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