Bug 1576507 - document FirstStartup module r=mythmon

Depends on D43366

Differential Revision: https://phabricator.services.mozilla.com/D44035

--HG--
extra : moz-landing-system : lando

toolkit/modules/docs/FirstStartup.rst

@@ -0,0 +1,81 @@
+.. _FirstStartup:
+
+==============
+FirstStartup
+==============
+
+``FirstStartup`` is a module which is invoked on application startup by the Windows Installer,
+to initialize services before the first application window appears.
+
+This is useful for:
+
+- one-time performance tuning
+- downloading critical data (hotfixes, experiments, etc)
+
+Blocking until the first Application window appears is important because the Installer
+will show a progress bar until this happens. This gives a user experience of:
+
+1. User downloads and starts the Windows Stub Installer.
+2. Progress bar advances while the application is downloaded and installed.
+3. Installer invokes the application with ``--first-startup``.
+4. Application window appears, and the installer window closes.
+
+Overall, the user experiences a very fast first-startup, with critical tasks that normally
+would be deferred until after UI startup already complete.
+
+.. _FirstStartup Architecture:
+
+FirstStartup: Example use case
+==============================
+
+An example use of the ``FirstStartup`` module is to invoke the Normandy client to download an experiment
+that will be used to customize the first-run page that Firefox shows.
+
+In this example, the first-run page would be loaded experimentally based on an attribution code provided
+by the Installer. The flow for this looks like:
+
+1. User clicks on download link containing an attribution (UTM) code(s).
+2. The download page serves a custom Windows Stub Installer with the appropriate attribution code embedded.
+3. The installer invokes Firefox with the `--first-startup` flag, which blocks the first window.
+4. Normandy is run by ``FirstStartup`` and downloads a list of available experiments, or "recipes".
+5. Recipes are evaluated and filtered based on local information, such as the OS platform and the attribution codes.
+6. A recipe is found which matches the current attribution code, and appropriate data is made available to the first-run page.
+7. ``FirstStartup`` completes and unblocks, which causes Firefox to show the first window and load the appropriate first-run data.
+
+List of phases
+==============
+
+``FirstStartup.NOT_STARTED``
+
+  The ``FirstStartup`` module has not been initalized (the ``init()``
+  function has not been called). This is the default state.
+
+``FirstStartup.IN_PROGRESS``
+
+  ``FirstStartup.init()`` has been called, and the event loop is
+  spinning. This state will persist until either all startup tasks
+  have finished, or time-out has been reached.
+
+  The time-out defaults to 5 seconds, but is configurable via the
+  ``first-startup.timeout`` pref, which is specified in milliseconds.
+
+``FirstStartup.TIMED_OUT``
+
+  The time-out has been reached before startup tasks are complete.
+
+  This status code is reported to Telemetry via the ``firstStartup.statusCode``
+  scalar.
+
+``FirstStartup.SUCCESS``
+
+  All startup tasks have completed successfully, and application startup may resume.
+
+  This status code is reported to Telemetry via the ``firstStartup.statusCode``
+  scalar.
+
+``FirstStartup.UNSUPPORTED``
+
+  No startup tasks are supported, and `FirstStartup` exited.
+
+  This status code is reported to Telemetry via the ``firstStartup.statusCode``
+  scalar.

toolkit/modules/docs/index.rst

@@ -8,3 +8,4 @@ The ``/toolkit/modules`` directory contains a number of self-contained toolkit m
   :maxdepth: 1
 
   AsyncShutdown
+  FirstStartup