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diff --git a/mobile/android/docs/shutdown.rst b/mobile/android/docs/shutdown.rst new file mode 100644 index 000000000..9ec0655f9 --- /dev/null +++ b/mobile/android/docs/shutdown.rst @@ -0,0 +1,77 @@ +.. -*- Mode: rst; fill-column: 100; -*- + +=================================== + Shutting down Firefox for Android +=================================== + +Background +========== + +Normally, apps on Android don't need to provide any support for explicit quitting, instead they are +just sent into the background where they eventually get killed by the OS's low-memory killer. + +Nevertheless, Firefox on Android allows explicit quitting to support the use case of users wanting +to clear part or all of their private data after finishing a browsing session. When this option to +"Clear private data on exit" is activated from the settings, a "Quit" button is provided in the menu. + +Because Firefox on Android uses a native UI (written in Java), which also holds some of the user's +browsing data, this creates some additional complications when compared to quitting on desktop. + +Technical details +================= + +When the "Quit" button is used, the UI sends a ``Browser:Quit`` notification to Gecko's ``BrowserApp``, +which initiates the normal Gecko shutdown procedure. At the same time however, the native UI needs to +shutdown as well, so as to + +1) provide an immediate visual feedback to the user that Firefox is indeed quitting + +2) avoid a state where the UI is still running "normally" while the rendering engine is already + shutting down, which could lead to loosing incoming external tabs if they were to arrive within + that period. + +Therefore, shutdown of the native UI was originally started simultaneously with notifying Gecko. +Because the clearing of private data during shutdown is handled by Gecko's ``Sanitizer``, while some +private data, e.g. the browsing history, is held in a database by the native UI, this means that +Gecko needs to message the native UI during shutdown if the user wants the browsing history to be +cleared on quitting. +Shutting down the UI simultaneously with Gecko therefore introduced a race condition where the data +clearing could fail because the native UI thread responsible for receiving Gecko's sanitization +messages had already exited by the time Gecko's ``Sanitizer`` was attempting to e.g. clear the +user's browsing history (for further reading, compare `bug 1266594 +<https://bugzilla.mozilla.org/show_bug.cgi?id=1266594>`_). + +To fix this issue, the native UI (in ``GeckoApp``) now waits for the ``Sanitizer`` to run and +message all necessary sanitization handlers and only starts its shutdown after receiving a +``Sanitize:Finished`` message with a ``shutdown: true`` parameter set. While this introduces a +certain delay in closing the UI, it is still faster than having to wait for Gecko to exit completely +before starting to close the UI. + +Currently, quitting Firefox therefore proceeds roughly as follows: + +1) The user presses the "Quit" button in the main menu, which sends a ``Browser:Quit`` notification + to ``BrowserApp``. This notification also contains additional parameters indicating which types + of private user data - if any - to clear during shutdown. + +2) ``BrowserApp.quit`` runs, which initiates Gecko shutdown by sending out a + ``quit-application-requested`` notification. + +3) If nobody cancelled shutdown in response to the ``quit-application-requested`` notification, + quitting proceeds and the ``SessionStore`` enters shutdown mode (``STATE_QUITTING``), which + basically means that no new (asynchronous) writes are started to prevent any interference with + the final flushing of data. + +4) ``BrowserApp`` calls the ``Sanitizer`` to clear up any private user data that might need cleaning. + After the ``Sanitizer`` has invoked all required sanitization handlers (including any on the + native Java UI side, e.g. for the browsing history) and finished running, it sends a + ``Sanitize:Finished`` message back to the native UI. + +5) On receiving the ``Sanitize:Finished`` message, ``GeckoApp`` starts the shutdown of the native UI + as well by calling ``doShutdown()``. + +6) After sending the ``Sanitize:Finished`` message, Gecko's ``Sanitizer`` runs the callback provided + by ``BrowserApp.quit``, which is ``appStartup.quit(Ci.nsIAppStartup.eForceQuit)``, thereby + starting the actual and final shutting down of Gecko. + +7) On receiving the final ``quit-application`` notification, the ``SessionStore`` synchronously + writes its current state to disk. |