Re: Any way to allow clobbering empty files when noclobber is set?

[Peter Stephenson <p.w.stephenson@ntlworld.com>]

On Sun, 2020-06-07 at 11:55 +0000, Daniel Shahaf wrote:
> My point here is really just the one I already made in 45976, and
> wasn't answered there: can't we avoid the close()-then-open() sequence
> that 45968 does?  That one seems to be an _avoidable_ race condition,
> unlike the above ones.

I think what it boils down to here is either you test using fstat() the
the file is empty, or you re-open using O_TRUNC, both of which ensure
the file is empty at that point.  Then at some later point, the file
will be written to.  Between the two it's there but empty, which is
unavoidable.  So my last change, closing and opening with O_TRUNC,
doesn't really gain anything over leaving it open after checking the
size with fstat(), I don't think, in which case the extra open() is
redundant and best removed (and that puts us in the fortuitous position
where we haven't actually added any system calls in adding
CLOBBER_EMPTY).  But I don't think taking it out actually removes a
race.

pws