For one thing, this makes missing-dependency failures into DistributionNotFound errors instead of ImportErrors, which might be more useful to the user. For another thing, if someone is using distributions that were installed with --multi-version, then they might be not importable until after require_auto_deps() has been run. (The docs claim that this would be the case, but we don't have an example of this happening at this time.)
It is currently hardcoded in setup.py to be 'allmydata-tahoe'. Ticket #556 is to make it configurable by a runtime command-line argument to setup.py: "--appname=foo", but I suddenly wondered if we really wanted that and at the same time realized that we don't need that for tahoe-1.3.0 release, so this patch just hardcodes it in setup.py.
setup.py inspects a file named 'src/allmydata/_appname.py' and assert that it contains the string "__appname__ = 'allmydata-tahoe'", and creates it if it isn't already present. src/allmydata/__init__.py import _appname and reads __appname__ from it. The rest of the Python code imports allmydata and inspects "allmydata.__appname__", although actually every use it uses "allmydata.__full_version__" instead, where "allmydata.__full_version__" is created in src/allmydata/__init__.py to be:
__full_version__ = __appname + '-' + str(__version__).
All the code that emits an "application version string" when describing what version of a protocol it supports (introducer server, storage server, upload helper), or when describing itself in general (introducer client), usese allmydata.__full_version__.
This fixes ticket #556 at least well enough for tahoe-1.3.0 release.
Using pkg_resources is probably better if it works -- zope.interface doesn't have a __version__ attribute that we can query, but pkg_resources knows zope.interface's version number, for one thing.
This code falls back to the old way -- looking at the __version__ attributes and __file__ attributes -- if the pkg_resources way doesn't answer.
Note that this patch also changes the capitalization of "Nevow", "Twisted", and "pyOpenSSL", and the spelling of "allmydata-tahoe". These changes are not frivolous: they are reflecting the fact that we are naming Python packages (technically called Python "distributions") instead of Python modules (technically and confusingly called Python "packages") here. The package ("distribution") is named "allmydata-tahoe". The module ("package") is named "allmydata".
This should make it sufficiently fast, while still giving a better answer on Ubuntu than platform.dist() currently does, and also falling back to lsb_release if platform.dist() says that it doesn't know.
Nowadays pkg_resources is a runtime requirement, and if there is something screwed up in the installation, we want an explicit ImportError exception as early as possible.
Using pkg_resources.require() like this also apparently allows people to install multiple different versions of packages on their system and tahoe (if pkg_resources is available to it) will import the version of the package that it requires. I haven't tested this feature.
