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

[Richard W.M. Jones]

On Wed, Mar 21, 2018 at 03:58:05PM -0500, Eric Blake wrote:
> On 03/21/2018 03:44 PM, Kevin Wolf wrote:
> >>>
> >>>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:
> 
> Is it a case where file locking actually works, and more than one
> process is trying to lock /dev/null at once?  (qemu-img info is
> short-lived, but could there be another longer-lived process also
> using /dev/null)?
> 
> Does using -r help (if the only reason you're telling qemu-img to
> operate on /dev/null is to probe qemu-img features, can you probe
> those same features without needing to write, which in turn requests
> less locking)?

The original test (before Pino's patch) runs ‘qemu-img info /dev/null’
purely as a way to fork qemu-img.  It's a regression test for some
problem we had with the code that confines qemu-img using resource
limits.  So really nothing much to do with qemu-img at all.  The fix
is to create a temporary file and run qemu-img against that.

Rich.

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