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Date: Thu, 15 Oct 2020 09:04:00 +0200
From: Fabian =?iso-8859-1?q?Gr=FCnbichler?= <f.gruenbichler@proxmox.com>
To: Proxmox Backup Server development discussion
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Subject: Re: [pbs-devel] [PATCH proxmox-backup 08/11] datastore: add
 manifest locking
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On October 15, 2020 7:25 am, Dietmar Maurer wrote:
> AFAIR flock with atomically replaced files has problems/races.
>=20
> Unfortunately, I do not remember why. Somebody remember why? Or is it rea=
lly safe?

you can obtain the flock a second time after replacing the original=20
(still locked) file. using a separate lock file that is not touched=20
other than for flocking solves this problem.

$ flock --verbose testfile -c 'sleep 60'
flock: getting lock took 0.000003 seconds
flock: executing /bin/zsh

while that is running and holding the lock:

$ flock --verbose --wait 5 testfile -c 'sleep 60'
flock: timeout while waiting to get lock

$ mv testfile2 testfile
$ flock --verbose --wait 5 testfile -c 'sleep 60'
flock: getting lock took 0.000003 seconds
flock: executing /bin/zsh
=