Upgrade to coverage 3.7.1 and have it auto-build itself on first use.

third_party/coverage-3.6/doc/subprocess.rst

-.. _subprocess:
-
-======================
-Measuring subprocesses
-======================
-
-:history: 20100224T201800, new for 3.3.
-:history: 20100725T211700, updated for 3.4.
-
-
-Complex test suites may spawn subprocesses to run tests, either to run them in
-parallel, or because subprocess behavior is an important part of the system
-under test. Measuring coverage in those subprocesses can be tricky because you
-have to modify the code spawning the process to invoke coverage.py.
-
-There's an easier way to do it: coverage.py includes a function,
-:func:`coverage.process_startup` designed to be invoked when Python starts.  It
-examines the ``COVERAGE_PROCESS_START`` environment variable, and if it is set,
-begins coverage measurement. The environment variable's value will be used as
-the name of the :ref:`configuration file <config>` to use.
-
-When using this technique, be sure to set the parallel option to true so that
-multiple coverage.py runs will each write their data to a distinct file.
-
-
-Configuring Python for subprocess coverage
-------------------------------------------
-
-Measuring coverage in subprocesses is a little tricky.  When you spawn a
-subprocess, you are invoking Python to run your program.  Usually, to get
-coverage measurement, you have to use coverage.py to run your program.
-Your subprocess won't be using coverage.py, so we have to convince Python
-to use coverage even when not explicitly invokved.
-
-To do that, we'll configure Python to run a little coverage.py code when it
-starts.  That code will look for an environment variable that tells it to
-start coverage measurement at the start of the process.
-
-To arrange all this, you have to do two things: set a value for the
-``COVERAGE_PROCESS_START`` environment variable, and then configure Python to
-invoke :func:`coverage.process_startup` when Python processes start.
-
-How you set ``COVERAGE_PROCESS_START`` depends on the details of how you create
-subprocesses.  As long as the environment variable is visible in your subprocess,
-it will work.
-
-You can configure your Python installation to invoke the ``process_startup``
-function in two ways:
-
-#. Create or append to sitecustomize.py to add these lines::
-
-    import coverage
-    coverage.process_startup()
-
-#. Create a .pth file in your Python installation containing::
-
-    import coverage; coverage.process_startup()
-
-The sitecustomize.py technique is cleaner, but may involve modifying an existing
-sitecustomize.py, since there can be only one.  If there is no sitecustomize.py
-already, you can create it in any directory on the Python path.
-
-The .pth technique seems like a hack, but works, and is documented behavior.
-On the plus side, you can create the file with any name you like so you don't
-have to coordinate with other .pth files.  On the minus side, you have to create
-the file in a system-defined directory, so you may need privileges to write it.