[issue39318] NamedTemporaryFile could cause double-close on an fd if _TemporaryFileWrapper throws

[Robert Xiao]

New submission from Robert Xiao <nneonneo at gmail.com>:

tempfile.NamedTemporaryFile creates its wrapper like so:

    try:
        file = _io.open(fd, mode, buffering=buffering,
                        newline=newline, encoding=encoding, errors=errors)

        return _TemporaryFileWrapper(file, name, delete)
    except BaseException:
        _os.unlink(name)
        _os.close(fd)
        raise

If _TemporaryFileWrapper throws any kind of exception (even KeyboardInterrupt), this closes `fd` but leaks a valid `file` pointing to that fd. The `file` will later attempt to close the `fd` when it is collected, which can lead to subtle bugs. (This particular issue contributed to this bug: https://nedbatchelder.com/blog/202001/bug_915_please_help.html)

This should probably be rewritten as:

    try:
        file = _io.open(fd, mode, buffering=buffering,
                        newline=newline, encoding=encoding, errors=errors)
    except:
        _os.unlink(name)
        _os.close(fd)
        raise

    try:
        return _TemporaryFileWrapper(file, name, delete)
    except BaseException:
        _os.unlink(name)
        file.close()
        raise

or perhaps use nested try blocks to avoid the _os.unlink duplication.

----------
components: Library (Lib)
messages: 359888
nosy: nneonneo
priority: normal
severity: normal
status: open
title: NamedTemporaryFile could cause double-close on an fd if _TemporaryFileWrapper throws
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39318>
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