User Interface Considerations

The actions system decouples in-tree changes from user interface changes by taking advantage of graceful degradation. User interfaces, when presented with an unfamiliar action, fall back to a usable default behavior, and can later be upgraded to handle that action with a more refined approach.

Default Behavior

Every user interface should support the following:

  • Displaying a list of actions relevant to each task, and displaying task-group tasks for the associated task-group.
  • Providing an opportunity for the user to enter input for an action. This might be in JSON or YAML, or using a form auto-generated from the action's JSON-schema. If the action has no schema, this step should be skipped. The user's input should be validated against the schema.

For action.kind = 'task':

  • Rendering the template using the JSON-e library, using the JSON-e context described in the specification.
  • Calling Queue.createTask with the resulting task, using the user's Taskcluster credentials. See the next section for some important security-related concerns.

For action.kind = 'hook':

  • Displaying the hookGroupId and hookId in the input form, if one is used.
  • Rendering the hookPayload using JSON-e with the context described in the specification.
  • Calling Hooks.triggerHook with the resulting hook identiifers and payload.

User interfaces should ignore actions with unrecognized kinds.

Security Concerns

When executing an action, a UI must ensure that the user is authorized to perform the action, and that the user is not being "tricked" into executing an unexpected action. The actions.json artifact should be treated as untrusted content!

To accomplish the first, the UI should create tasks with the user's Taskcluster credentials. Do not use credentials configured as part of the service itself!

"task" Actions

To accomplish the second for "task" actions, use the decision tasks' scopes property as the authorizedScopes for the Queue.createTask call. This prevents action tasks from doing anything the original decision task couldn't do.

"hook" Actions

Hook actions are a bit different, as they are intended to allow actions the decision task does not have scopes to perform. For example, while many developers may be able to push to the master branch of a repository (and thus create a decision task), a "release" action might be limited to only a few project members.

For an action that will trigger a hook <hookGroupId>/<hookId>, the user interface must verify that the decision task's scopes satisfy in-tree:hook-action:<hookGroupId>/<hookId>. While a decision task does not itself exercise this scope, the check serves to verify that the repository for which the decision task was made had this in-tree:hook-action:.. scope, and thus that the hook was designed to be triggered for that repository.

The check can be carried out by fetching the decision task, passing task.scopes the Auth service's expandScopes API method, and then using taskcluster-lib-scopes' satisfiesExpression:

const expansion = await auth.expandScopes({scopes: task.scopes});
if (satisfiesExpression(
  expansion.scopes, `in-tree:hook-action:${action.hookGroupId}/${action.hookId}`)) {
  // call triggerHook

Specialized Behavior

The default behavior is too clumsy for day-to-day use for common actions. User interfaces may want to provide a more natural interface that still takes advantage of the actions system.

Specialized Input

A user interface may provide specialized input forms for specific schemas. The input generated from the form must conform to the schema.

To ensure that the schema has not changed, implementers should do a deep comparison between a schema for which a hand-written form exists, and the schema required by the action. If the two differ, the default behavior should be used instead.

Specialized Triggering

A user interface may want to trigger a specific action using a dedicated UI element. For example, an "start interactive session" button might be placed next to each failing test in a list of tests.

User interfaces should look for the desired action by name. The UI should check that there is exactly one matching action available for the given task or task-graph. If multiple actions match, the UI should treat that as an error (helping to avoid actions being surreptitiously replaced by similarly-named, malicious actions).

Having discovered the task, the user interface has a choice in how to provide its input. It can use the "specialized input" approach outlined above, providing a customized form if the action's schema is recognized and gracefully degrading if not.

But if the user interface is generating the input internally, it may instead validate that generated input against the action's schema as given, proceeding if validation succeeds. In this alternative, there is no need to do a deep comparison of the schema. This approach allows in-tree changes that introduce backward-compatible changes to the schema, without breaking support in user interfaces. Of course, if the changes are not backward-compatible, breakage will ensue.

Skipping Confirmation

A user interface that specializes for an action may skip the user-confirmation process for hook actions if any of

  • it can whitelist specific hooks -- for example, a retrigger action might always use a hook with a specific name or pattern;

  • it can determine that the actions.json is trusted -- for example, if the task is definitively associated with a commit to a trusted repository; or

  • it can limit the scopes available using authorizedScopes based on some other definitive information about the task -- for example, actions on a task not created from a Github "master" branch might use authorizedScopes to limit access to only pull-request-related actions.