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: Sat, 6 Jun 2020 04:55:31 +0000 [thread overview]
Message-ID: <20200606045531.4b506c8e@tarpaulin.shahaf.local2> (raw)
In-Reply-To: <CAH+w=7ZHapcvCZ=QmV2QEBxi1HGwfKOd4dJ47Ndi+5R-V2VDSg@mail.gmail.com>
Bart Schaefer wrote on Fri, 05 Jun 2020 18:41 -0700:
> On Thu, Jun 4, 2020 at 9:39 PM Roman Perepelitsa
> <roman.perepelitsa@gmail.com> wrote:
> >
> > 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"
>
> I don't think that's the intended typical usage of noclobber. It's
> not set by default, and it can't have any effect outside the local
> shell. It's meant to keep you from making silly mistakes when
> interacting with the interpreter, not as a concurrent programming
> tool.
NO_CLOBBER causes open() to be called with the O_EXCL bit, which does
affect other processes as well.
> Nevertheless:
>
> > Is there a way to provide these guarantees with clobber_empty?
>
I don't understand the question, really. If you can get these
semantics with NO_CLOBBER, why does it matter whether or not
CLOBBER_EMPTY can provide them or not? You can always leave it off (or
locally unset it).
Or use mkdir without the -p flag as a poor man's mutex.
> With clobber, you have #2, but not #1,
Doesn't O_EXCL achieve #1?
> so I think we should focus on
> whether clobber_empty can guarantee #2. If you want #1, there are
> other ways (but I suspect it arises from the implementation of #2 even
> so).
>
> I think the answer is that #2 can be guaranteed if and only if
> fcntl()-based locking can be applied, or some equivalent kernel
> mechanism such that no more than closing of the (last dup of the)
> descriptor is necessary to release the lock. The steps would have to
> be:
>
> 1. Open the file for write without truncate.
> 2. Attempt lock, non-blocking, and immediately close/fail if not locked.
> 3. Check for zero size, and immediately unlock/close/fail when nonzero.
> 4. Return the locked descriptor.
Cheers,
Daniel
next prev parent reply other threads:[~2020-06-06 4:56 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 [this message]
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
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