remove the version-checking code from `__init__.py`

[warner]

I think it's time we removed the (large!) chunk of code in src/allmydata/__init__.py that measures and checks versions of our dependencies at startup.

I'd like to see a small subset of this code move into a utility function (maybe src/allmydata/version_checks.py): just enough to pull version strings for our known dependencies. This would support the extended form of tahoe --version. I think this code should *not* know what the setup.py install_requires= is, nor should it make any attempt to pass judgement upon whether the versions it finds are good enough.

Then I'd like to delete everything else from __init__.py except for the Versioneer setup clause (#2748).

pip now has responsibility for ensuring that the target virtualenv has the right code at install time, and (I think) the entrypoint script has that responsibility at runtime. The code in __init__.py duplicates this, plus it frequently misfires, emitting a traceback-like warning that many users mistake for a fatal exception.

This would let us move the list of install_requires= into setup.py where it belongs, rather than the current arrangement where it lives in src/ but gets execfile()ed by setup.py (which is causing problems with py3-based tox runs, #2747).

One counter-argument I can think of: you pip install --editable . from a git checkout into a virtualenv, work for a while and everything is fine, then you do an update and get a setup.py that bumps the dependency on something. If you don't update your virtualenv every once in a while (pip list -o, pip install -U on anything it says is out-of-date), then eventually Tahoe will want more than what you've got.

I think the entrypoint script is supposed to do this checking, but you'd need to at least re-run setup.py egg_info to update the list of dependencies that it looks at. Having something at runtime would guard against this, and the shared _auto_deps.py is the cleanest way to accomplish that.

But I still think our __init__.py is too big, and I don't like having all that code run at import, and I don't think the git-update case is worth protecting against.

[warner]

#2873 brought up a case where those checks do not know as much about version strings as the real setuptools does, blocking us from changing our dependency on twisted[tls] to twisted[tls,conch] because it makes check_all_requirements() crash.

[GitHub <noreply@…>]

In 15af2bb/trunk:

Merge pull request #643 from tahoe-lafs/2749.remove-init-version-checking

Remove version checking and other code from allmydata/__init__.py

Fixes: ticket:2749

[warner]

YAY!!