This package is needed in QA tests of telemetry.

gps, i was told you would probably know what needs to be done here. We are adding python-/marionette-based testing which partially depends on the lz4 module. Apparently we can use the <obj>/_virtualenv setup we have in-tree for that. What steps are needed to get the lz4 module covered by that too?

See https://hg.mozilla.org/mozilla-central/rev/c1cbf483a450 and the build/virtualenv_packages.txt file. Although this may be slightly more complicated than that: IIRC lz4 requires a Python C extension and we currently don't guarantee that all clients are able to build Python C extensions. The one Python C extension I know we use in-tree (psutil) is treated as optional by most tools. Requiring a Python C extension for local builds could be a PITA. For automation, we'll likely need to upload some packaged wheels for Windows (OS X and Linux are able to build Python C extensions). This was recently done in bug 1192971. Since this will be testing only, this probably entails adding the package to the testing environment via mozharness changes + releng wheel deploy and adding detection of the package / skipping so local test runs behave properly. You've found yourself in a hornet's nest.

Ok, thanks - that sounds like a not-so-productive road right now. We'll shift the validations for now and might revisit this later.