Firefox/Go Faster/Client Implementation Plan

This is a straw-man client implementation plan that covers the main Client Requirements. The rough set of bugs needed to implement this are called out.

Known-good sets

To reduce the number of possible configurations thus simplifying QA and dependency issues the client implementation uses the concept of a "known-good set" of system add-ons. This is a list of add-on IDs and versions to run for a particular application.

Every application package and update will be distributed with a known-good set of system add-ons for that application. Periodically the application will attempt to discover a new known-good set to be installed and used.

The application will only run with a known-good set of system add-ons enabled or it will not enable any system add-ons and run standalone though since the application ships with a known-good set we should generally never be in this state.

The only exception is in the case where a developer has installed a version of a system add-on in the profile for testing, in this case it essentially overrides the known-good version for that add-on.

Running system add-ons

System add-ons will be installed in the user's profile. This assures that client code can install and update add-ons without needing special privileges and that there can only be one client application accessing the add-ons at a time.

They will be installed in a directory used as a regular add-on install location with a lower priority than the profile install location. This allows developers to test out new versions of system add-ons by installing them using the regular add-ons manager.

The specific path to the install location will be <profile>/features/<app path hash>. "app path hash" is a simple hash of the client's install directory. This means that if a user is using two different versions of the client application (say a developer version and a release version) with the same profile they will use two different directories for the system add-ons allowing them to be updated independently and retain the known-good set for each build. The majority of users use a single build and so will have just one directory in use.

The hash can be short to avoid causing path length problems and quick to generate. Collisions aren't a big problem, the client will recover gracefully automatically.

• Bug: Create a features install location with a lower priority to the profile location
• Bug: Hide system add-ons in the features location from the add-ons manager UI

If a user uninstalls an old build they will be left with a now useless set of system add-ons. Periodically we should check <profile>/features for unused directories. We can write the current time to the current location and for any locations that are older than a certain date delete them.

• Bug: Shortly after startup write the current time to the system add-on install location and purge older locations

Discovering system add-ons

By default the add-ons manager will attempt to update system add-ons automatically, this must be disabled so a slightly different mechanism for finding updates can be used.

• Bug: Disable automated updates for the system add-ons location

During runtime the client will periodically make a HTTPS request to a specific URL including data on the application channel, version and OS The manifest returned lists the specific system add-ons that should be used with that application.

An example of the data the manifest must include:

{
  "<addon-1-ID>": {
    "version": "1.2.0",
    "xpiURL": "http://mozilla.com/foo/bar/addon1.xpi",
    "xpiHash": "sha256:38974658476582645723465802458972345",
  },
  "<addon-2-ID>": {
    "version": "1.0.0",
    "xpiURL": "http://mozilla.com/foo/bar/addon2.xpi",
    "xpiHash": "sha256:45676878579689577567674563576746746",
  }
}

This manifest could be a standalone file or embedded in the existing update.xml file somewhere. The former is a little easier to implement on the client side.

Any add-on versions that are not already available locally are downloaded and checked against the given hash (is this needed given that we're signing the add-ons too?).

• Bug: Download and install new system add-ons

Once all the add-on versions listed in the manifest are available locally this list replaces the current known-good set, any enabled system add-ons not in the set are disabled, any system add-ons in the set not enabled are enabled.

• Bug: Replace the current set of system add-ons with the new known-good set

Securing system add-ons

System add-ons will need to be signed in a way that differentiates them from regular add-ons. This means either using a custom signing certificate or using the AMO signing service. AMO currently has two signing servers set up two sign preliminarily reviewed add-ons and fully reviewed add-ons. Clients can distinguish between the two based on a special string added to the Organizational Unit (OU) of the signing certificate for each add-on. Adding a third signing server that adds a new string to the OU is straightforward but AMO would need some updates to know to pass certain add-ons to that server.

• Bug: Add-ons manager signature checks should enforce special rules for add-ons installed in the system add-ons location

The root manifest and update manifests should be protected by https at a minimum (no certificate overrides allowed). As they can only point to signed XPIs there may be no need to do any additional security here but we should consult with the security team.

Bootstrapping

In order to give a good experience to users using a new profile or updating after not having used the client in a while the application should ship with a known-good set of system add-ons for that build. This means including the add-ons in installers, packages, dmgs and the update mars for the application.

• Bug: Include system add-ons in application packages and updates

On the startup of a new version of an application the client will find the add-ons included with the application, copy them to the install location and then use them as the known-good set.

• Bug: On startup with a new application version install all bundled system add-ons into the system add-on install location"

Add-on Dependencies

Declaring dependencies between add-ons isn't supported. Each add-on is downloaded individually and if there are network issues it will be possible for one system add-on to update when another doesn't. For now if add-ons have dependencies they should either be shipped in Firefox so we can use version checks to enforce them or the add-on should programmatically check that the necessary dependencies are installed.