replace _auto_deps with a pkg_resources API call

[zooko]

In setup.py we use a separate file named _auto_deps.py so that both the build system that is executing setup.py at build-time and the running Tahoe-LAFS app itself at run-time can both query what the requirements are, while respecting the DRY (Don't Repeat Yourself) principle.

[zooko]

dstufft explained on IRC that you can get this information at run-time with:

     pkg_resources.get_distribution("allmydata-tahoe").requires()

[daira]

That seems to work (that is, it gives a list of Requirement objects corresponding to install_requires). So it's a valid alternative approach to what we have now, but I'm not sure it actually helps all that much. We Don't Repeat Ourselves already -- this would just mean we could choose to move the definition of install_requires to setup.py, say, which wouldn't by itself reduce complexity. (The install_requires definition is about half of _auto_deps.py.)

[warner]

I like this, as I'd like to see that funky execfile in setup.py go away. I'd like to do this at the same time as #2749, and move the make-sure-we-have-what-we-think-we-need code into a separate diagnostic command (maybe tahoe --check-versions?).

[warner]

moving most tickets from 1.12 to 1.13 so we can release 1.12 with magic-folders

[warner]

Would this really mean replacing a lot of the diagnostic "what version got installed, and does it meet our requirements?" code in __init__.py? I think *that*'s the deeper problem. I agree with Daira that _auto_deps.py isn't all that complicated (but I strongly feel that its install_requires contents should be moved into setup.py).

What if we made a module named allmydata.versioncheck? We would move all the non-install_requires stuff from _auto_deps.py into there, we'd get rid of the deprecation-suppression and fix those warnings directly (I have to imagine that most of them are resolved by now), we'd move all the check-and-compare-versions stuff from __init__.py into versioncheck too, make it a function, that gets called by tahoe --check-versions, and then part of the troubleshooting process is to run this command, maybe do it automatically when you do something early, like tahoe --version or tahoe create-client.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/646

[GitHub <noreply@…>]

In eb751ba/trunk:

Merge pull request #646 from tahoe-lafs/2283.use-pkg_resources

Use pkg_resources instead of _auto_deps.install_requires

Fixes: ticket:2283