Re: [Qemu-devel] [Qemu-block] [Libguestfs] [PATCH] tests: regressions: make test-big-heap use a temporary empty file

[Kevin Wolf <kwolf@redhat.com>]

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Am 21.03.2018 um 14:48 hat Pino Toscano geschrieben:
> On Wednesday, 21 March 2018 14:45:38 CET Eric Blake wrote:
> > [adding qemu lists]
> > 
> > On 03/21/2018 07:51 AM, Richard W.M. Jones wrote:
> > > On Wed, Mar 21, 2018 at 01:44:17PM +0100, Pino Toscano wrote:
> > >> Newer versions of qemu use file locking for the images by default, and
> > >> apparently that does not work with /dev/null.  Since this test just
> > >> calls qemu-img to get the format of an empty image, create a temporary
> > >> one instead.
> > > 
> > > ACK, but feels like this is a bug in qemu-img to me.
> > 
> > You're right that file locking on a character device like /dev/null is 
> > not going to work as expected, but is it a case where fcntl() actually 
> > fails, or is it worse where the fcntl() claiming the locks "succeeds" 
> > but doesn't do what we want?  That is, what were the actual error 
> > messages you ran into?
> 
> $ qemu-img --version
> qemu-img version 2.10.1(qemu-2.10.1-2.fc27)
> Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
> $ qemu-img info /dev/null 
> qemu-img: Could not open '/dev/null': Failed to get "consistent read" lock
> Is another process using the image?

Not sure where the difference is, but I can't reproduce this on
upstream, neither git master nor the v2.10.1 tag:

$ ./qemu-img --version
qemu-img version 2.10.1 (v2.10.1-dirty)
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
$ ./qemu-img info /dev/null
image: /dev/null
file format: raw
virtual size: 0 (0 bytes)
disk size: 0

Also with strace:

open("/dev/null", O_RDONLY|O_CLOEXEC)   = 10
fcntl(10, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, l_len=1}) = 0
...

So my kernel (4.15.9-200.fc26.x86_64) seems to have no problems with
locking /dev/null.

Kevin

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