[Python-checkins] python/dist/src/Lib/plat-os2emx _emx_link.py, NONE, 1.1

Tue Dec 2 07:31:11 EST 2003

Update of /cvsroot/python/python/dist/src/Lib/plat-os2emx
In directory sc8-pr-cvs1:/tmp/cvs-serv5006
Added Files:
	_emx_link.py 
Log Message:
EMX lacks an implementation of link(). As Mailman wants os.link() to
implement its locking scheme, this module implements a crude link() by
way of copying the source to the destination provided the destination
doesn't already exist.
--- NEW FILE: _emx_link.py ---
# _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])