nevow error against twisted-trunk

[warner]

The "deprecations" buildbot picked up a test failure (which will probably also be a runtime error) when running against Twisted trunk:

[ERROR]
Traceback (most recent call last):
  File "/home/bb-tahoe/bb-tahoe/deprecations/build/.tox/upcoming-deprecations/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 150, in maybeDeferred
    result = f(*args, **kw)
  File "/home/bb-tahoe/bb-tahoe/deprecations/build/.tox/upcoming-deprecations/local/lib/python2.7/site-packages/nevow/static.py", line 311, in renderHTTP
    if request.setLastModified(self.fp.getmtime()) is http.CACHED:
exceptions.AttributeError: 'FilePath' object has no attribute 'getmtime'

allmydata.test.test_system.SystemTest.test_filesystem
allmydata.test.web.test_web.Web.test_CSS_FILE

(see the log in ​https://tahoe-lafs.org/buildbot-tahoe-lafs/builders/deprecations/builds/140/steps/upcoming-deprecations/logs/stdio)

This is using tahoe at [5a5da0de2a4b23c6b9b5540e4675ee532a8addb2], Twisted at "1.6.0dev0" (they don't use Versioneer, but it's probably ​0384682cad94bfa1d16a6fc80e795fa6765d7454), and Nevow-0.14.2 .

Twisted just removed several methods from FilePath which had been (informally) deprecated back in 2007 (twisted ​a8805a0162d8c67830c5353caa18b5e387cf8ea6), which would be the Twisted-2.6/8.0 era. Unfortunately, it appears that no actual DeprecationWarning was raised for these methods, so we didn't see any notice that they'd be going away: the only indication was in the docstring. The specific method that Nevow-0.14.2 is using is getmtime, which was replaced with a higher-precision variant named getModificationTime.

(interesting trivia note: they replaced getctime with getStatusChangeTime, which would help correct my perpetual misbelief that the c stands for creation).

This is a Nevow bug, but it's present in the most recent release (0.14.2), and the ​Nevow repo has no further commits. So either we need to help get a fixed Nevow release out ASAP, or Tahoe needs to avoid using Nevow in a way that triggers this code path. Otherwise Tahoe will stop working as soon as the next Twisted release comes out.

[warner]

Just filed a Nevow bug: ​https://github.com/twisted/nevow/issues/96

[warner]

Deprioritizing this one: Twisted is going to revert the change, and start again with a real deprecation period. Nevow is going to stop using the Twisted API in question. That means we don't need to do anything. I'll leave it open for a while just in case something goes wrong, and we need to be able to find the context. Let's close it when Nevow has switched to getModificationTime, and we're not in danger of seeing any warnings.

[exarkun]

The Nevow bug is fixed.