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From: Roman Perepelitsa <roman.perepelitsa@gmail.com>
Date: Fri, 5 Jun 2020 06:38:46 +0200
Message-ID: <CAN=4vMoC8aNBz=ZAW-BdA9W6MaBXN3SNBPpZZQobVRwjUg2wGQ@mail.gmail.com>
Subject: Re: Any way to allow clobbering empty files when noclobber is set?
To: Daniel Shahaf <d.s@daniel.shahaf.name>
Cc: "zsh-workers@zsh.org" <zsh-workers@zsh.org>, Martin Tournoij <martin@arp242.net>
Content-Type: text/plain; charset="UTF-8"

On Fri, Jun 5, 2020 at 4:07 AM Daniel Shahaf <d.s@daniel.shahaf.name> wrote:
>
> Roman Perepelitsa wrote on Thu, 04 Jun 2020 07:41 +0200:
>
> > 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.

Consider the following program:

    write() print -rn -- $1
    rm -f foo
    write hello >foo &
    write bye >foo &
    wait

With regular no_clobber it has the following guarantees:

1. `write` is executed exactly once
2. once `wait` completes, `foo` contains either "hello" or "bye"

Is there a way to provide these guarantees with clobber_empty? (2) is
especially important. Seeing "byelo" or "hellobye" in `foo` would be
quite confusing. It would look like clobbering.

Roman.