zsh-workers
 help / color / mirror / code / Atom feed
From: Daniel Shahaf <d.s@daniel.shahaf.name>
To: Roman Perepelitsa <roman.perepelitsa@gmail.com>
Cc: "zsh-workers@zsh.org" <zsh-workers@zsh.org>,
	Martin Tournoij <martin@arp242.net>
Subject: Re: Any way to allow clobbering empty files when noclobber is set?
Date: Fri, 5 Jun 2020 02:07:48 +0000	[thread overview]
Message-ID: <20200605020748.635b9bb3@tarpaulin.shahaf.local2> (raw)
In-Reply-To: <CAN=4vMqBCZqD04QO-hw0DwipmCVkdq+QNOEGNaSEfNxyRBqiiw@mail.gmail.com>

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

  reply	other threads:[~2020-06-05  2:08 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20200603020919eucas1p13e26ebcbb335784d14bfb97b137f385a@eucas1p1.samsung.com>
2020-06-03  2:08 ` Martin Tournoij
2020-06-03 12:04   ` Peter Stephenson
2020-06-04  1:48     ` Daniel Shahaf
2020-06-04  2:43       ` Bart Schaefer
2020-06-04  4:06         ` Daniel Shahaf
2020-06-04  5:00           ` Bart Schaefer
2020-06-05  3:10             ` Daniel Shahaf
2020-06-05  3:18               ` Daniel Shahaf
2020-06-06  1:07               ` Bart Schaefer
2020-06-06  4:48                 ` Daniel Shahaf
2020-06-06  7:04                   ` Bart Schaefer
2020-06-04  6:31       ` Martin Tournoij
2020-06-05  2:22         ` Daniel Shahaf
2020-06-04  2:13     ` Vin Shelton
2020-06-04  2:35       ` Bart Schaefer
2020-06-04  2:36       ` Daniel Shahaf
2020-06-04 11:57         ` Vin Shelton
2020-06-04  5:06     ` Bart Schaefer
2020-06-04  5:41       ` Roman Perepelitsa
2020-06-05  2:07         ` Daniel Shahaf [this message]
2020-06-05  4:38           ` Roman Perepelitsa
2020-06-06  1:41             ` Bart Schaefer
2020-06-06  4:55               ` Daniel Shahaf
2020-06-06  6:25                 ` Roman Perepelitsa
2020-06-06  7:08                 ` Bart Schaefer
2020-06-06  8:03                   ` Daniel Shahaf
     [not found]           ` <1941572212.466119.1591360860372@mail2.virginmedia.com>
     [not found]             ` <e7f7dfe2-eb4a-457b-85fb-091935a74c0e@www.fastmail.com>
2020-06-06 11:57               ` Peter Stephenson
2020-06-06 12:48                 ` Roman Perepelitsa
2020-06-06 15:24                   ` Bart Schaefer
2020-06-06 16:24                     ` Peter Stephenson
2020-06-07 11:55                       ` Daniel Shahaf
2020-06-07 17:00                         ` Peter Stephenson
2020-06-08  3:27                           ` Daniel Shahaf
2020-06-08  9:30                             ` Peter Stephenson
2020-06-07 17:20                         ` Bart Schaefer
2020-06-06 15:09                 ` Bart Schaefer
2020-06-04  6:47       ` Martin Tournoij
2020-06-04  9:42       ` Peter Stephenson
2020-06-04 12:20         ` Roman Perepelitsa
2020-06-04 12:26           ` Peter Stephenson
2020-06-04 12:15     ` Peter Stephenson
2020-06-04 20:35       ` Bart Schaefer
2020-06-05  1:59         ` Daniel Shahaf

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200605020748.635b9bb3@tarpaulin.shahaf.local2 \
    --to=d.s@daniel.shahaf.name \
    --cc=martin@arp242.net \
    --cc=roman.perepelitsa@gmail.com \
    --cc=zsh-workers@zsh.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://git.vuxu.org/mirror/zsh/

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).