Add coverage

tools/telemetry/third_party/coverage/doc/trouble.rst

Index: tools/telemetry/third_party/coverage/doc/trouble.rst
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+.. Licensed under the Apache License: http://www.apache.org/licenses/LICENSE-2.0
+.. For details: https://bitbucket.org/ned/coveragepy/src/default/NOTICE.txt
+
+.. _trouble:
+
+=========================
+Things that cause trouble
+=========================
+
+.. :history: 20121231T085200, brand new docs.
+.. :history: 20150124T160800, remove obsolete stuff.
+
+
+Coverage.py works well, and I want it to properly measure any Python program,
+but there are some situations it can't cope with.  This page details some known
+problems, with possible courses of action, and links to coverage.py bug reports
+with more information.
+
+I would love to :ref:`hear from you <contact>` if you have information about
+any of these problems, even just to explain to me why you want them to start
+working properly.
+
+If your problem isn't discussed here, you can of course search the `coverage.py
+bug tracker`_ directly to see if there is some mention of it.
+
+.. _coverage.py bug tracker: https://bitbucket.org/ned/coveragepy/issues?status=new&status=open
+
+
+Things that don't work
+----------------------
+
+There are a number of popular modules, packages, and libraries that prevent
+coverage.py from working properly:
+
+* `execv`_, or one of its variants.  These end the current program and replace
+  it with a new one.  This doesn't save the collected coverage data, so your
+  program that calls execv will not be fully measured.  A patch for coverage.py
+  is in `issue 43`_.
+
+* `thread`_, in the Python standard library, is the low-level threading
+  interface.  Threads created with this module will not be traced.  Use the
+  higher-level `threading`_ module instead.
+
+* `sys.settrace`_ is the Python feature that coverage.py uses to see what's
+  happening in your program.  If another part of your program is using
+  sys.settrace, then it will conflict with coverage.py, and it won't be
+  measured properly.
+
+.. _execv: http://docs.python.org/library/os#os.execl
+.. _sys.settrace: http://docs.python.org/library/sys.html#sys.settrace
+.. _thread: http://docs.python.org/library/thread.html
+.. _threading: http://docs.python.org/library/threading.html
+.. _issue 43: https://bitbucket.org/ned/coveragepy/issue/43/coverage-measurement-fails-on-code
+
+
+Things that require --timid
+---------------------------
+
+Some packages interfere with coverage measurement, but you might be able to
+make it work by using the ``--timid`` command-line switch, or the ``[run]
+timid=True`` configuration option.
+
+* `DecoratorTools`_, or any package which uses it, notably `TurboGears`_.
+  DecoratorTools fiddles with the trace function.  You  will need to use
+  ``--timid``.
+
+.. _DecoratorTools: http://pypi.python.org/pypi/DecoratorTools
+.. _TurboGears: http://turbogears.org/
+
+
+Still having trouble?
+---------------------
+
+If your problem isn't mentioned here, and isn't already reported in the
+`coverage.py bug tracker`_, please :ref:`get in touch with me <contact>`,
+we'll figure out a solution.