1.8beta is incompatible with existing node directories due to change of appname

[davidsarah]

(​Reported to tahoe-dev by Nathan Eisenberg.)

In the 1.8beta release, when a user attempts to start a node using a node directory generated by an earlier version (or trunk), it will fail to start and will report a traceback similar to:

~/tahoe-lafs-ticket798-1.7.1-r4653/bin# ./tahoe start
STARTING '/root/.tahoe'
Traceback (most recent call last):
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/application/app.py", line 626, in run
    runApp(config)
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/scripts/twistd.py", line 23, in runApp
    _SomeApplicationRunner(config).run()
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/application/app.py", line 374, in run
    self.application = self.createOrGetApplication()
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/application/app.py", line 439, in createOrGetApplication
    application = getApplication(self.config, passphrase)
--- <exception caught here> ---
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/application/app.py", line 450, in getApplication
    application = service.loadApplication(filename, style, passphrase)
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/application/service.py", line 390, in loadApplication
    application = sob.loadValueFromFile(filename, 'application', passphrase)
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/Twisted-10.1.0-py2.5-linux-x86_64.egg/twisted/persisted/sob.py", line 210, in loadValueFromFile
    exec fileObj in d, d
  File "tahoe-client.tac", line 5, in <module>
    pkg_resources.require('allmydata-tahoe')
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/setuptools-0.6c16dev.egg/pkg_resources.py", line 666, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/root/tahoe-lafs-ticket798-1.7.1-r4653/setuptools-0.6c16dev.egg/pkg_resources.py", line 565, in resolve
    raise DistributionNotFound(req)  # XXX put more info here
pkg_resources.DistributionNotFound: allmydata-tahoe

This happens because the appname was changed to "tahoe-lafs-ticket798" for the beta [4595/ticket798], but existing .tac files contain references to the previous "allmydata-tahoe" appname. For example, tahoe-client.tac typically contains:

# -*- python -*-

import pkg_resources
pkg_resources.require('allmydata-tahoe')
pkg_resources.require('twisted')
from allmydata import client
from twisted.application import service

c = client.Client()

application = service.Application("allmydata_client")
c.setServiceParent(application)

Note that the appname change has not been made on trunk, only on the ticket798 branch from which the beta was built.

We should therefore withdraw the beta, because otherwise users may generate new node directories that will not work in future versions (due to the opposite problem, i.e. DistributionNotFound: tahoe-lafs-ticket798).

This failed to be detected because there are no tests that check compatibility with a node directory generated by a previous version. To prevent this happening again, we need the test suite to run at least a smoke test with a node directory that looks like one generated by an earlier version (perhaps check one in as a resource, with a big-fat-warning not to change it).