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From: Bart Schaefer <schaefer@brasslantern.com>
Date: Thu, 4 Jun 2020 13:35:22 -0700
Message-ID: <CAH+w=7Yk6gbTKjzQ-N91X_hR_24wnBbxE+7TqrtoTc6cyocqmQ@mail.gmail.com>
Subject: Re: Any way to allow clobbering empty files when noclobber is set?
To: "zsh-workers@zsh.org" <zsh-workers@zsh.org>
Cc: Martin Tournoij <martin@arp242.net>
Content-Type: text/plain; charset="UTF-8"

On Thu, Jun 4, 2020 at 5:16 AM Peter Stephenson
<p.w.stephenson@ntlworld.com> wrote:
>
> One thing I missed is that we already open the file and run fstat to
> check if it's regular.  We can simply check if it's empty at the
> same point

Exactly where my earlier question came from.

> +           if (isset(CLOBBEREMPTY) && buf.st_size == 0)
> +           {
> +               close(fd);
> +               return open(ufname, O_WRONLY | O_CREAT | O_TRUNC | O_NOCTTY,
> +                           0666);
> +           }

Considering the concurrent-openers situation that Roman mentioned, I'm
debating whether there is any benefit to doing:

  int newfd = open(ufname, O_WRONLY | O_CREAT | O_TRUNC | O_NOCTTY, 0666);
  close(fd);
  return newfd;

I have a vague sense that keeping the descriptor open until the new
file is created might prevent some races, but I can't recite an
example.