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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Feb 15, 2022 at 02:28:24PM +0100, Thomas Huth wrote:
> On 11/02/2022 17.48, Thomas Huth wrote:
> > 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.
> 
> Now that I stepped through the list, the other major part that is failing on
> non-GNU seds are the statements that use "\r" or "\n" or "\e" to replace
> special characters. That seems to be a non-POSIX extension, too.
> 
> But for running with Alpine, there is also the additional problems that the
> libc uses slightly different error strings, e.g. "I/O error" instead of
> "Input/output error", see e.g.:
> 
>  https://gitlab.com/thuth/qemu/-/jobs/2094869856
> 
> Maybe it could be fixed with some extensions to the filters, but I'm not
> sure whether we really want to go down that road...?

AFAIK, errno strings are not standardized by POSIX, so I presume this
problem will apply to running I/O tests on any non-Linux system too.

With this in mind I think we should consider what a portable solution
looks like. We can't simply match the Alpine strings and turn them
into GLibC strings, as that does nothing to help portability on *BSD,
macOS, Windows, etc. We would need to figure out how to blank out
arbitrary input error message strings.

Regards,
Daniel
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