As suggested by Toshio Kuratomi at https://bugs.python.org/issue36656, I
am raising this here for inclusion in the shutil module.
Mimicking POSIX, os.symlink() will raise FileExistsError if the link
name to be created already exists.
A common use case is overwriting an existing file (often a symlink) with
a symlink. Naively, one would delete the file named link_name file if it
exists, then call symlink(). This "solution" is already 3 lines of code,
and without exception handling it introduces the race condition of a
file named link_name being created between unlink and symlink.
Depending on the functionality required, I suggest:
* os.symlink() - the new link name is expected to NOT exist
* shutil.symlink() - the new symlink replaces an existing file
Handling all possible race conditions (some detailed in issue36656) is
non-trivial, however this is the best that I have come up with so far:
==========================================================================
import os, tempfile
def symlink(target, link_name):
'''Create a symbolic link link_name pointing to target.
Overwrites link_name if it exists. '''
# os.replace() may fail if files are on different filesystems
link_dir = os.path.dirname(link_name)
# Link to a temporary filename that doesn't exist
while True:
temp_link_name = tempfile.mktemp(dir=link_dir)
# os.* functions mimic as closely as possible system functions
# The POSIX symlink() returns EEXIST if link_name already exists
#
https://pubs.opengroup.org/onlinepubs/9699919799/functions/symlink.html
try:
os.symlink(target, temp_link_name)
break
except FileExistsError:
pass
# Replace link_name with temp_link_name
try:
# Pre-empt os.replace on a directory with a nicer message
if os.path.isdir(link_name):
raise IsADirectoryError(f"Cannot symlink over existing
directory: '{link_name}'")
os.replace(temp_link_name, link_name)
except:
if os.path.islink(temp_link_name):
os.remove(temp_link_name)
raise
==========================================================================
The documentation (https://docs.python.org/3/library/shutil.html) I
suggest for this is:
shutil.symlink(target, link_name)
Create a symbolic link named link_name pointing to target, overwriting
target if it exists. If link_name is a directory, IsADirectoryError is
raised. To not overwrite target, use os.symlink()
==========================================================================
It would be tempting to do:
while True:
try:
os.symlink(target, link_name)
break
except FileExistsError:
os.remove(link_name)
But this has a race condition when replacing a symlink should should
*always* exist, eg:
/lib/critical.so -> /lib/critical.so.1.2
When upgrading by:
symlink('/lib/critical.so.2.0', '/lib/critical.so')
There is a point in time when /lib/critical.so doesn't exist.
==========================================================================
One issue I see with my suggested code is that the file at
temp_link_name could be changed before target is replaced with it. This
is mitigated by the randomness introduced by mktemp().
While it is far less likely that a file is accessed with a random and
unknown name than with an existing known name, I seek input on a
solution if this is an unacceptable risk.
Prior art:
* https://bugs.python.org/issue36656 (already mentioned above)
* https://stackoverflow.com/a/55742015/5353461
* https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/ln.c
--
Tom Hale
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