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From: Daniel Shahaf <d.s@daniel.shahaf.name>
To: Bart Schaefer <schaefer@brasslantern.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 01:59:48 +0000	[thread overview]
Message-ID: <20200605015948.0c26ca38@tarpaulin.shahaf.local2> (raw)
In-Reply-To: <CAH+w=7Yk6gbTKjzQ-N91X_hR_24wnBbxE+7TqrtoTc6cyocqmQ@mail.gmail.com>

Bart Schaefer wrote on Thu, 04 Jun 2020 13:35 -0700:
> 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.

This wouldn't prevent races, because the filename might be unlinked
between the first and second open(2) calls.

I think the right fix here is to simply change the body of the quoted
if statement to «return fd».  What do we gain by re-opening the file
with the O_TRUNC flag set when we already know the file is
zero-length?  It seems to me we're just introducing a race condition
for no reason.

      reply	other threads:[~2020-06-05  2:00 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
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 [this message]

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