[ python-Bugs-1546442 ] subprocess.Popen can't read file object as stdin after seek

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Bugs item #1546442, was opened at 2006-08-25 15:52
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: GaryD (gazzadee)
Assigned to: Nobody/Anonymous (nobody)
Summary: subprocess.Popen can't read file object as stdin after seek

Initial Comment:
When I use an existing file object as stdin for a call
to subprocess.Popen, then Popen cannot read the file if
I have called seek on it more than once.

eg. in the following python code:

>>> import subprocess
>>> rawfile = file('hello.txt', 'rb')
>>> rawfile.readline()
'line 1\n'
>>> rawfile.seek(0)
>>> rawfile.readline()
'line 1\n'
>>> rawfile.seek(0)
>>> process_object = subprocess.Popen(["cat"],
stdin=rawfile, stdout=subprocess.PIPE,
stderr=subprocess.PIPE)

process_object.stdout now contains nothing, implying
that nothing was on process_object.stdin.

Note that this only applies for a non-trivial seek (ie.
where the file-pointer actually changes). Calling
seek(0) multiple times in a row does not change
anything (obviously).

I have not investigated whether this reveals a problem
with seek not changing the underlying file descriptor,
or a problem with Popen not handling the file
descriptor properly.

I have attached some complete python scripts that
demonstrate this problem. One shows cat working after
calling seek once, the other shows cat failing after
calling seek twice.

Python version being used:
Python 2.4.2 (#1, Nov  3 2005, 12:41:57)
[GCC 3.4.3-20050110 (Gentoo Linux 3.4.3.20050110,
ssp-3.4.3.20050110-0, pie-8.7 on linux2


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