subprocess.Popen.stdout.flush fails os OS-X 10.6.1

[petegibson]

BPO	7240
Nosy	@ronaldoussoren, @pjenvey, @ned-deily

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ronaldoussoren'
closed_at = <Date 2009-11-03.20:57:24.046>
created_at = <Date 2009-10-30.05:15:47.705>
labels = ['OS-mac', 'invalid', 'library', 'type-crash']
title = 'subprocess.Popen.stdout.flush fails os OS-X 10.6.1'
updated_at = <Date 2009-11-03.20:57:24.045>
user = 'https://bugs.python.org/petegibson'

bugs.python.org fields:

activity = <Date 2009-11-03.20:57:24.045>
actor = 'pjenvey'
assignee = 'ronaldoussoren'
closed = True
closed_date = <Date 2009-11-03.20:57:24.046>
closer = 'pjenvey'
components = ['Library (Lib)', 'macOS']
creation = <Date 2009-10-30.05:15:47.705>
creator = 'petegibson'
dependencies = []
files = []
hgrepos = []
issue_num = 7240
keywords = []
message_count = 5.0
messages = ['94694', '94695', '94696', '94705', '94858']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'pjenvey', 'ned.deily', 'petegibson']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue7240'
versions = ['Python 2.6']

[petegibson]

subprocess.Popen.stdout.flush() fails on OS-X 10.6.1 under the bundled
Python 2.6.1 and 2.6.3 from Macports.

>>> from subprocess import Popen, PIPE
>>> p = Popen('cat', stdin=PIPE, stdout=PIPE)
>>> p.stdout.flush()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor

However it works on Python 2.6.2 on Linux.

[pjenvey]

Why are you flushing stdout? It's read-only and flush is for writing. This
behavior is dependent on the underlying platform's fflush, which really
*should* be raising EBADF when fflushing a read only file, anyway

[ned-deily]

Philip is correct:

>>> p.stdout.flush()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor
>>> p.stdout
<open file '<fdopen>', mode 'rb' at 0x100527470>

You'll get the same error on OS X (at least as far back as Python 2.3.5
on OS X 10.4) if you try to flush a disk read-only file:

>>> f = open('a.txt', 'rb')
>>> f.flush()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IOError: [Errno 9] Bad file descriptor

Note, both the OS X and Linux 2.6 fflush(3) man pages clearly state that
EBADF can be returned if the stream is not open for writing but there
seems to be a difference in behavior between the two OS's.

As this doesn't seem to be a new issue and can easily be avoided (don't
flush a read-only file), I suggest closing the issue.

[petegibson]

Not my code, but as it's using a pipe to communicate with another
process, I assume that the flush call is intended to discard any
unwanted output prior to sending a command and processing the result.

Is there another way to achieve the same effect, such as reading and
remaining characters in the buffer?

[ronaldoussoren]

I don't think so. stdio on OSX has a fdiscard function, but that's not
exposed to Python.

I tend to explicitly synchronize on prompts when communicating with an
interactive program over a pipe. That is, read until you found the prompt,
then send a command, then read again until getting to the prompt, ...