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

[Thomas Huth]

On 11/02/2022 17.14, Eric Blake wrote:
> 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).

shopt -s expand_aliases
... as I just learnt the hard way ;-)

> 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.

The most obvious part are the filter functions in common.filter - we're 
using "-r" here that is not part of the POSIX sed as far as I can see.

Not sure whether anybody really wants to rewrite all sed statements for full 
portability, but maybe we could also introduce a wrapper for GNU sed like this:

if ! command -v gsed >/dev/null 2>&1; then
    if sed --version | grep -v 'not GNU sed' | grep 'GNUx sed' \
       > /dev/null 2>&1;
    then
        gsed()
        {
            sed $*
        }
    else
        gsed()
        {
            _notrun "GNU sed not available"
        }
    fi
fi

... then we could simply use "gsed" everywhere we depend on the GNU 
behavior, and the tests don't look as ugly as with the $SED ?

 Thomas