# _emx_link.py

# Written by Andrew I MacIntyre, December 2002.

"""_emx_link.py is a simplistic emulation of the Unix link(2) library routine

for creating so-called hard links.  It is intended to be imported into

the os module in place of the unimplemented (on OS/2) Posix link()

function (os.link()).

We do this on OS/2 by implementing a file copy, with link(2) semantics:-

  - the target cannot already exist;

  - we hope that the actual file open (if successful) is actually

    atomic...

Limitations of this approach/implementation include:-

  - no support for correct link counts (EMX stat(target).st_nlink

    is always 1);

  - thread safety undefined;

  - default file permissions (r+w) used, can't be over-ridden;

  - implemented in Python so comparatively slow, especially for large

    source files;

  - need sufficient free disk space to store the copy.

Behaviour:-

  - any exception should propagate to the caller;

  - want target to be an exact copy of the source, so use binary mode;

  - returns None, same as os.link() which is implemented in posixmodule.c;

  - target removed in the event of a failure where possible;

  - given the motivation to write this emulation came from trying to

    support a Unix resource lock implementation, where minimal overhead

    during creation of the target is desirable and the files are small,

    we read a source block before attempting to create the target so that

    we're ready to immediately write some data into it.

"""

import os

import errno

__all__ = ['link']

def link(source, target):

    """link(source, target) -> None

    Attempt to hard link the source file to the target file name.

    On OS/2, this creates a complete copy of the source file.

    """

    s = os.open(source, os.O_RDONLY | os.O_BINARY)

    if os.isatty(s):

        raise OSError, (errno.EXDEV, 'Cross-device link')

    data = os.read(s, 1024)

    try:

        t = os.open(target, os.O_WRONLY | os.O_BINARY | os.O_CREAT | os.O_EXCL)

    except OSError:

        os.close(s)

        raise

    try:

        while data:

            os.write(t, data)

            data = os.read(s, 1024)

    except OSError:

        os.close(s)

        os.close(t)

        os.unlink(target)

        raise

    os.close(s)

    os.close(t)

if __name__ == '__main__':

    import sys

    try:

        link(sys.argv[1], sys.argv[2])

    except IndexError:

        print 'Usage: emx_link <source> <target>'

    except OSError:

        print 'emx_link: %s' % str(sys.exc_info()[1])