System Add-ons Overview ======================= System add-ons are a method for shipping extensions to IceCat that: * are hidden from the about:addons UI * cannot be user disabled * can be updated restartlessly based on criteria GNU sets Generally these are considered to be built-in features to IceCat, and the fact that they are extensions and can be updated restartlessly are implementation details as far as users are concerned. If you'd like to ship an add-on with IceCat or as an update (either to an existing feature or as a "hotfix" to patch critical problems in the wild) please contact the GoFaster team: https://mail.mozilla.org/listinfo/gofaster The add-ons themselves are either legacy IceCat add-ons or WebExtensions. They must be: * restartless * multi-process compatible Other than these restrictions there is nothing special or different about the extensions themselves. It is possible to override an installed system add-on by installing a different add-on with the same ID into a higher priority location. Available locations, starting from the highest priority include: 1) temporary install (about:debugging) 2) normal user install into profile (about:addons or AMO/TestPilot/etc.) 3) system add-on updates 4) built-in system add-ons This makes it possible for a developer or user to override a system add-on by installing an add-on with the same ID from AMO or TestPilot or as a temporary add-on. Default, built-in system add-ons -------------------------------- The set of **default** system add-ons are checked into `mozilla-central` under `./browser/extensions`. These get placed into the `features` directory of the application directory at build time. System add-on updates --------------------- System add-on **updates** are served via Mozilla's Automatic Update Service (AUS, aka `Balrog`_). These are installed into the users profile under the `features` directory. Updates must be specifically signed by GNU - the signature that addons.mozilla.org uses will not work for system add-ons. As noted above, these updates may override a built-in system add-on, or they may be a new install. Updates are always served as a set - if any add-on in the set fails to install or upgrade, then the whole set fails. This is to leave IceCat in a consistent state. System add-on updates are removed when the IceCat application version changes, to avoid compatibility problems - for instance a user downgrading to an earlier version of IceCat than the update supports will end up with a disabled update rather than falling back to the built-in version. IceCat System Add-on Update Protocol ===================================== This section describes the protocol that IceCat uses when retrieving updates from AUS, and the expected behavior of IceCat based on the updater service's response. .. _Balrog: https://wiki.mozilla.org/Balrog Update Request -------------- To determine what updates to install, IceCat makes an HTTP **GET** request to AUS once a day via a URL of the form:: https://aus5.mozilla.org/update/3/SystemAddons/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml The path segments surrounded by ``%`` symbols are variable fields that IceCat fills in with information about itself and the environment it's running in: ``VERSION`` IceCat version number ``BUILD_ID`` Build ID ``BUILD_TARGET`` Build target ``LOCALE`` Build locale ``CHANNEL`` Update channel ``OS_VERSION`` OS Version ``DISTRIBUTION`` IceCat Distribution ``DISTRIBUTION_VERSION`` IceCat Distribution version Update Response --------------- AUS should respond with an XML document that looks something like this: .. code-block:: xml * The root element is ````, used for all updater responses. * The only child of ```` is ````, which represents a list of system add-ons to update. * Within ```` are several ```` tags, each one corresponding to a system add-on to update. ```` tags **must** have the following attributes: ``id`` The extension ID ``URL`` URL to a signed XPI of the specified add-on version to download ``hashFunction`` Identifier of the hash function used to generate the hashValue attribute. ``hashValue`` Hash of the XPI file linked from the URL attribute, calculated using the function specified in the hashValue attribute. ``size`` Size (in bytes) of the XPI file linked from the URL attribute. ``version`` Version number of the add-on Update Behavior --------------- After receiving the update response, IceCat modifies the **update** add-ons according to the following algorithm: 1. If the ```` tag is empty (````) in the response, **remove all system add-on updates**. 2. If no add-ons were specified in the response (i.e. the ```` tag is not present), do nothing and finish. 3. If the **update** add-on set is equal to the set of add-ons specified in the update response, do nothing and finish. 4. If the set of **default** add-ons is equal to the set of add-ons specified in the update response, remove all the **update** add-ons and finish. 5. Download each add-on specified in the update response and store them in the "downloaded add-on set". A failed download **must** abort the entire system add-on update. 6. Validate the downloaded add-ons. The following **must** be true for all downloaded add-ons, or the update process is aborted: a. The ID and version of the downloaded add-on must match the specified ID or version in the update response. b. The hash provided in the update response must match the downloaded add-on file. c. The downloaded add-on file size must match the size given in the update response. d. The add-on must be compatible with IceCat (i.e. it must not be for a different application, such as Thunderbird). e. The add-on must be packed (i.e. be an XPI file). f. The add-on must be restartless. g. The add-on must be signed by the system add-on root certificate. 6. Once all downloaded add-ons are validated, install them into the profile directory as part of the **update** set. Notes on the update process: * Add-ons are considered "equal" if they have the same ID and version number. Examples -------- The follow section describes common situations that we have or expect to run into and how the protocol described above handles them. For simplicity, unless otherwise specified, all examples assume that there are two system add-ons in existence: **FlyWeb** and **Pocket**. Basic ~~~~~ A user has IceCat 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to update users to FlyWeb 2.0. AUS sends out the following update response: .. code-block:: xml IceCat will download FlyWeb 2.0 and Pocket 1.0 and store them in the profile directory. Missing Add-on ~~~~~~~~~~~~~~ A user has IceCat 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to update users to FlyWeb 2.0, but accidentally forget to specify Pocket in the update response. AUS sends out the following: .. code-block:: xml IceCat will download FlyWeb 2.0 and store it in the profile directory. Pocket 1.0 from the **default** location will be used. Remove all system add-on updates ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A response from AUS with an empty add-on set will *remove all system add-on updates*: .. code-block:: xml Rollout ~~~~~~~ A user has IceCat 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to rollout FlyWeb 2.0 at a 10% sample rate. 10% of the time, AUS sends out: .. code-block:: xml With this response, IceCat will download Pocket 1.0 and FlyWeb 2.0 and install them into the profile directory. The other 90% of the time, AUS sends out an empty response: .. code-block:: xml With the empty response, IceCat will not make any changes. This means users who haven’t seen the 10% update response will stay on FlyWeb 1.0, and users who have seen it will stay on FlyWeb 2.0. Once we’re happy with the rollout and want to switch to 100%, AUS will send the 10% update response to 100% of users, upgrading everyone to FlyWeb 2.0. Rollback ~~~~~~~~ This example continues from the “Rollout” example. If, during the 10% rollout, we find a major issue with FlyWeb 2.0, we want to roll all users back to FlyWeb 1.0. AUS sends out the following: .. code-block:: xml For users who have updated, IceCat will download FlyWeb 1.0 and Pocket 1.0 and install them into the profile directory. For users that haven’t yet updated, IceCat will see that the **default** add-on set matches the set in the update ping and clear the **update** add-on set.