stdio_bufmode.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2025  Nexedi SA and Contributors.
+#                     Kirill Smelkov <[email protected]>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Program print_stdio_bufmode prints information about stdout/stderr buffering mode."""
+
+from __future__ import print_function, absolute_import
+
+import sys, os
+
+
+def main():
+    null = os.open(os.devnull, os.O_WRONLY)
+
+    def check(subj, ioobj):
+        ioobj.write('%s: unbuffered if you see the next line; buffered otherwise\n' % subj)
+        ioobj.flush()
+        ioobj.write('%s: unbuffered' % subj)  # NOTE: no \n to avoid flush even on line-bufferring
+        os.close(ioobj.fileno())
+        os.dup2(null, ioobj.fileno())   # not to hit an error when ioobj is closed at the end
+
+    check('stdout', sys.stdout)
+    check('stderr', sys.stderr)
+
+
+if __name__ == '__main__':
+    main()