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

From: Eric Blake
Date: Fri Oct 28 2011 - 11:58:35 EST

On 10/28/2011 09:42 AM, Eric Blake wrote:
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.

Contrast this to the POSIX 2001 wording for mv:

2. The mv utility shall perform actions equivalent to the rename() function defined in the System Interfaces volume of IEEE Std 1003.1-2001,...

with no escape clause for identical files. That is, POSIX 2008 specifically inserted a new step 2 (renumbering the step 2 from POSIX 2001 to step 3 in POSIX 2008) at GNU Coreutils' insistence that the requirements were wrong. See also, and search for AI-169, which was where the change in wording was made; including this rationale:

Note that ERN 88 was rejected in spite of the fact that
claims the original intent was to have rename detect identical
directory entries rather than identical files. One of the arguments
presented for rejecting ERN 88 was that rename(2) has the property
of preserving link count, whereas having rename("a", "b") unlink
"a" when the two names are hard links would not.

Unfortunately, that link no longer works, so I can't point you to the historical conversation at the time where it was argued that changing rename(2) behavior would be detrimental. But it does give enough context to provide an alternate interpretation of rename() semantics that fit the standardized behavior: a successful rename("a","b") guarantees that "b" now has the same link count as "a" had prior to the rename call, even if that meant a hard-linked "a" had to be left untouched to avoid changing the link count of hard-linked "b".

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