Working on some deduplication code, I want do my my best at
performing an atomic re-hard-linking atop an existing file, akin to
"ln -f source.txt dest.txt"
However, when I issue
os.link("source.txt", "dest.txt")
it fails with an OSError (EEXISTS). This isn't surprising as it's
documented. Unfortunately, os.link doesn't support something like
os.link("source.txt", "dest.txt", force=True)
However, I don't want to
os.unlink("dest.txt")
os.link("source.txt", "dest.txt")
in the event the power goes out between the unlink() and the link(),
leaving me in a state where dest.txt is deleted but the link hasn't
yet happened.
So my plan was to do something like
temp_name = tempfile.mktemp(dir=DIRECTORY_CONTAINING_SOURCE_TXT)
os.link("source.txt", temp_name)
try:
os.rename(temp_name, "dest.txt") # docs guarantee this is atomic
except OSError:
os.unlink(temp_name)
There's still the potential leakage if a crash occurs, but I'd rather
have an extra hard-link floating around than lose an original
file-name.
Unfortunately, tempfile.mktemp() is described as deprecated
since 2.3 (though appears to still exist in the 3.4.2 that is the
default Py3 on Debian Stable). While the deprecation notice says
"In version 2.3 of Python, this module was overhauled for enhanced
security. It now provides three new functions, NamedTemporaryFile(),
mkstemp(), and mkdtemp(), which should eliminate all remaining need
to use the insecure mktemp() function", as best I can tell, all of
the other functions/objects in the tempfile module return a file
object, not a string suitable for passing to link().
So which route should I pursue?
- go ahead and use tempfile.mktemp() ignoring the deprecation?
- use a GUID-named temp-file instead for less chance of collision?
- I happen to already have a hash of the file contents, so use
the .hexdigest() string as the temp-file name?
- some other solution I've missed?
Thanks,
-tkc
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