Re: rename("a", "b") would not always remove "a" on success. ?!!

From: Eric Blake
Date: Fri Oct 28 2011 - 11:42:18 EST

On 10/28/2011 09:32 AM, Pádraig Brady wrote:

'If the old argument and the new argument resolve to either .... or different
directory entries for the same existing file, rename() shall return
successfully and perform no other action.'

It's incredible they had audacity to put such nonsense into standard.

The page says in "RATIONALE" section:

'The specification that if old and new refer to the same file is
intended to guarantee that:

rename("x", "x");

does not remove the file.'

Why didn't they just explicitly say that they actually want THIS
particular case to work correctly, not OTHER cases to be fucked up?!

Because it is historical precedent, and changing it now would break software that has come to expect this behavior on hard links.

Anyway. My question is, does it really need to be implemented in Linux?
It looks bogus to me, and it basically requires any program
to contain a work-around for this case. For example, mv from util-linux
apparently already has a workaround:

$ touch a; ln a b
$ strace mv a b
stat64("b", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lstat64("a", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lstat64("b", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
geteuid32() = 0
unlink("a") = 0
close(0) = 0
close(1) = 0
close(2) = 0
exit_group(0) = ?

mv is from coreutils BTW.
Here is the related comment from the source:

"Set *UNLINK_SRC if we've determined that the caller wants to do
`rename (a, b)' where `a' and `b' are distinct hard links to the same
file. In that case, the caller should try to unlink `a' and then return
successfully. Ideally, we wouldn't have to do that, and we'd be
able to rely on rename to remove the source file. However, POSIX
mistakenly requires that such a rename call do *nothing* and return

Perhaps it could be brought up as an issue with the standards guys?

We already have. And POSIX 2008 already acted on that. While you quoted rename(2) (which was intentionally not changed), you forgot to also read the POSIX wording on mv(1):

2. If the source_file operand and destination path name the same existing file, then the destination path shall not be removed, and one of the following shall occur:

a. No change is made to source_file, no error occurs, and no diagnostic is issued.
b. No change is made to source_file, a diagnostic is issued to standard error identifying the two names, and the exit status is affected.
c. If the source_file operand and destination path name distinct directory entries, then the source_file operand is removed, no error occurs, and no diagnostic is issued.

2a is the naive approach (using rename(2) semantics)
2b is the typical mv(1) approach (IIRC, both Solaris and BSD mv independently implemented this mode)
2c is the GNU coreutils approach (make mv(1) do what you meant, even though rename(2) is _required_ to do nothing)

Do NOT change the Linux kernel to "fix" rename(2); that will only cause more heartache by deviating from the standard.

Eric Blake eblake@xxxxxxxxxx +1-801-349-2682
Libvirt virtualization library
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