Shutting down Firefox for Android


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 losing 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).

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.