Re: [PATCH 1/6] tests/qemu-iotests: Improve the check for GNU sed

[Eric Blake]

On Tue, Feb 08, 2022 at 03:52:19PM +0100, Thomas Huth wrote:
> > > The current code with $SED has been introduced almost three years
> > > ago already...
> > > 
> > > >   Can’t we just do `alias sed=gsed`?
> > > 
> > > Maybe ... but let's ask Philippe and Kevin first, who Signed-off
> > > commit bde36af1ab4f476 that introduced the current way with $SED:
> > > What's your opinion about this?
> > 
> > This commit was to have check-block working on the OpenBSD VM image.
> 
> Sure. The question was whether using an alias as suggested by Hanna would be
> nicer instead of using $SED ?

Scripting with aliases becomes a nightmare to debug, since it is
relatively uncommon.  In particular, in bash, you have to explicitly
opt in to using aliases (contrary to POSIX sh where aliases are
available to scripts at startup).  Using $SED everywhere may require
more hunting, but it is more obvious when reading a test that "oh
yeah, I might be using extensions that the default 'sed' can't
support" than a script that blindly uses 'sed' and depends on it
aliasing to a more-capable sed at a distance.

The other question is how many GNU sed features are we actually
depending on?  Which tests break if we have BSD sed or busybox sed?
Can we rewrite those sed scripts to avoid GNU extensions?  But
auditing for s/sed/$SED/ seems easier than auditing for which
non-portable sed extensions we depend on.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org