tox: add "coverage" environment

Run with "tox -e coverage". Uses a new helper
module (allmydata.test.run_trial) to let us import+execute trial without
knowing exactly where the "trial" binary lives, which helps with using
"coverage run" under tox.

.coveragerc

@@ -0,0 +1,10 @@
+# -*- mode: conf -*-
+
+[run]
+# only record trace data for allmydata.*
+source =
+       allmydata
+# and don't trace the test files themselves, or generated files
+omit =
+     */allmydata/test/*
+     */allmydata/_version.py

.gitignore

@@ -37,3 +37,4 @@ zope.interface-*.egg
 /violations.txt
 /.tox/
 /docs/_build/
+/coverage.xml

src/allmydata/test/run_trial.py

@@ -0,0 +1,16 @@
+
+# This is a tiny helper module, to let "python -m allmydata.test.run_trial
+# ARGS" does the same thing as running "trial ARGS" (unfortunately
+# twisted/scripts/trial.py does not have a '__name__=="__main__"' clause).
+#
+# This makes it easier to run trial under coverage from tox:
+# * "coverage run trial ARGS" is how you'd usually do it
+# * but "trial" must be the one in tox's virtualenv
+# * "coverage run `which trial` ARGS" works from a shell
+# * but tox doesn't use a shell
+# So use:
+#  "coverage run -m allmydata.test.run_trial ARGS"
+
+from twisted.scripts.trial import run
+
+run()

tox.ini

@@ -15,6 +15,15 @@ commands =
          tahoe --version
          trial --rterrors {posargs:allmydata}
 
+[testenv:coverage]
+# coverage (with --branch) takes about 65% longer to run
+passenv = USERPROFILE HOMEDRIVE HOMEPATH
+skip_install = True
+deps = --editable=.[test]
+commands =
+         tahoe --version
+         coverage run --branch -m allmydata.test.run_trial --rterrors --reporter=timing {posargs:allmydata}
+
 [testenv:deprecations]
 basepython=python2.7
 passenv = USERPROFILE HOMEDRIVE HOMEPATH