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

[Daniel Shahaf <d.s@xxxxxxxxxxxxxxxxxx>]

Roman Perepelitsa wrote on Thu, 04 Jun 2020 07:41 +0200:
> The regular noclobber can use O_CREAT | O_EXCL to avoid races. How
> would any of the proposed modifications work? That is, how to
> implement unlink-if-empty without ever unlinking non-empty files?

I don't think that's possible: it'd require an atomic
stat-and-conditionally-unlink operation, and I don't think there's
a syscall for this.  fstatat(2)-then-unlinkat(2) gets close, but it's
still racy.

Good point.

> Or how to implement clobber-if-empty without ever clobbering non-empty
> files?

By using a non-clobbering open(), then calling fstat() on the open fd.
If it's zero length, just carry on.  If it's not zero length, close()
it and report an error.

Cheers,

Daniel