akira added the comment: It looks like a bug in the subprocess module e.g., if child process does:
sys.stdout.write(sys.stdin.readline()) sys.stdout.flush() and the parent: p.stdin.write(line) #NOTE: no flush line = p.stdout.readline() then a deadlock may happen with bufsize=1 (because it is equivalent to bufsize=-1 in the current code) Surprisingly, it works if universal_newlines=True but only for C implementation of io i.e., if C extension is disabled: import io, _pyio io.TextIOWrapper = _pyio.TextIOWrapper then it blocks forever even if universal_newlines=True with bufsize=1 that is clearly wrong (<-- bug) e.g., `test_universal_newlines` deadlocks with _pyio.TextIOWrapper C implementation works because it sets `needflush` flag even if only `write_through` is provided [1]: if (self->write_through) needflush = 1; else if (self->line_buffering && (haslf || PyUnicode_FindChar(text, '\r', 0, PyUnicode_GET_LENGTH(text), 1) != -1)) needflush = 1; [1]: http://hg.python.org/cpython/file/0b2e199ad088/Modules/_io/textio.c#l1333 Python io implementation doesn't flush with only `write_through` flag. It doesn't deadlock if bufsize=0 whether universal_newlines=True or not. Note: Python 2.7 doesn't deadlock with bufsize=0 and bufsize=1 in this case as expected. What is not clear is whether it should work with universal_newline=False and bufsize=1: both current docs and Python 2.7 behaviour say that there should not be a deadlock. I've updated the docs to mention that bufsize=1 works only with universal_newlines=True and added corresponding tests. I've also updated the subprocess' code to pass line_buffering explicitly. Patch is uploaded. ---------- keywords: +patch type: -> behavior Added file: http://bugs.python.org/file35109/subprocess-line-buffering-issue21332.patch _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue21332> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com