[docs] Improve guidance on installing the correct buildbot-worker version

[asb]

Summary

The docs currently state that buildbot-worker 2.8.5 is used. However, looking at the buildbot release notes, this release doesn't seem to exist (it's not on PyPI either). The document also links to http://trac.buildbot.net/ as a source to retrieve an old version and install it manually, but this is dead.

Instead, drop that advice and suggest pip3 install buildbot-worker==2.8.4, as done by the Dockerfiles in llvm-zorg/buildbot/google/docker/*.

[gkistanova]

More recent versions of buildbot-worker would work too. But you are right, we shall recommend what would work for sure and what would require less effort to setup.

[asb]

Thanks, although I have a follow-up question I've committed as this patch is at least an incremental improvement. Is the recommendation of a certain buildbot version a holdover from an earlier time that should be removed? If we expect the latest version of buildbot-worker to "just work", perhaps we should recommend that?

[gkistanova]

There is no promise of forward compatibility as far as I know. Thus far I have seen working workers from version 0.8.4 to version 3.7.0. I have also seen people setting up workers from the source code and not any particular release. All this is fine. I don't think we should force everyone to use a particular version as long as that does not affect reliability of the production buildbot, and experiments with newer versions do not require much attention and troubleshooting on the server side. Being flexible with the requirements to workers help bot owners to keep them on their own schedule.

That's a long way of saying that the current wording is Ok, I think. This is mostly for people who are about to set their first worker and looking for a safe bet.

llvm/docs/HowToAddABuilder.rst

#. Check the existing build configurations to make sure the one you are
    will get feedback sooner after changes get committed.
 
 #. The computer you will be registering with the LLVM buildbot
    infrastructure should have all dependencies installed and you can
    actually build your configuration successfully. Please check what degree
    of parallelism (-j param) would give the fastest build.  You can build
    multiple configurations on one computer.
 
-#. Install buildbot-worker (currently we are using buildbot version 2.8.5).
-   Depending on the platform, buildbot-worker could be available to download and
-   install with your package manager, or you can download it directly from
-   `<http://trac.buildbot.net>`_ and install it manually.
+#. Install buildbot-worker (currently we are using buildbot version 2.8.4).
+   This specific version can be installed using ``pip``, with a command such
+   as ``pip3 install buildbot-worker==2.8.4``.
 
 #. Create a designated user account, your buildbot-worker will be running under,
    and set appropriate permissions.
 
 #. Choose the buildbot-worker root directory (all builds will be placed under
    it), buildbot-worker access name and password the build master will be using
    to authenticate your buildbot-worker.