interfaceutil: module to stub out zope.interface The startup time of `hg` increased during the 4.6 development cycle. A cause of that was importing more modules and doing more work at module import time. The import of zope.interface and the declaring of various interfaces is partially responsible for the startup time regression. Our current usage of zope.interface doesn't do much at run time: we are merely declaring interfaces and stating that certain types implement various interfaces. Core Mercurial is not (yet) using of any of zope.interface features that actually require that interface plumbing be defined. The only place we actually need the interface metadata is in test-check-interfaces.py. This commit establishes a new interfaceutil module. It exposes the subset of the zope.interface API that we currently use. By default, the APIs no-op. But if an environment variable is set, we export the real zope.interface APIs. Existing importers of zope.interface have been converted to use the new module. test-check-interfaces.py has been updated to define the environment variable so the real zope.interface is used. The net effect of this change is we stop importing 9 zope.interface.* modules and we no longer perform interface bookkeeping when registering interfaces. On my i7-6700K on Linux, a shell loop that runs `hg log -r .` 300 times on a repo with 1 commit shows a significant CPU time improvement (average of 4 runs): 4.5: 14.814s before: 19.028s after: 16.945s And with `run-tests.py -j10` (single run): 4.5: ~3100s (~51.7m) before: ~4450s (~74.2m) after: ~3980s (~66.3m) So this claws back about half of the regressions in 4.6. Differential Revision: https://phab.mercurial-scm.org/D3419