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+# Testing with one-click loaners
+
+[Taskcluster] is the task execution framework that supports Mozilla's
+continuous integration and release processes.
+
+Build and test jobs (like Marionette) are executed across all supported
+platforms, and job results are pushed to [Treeherder] for observation.
+
+The best way to debug issues for intermittent test failures of
+Marionette tests for Firefox and Fennec (Android) is to use a
+one-click loaner as provided by Taskcluster. Such a loaner creates
+an interactive task you can interact with via a shell and VNC.
+
+To create an interactive task for a Marionette job which is shown
+as failed on Treeherder, select the job, click the ellipse in the lower
+left pane, and choose `Create Interactive Task`.
+
+Please note that you need special permissions to actually request
+such a loaner.
+
+When the task has been created you will receive an email with the connection
+details. Open the referenced shell and you will be connected via a WebSocket.
+Once that has been done a wizard will automatically launch and
+provide some options. Best here is to choose the second option,
+which will run all the setup steps, installs the Firefox or Fennec
+binary, and then exits.
+
+[Taskcluster]: https://docs.taskcluster.net/
+[Treeherder]: https://treeherder.mozilla.org
+
+## Setting up the Marionette environment
+
+Best here is to use a virtual environment, which has all the
+necessary packages installed. If no modifications to any Python
+package will be done, the already created environment by the
+wizard can be used:
+
+    % cd /builds/worker/workspace/build
+    % source venv/bin/activate
+
+Otherwise a new virtual environment needs to be created and
+populated with the mozbase and marionette packages installed:
+
+    % cd /builds/worker/workspace/build && rm -r venv
+    % virtualenv venv && source venv/bin/activate
+    % cd tests/mozbase && ./setup_development.py
+    % cd ../marionette/client && python setup.py develop
+    % cd ../harness && python setup.py develop
+    % cd ../../../
+
+## Running Marionette tests
+
+### Firefox
+
+To run the Marionette tests execute the `runtests.py` script. For all
+the required options as best search in the log file of the failing job
+the interactive task has been created from.  Then copy the complete
+command and run it inside the already sourced virtual environment:
+
+    % /builds/worker/workspace/build/venv/bin/python -u /builds/worker/workspace/build/tests/marionette/harness/marionette_harness/runtests.py --gecko-log=- -vv --binary=/builds/worker/workspace/build/application/firefox/firefox --address=127.0.0.1:2828 --symbols-path=https://queue.taskcluster.net/v1/task/GSuwee61Qyibujtxq4UV3A/artifacts/public/build/target.crashreporter-symbols.zip /builds/worker/workspace/build/tests/marionette/tests/testing/marionette/harness/marionette_harness/tests/unit-tests.ini
+
+### Fennec
+
+The Marionette tests for Fennec are executed by using an Android
+emulator which runs on the host platform. As such some extra setup
+steps compared to Firefox on desktop are required.
+
+The following lines set necessary environment variables before
+starting the emulator in the background, and to let Marionette
+know of various Android SDK tools.
+
+    % export ADB_PATH=/builds/worker/workspace/build/android-sdk-linux/platform-tools/adb
+    % export ANDROID_AVD_HOME=/builds/worker/workspace/build/.android/avd/
+    % /builds/worker/workspace/build/android-sdk-linux/tools/emulator -avd test-1 -show-kernel -debug init,console,gles,memcheck,adbserver,adbclient,adb,avd_config,socket &
+
+The actual call to `runtests.py` is different per test job because
+those are using chunks on Android. As best search for the command
+and its options in the log file of the failing job the interactive
+task has been created from. Then copy the complete command and run
+it inside the already sourced virtual environment.
+
+Here an example for chunk 1 which runs all the tests in the current
+chunk with some options for logs removed:
+
+    % /builds/worker/workspace/build/venv/bin/python -u /builds/worker/workspace/build/tests/marionette/harness/marionette_harness/runtests.py --emulator --app=fennec --package=org.mozilla.fennec_aurora --address=127.0.0.1:2828 /builds/worker/workspace/build/tests/marionette/tests/testing/marionette/harness/marionette_harness/tests/unit-tests.ini --gecko-log=- --symbols-path=/builds/worker/workspace/build/symbols --startup-timeout=300 --this-chunk 1 --total-chunks 10
+
+To execute a specific test only simply replace `unit-tests.ini`
+with its name.