More flexible workflow templates ================================ :ref:`Workflow templates ` can specify a subset of the input parameters for their workflow that cannot be overridden by the user starting the workflow; the parameters not set in the workflow template are called run-time parameters, and can be provided by the user. This is adequate for some use cases, and it is a good fit for situations where some of the input parameters are security-sensitive. However, it is not enough for others. For example: * Create a :workflow:`debian_pipeline` workflow template that defaults to uploading the package to an upload target, but allow the user to skip the upload step * Default ``autopkgtest_backend`` to ``unshare``, but allow the user to select another backend, perhaps with some restrictions (compare :issue:`803`) * Forbid setting a particular parameter, while relying on the workflow's default value rather than needing to specify one * Ensure that changes to the implementation of a workflow don't unexpectedly make new parameters available Explicit runtime parameters --------------------------- Most of these requirements can be satisfied by adding a new ``runtime_parameters`` field to ``WorkflowTemplate``. This contains a dictionary mapping ``task_data`` parameter names to lists of allowed choices for each parameter, or to None. When starting a workflow, a user may only set a ``task_data`` parameter if that parameter appears in ``runtime_parameters``, and if the value is one of the allowed choices for that parameter. Setting the allowed choices for a parameter to the string ``any`` (as opposed to the normal list of strings) allows users to set that parameter to any value; similarly, setting the entire ``runtime_parameters`` field to the string ``any`` (as opposed to the normal dictionary) allows users to set any parameter to any value. Since this weakens the connection between ``WorkflowTemplate.task_data`` and ``WorkRequest.task_data``, we rename ``WorkflowTemplate.task_data`` to ``WorkflowTemplate.static_parameters``. A data migration sets ``runtime_parameters`` for each workflow template to ``field: None`` for each field in the given workflow's task data model that is not set in the template's ``static_parameters``. This preserves the previous behaviour of workflow templates. After this change, workflow templates will need to be explicitly adjusted to make new parameters available, unless they use ``runtime_parameters: None``. Examples ~~~~~~~~ A template with an overridable ``enable_upload`` parameter, providing a default but also allowing the user to set it when starting a workflow: .. code-block:: yaml static_parameters: enable_upload: true runtime_parameters: enable_upload: "any" For a task with ``vendor``, ``codename``, and ``backend`` parameters, a template that sets ``vendor`` without allowing the user to override it, allows ``codename`` to be set by the user to one of a limited set of values, and relies on the workflow's default value for ``backend`` without allowing the user to set it: .. code-block:: yaml static_parameters: vendor: "debian" runtime_parameters: codename: ["bookworm", "trixie"] A template that allows setting ``boring_parameter``, but if an ``amazing_parameter`` option is later added to its target workflow then it will not allow setting it unless ``runtime_parameters`` is changed: .. code-block:: yaml runtime_parameters: boring_parameter: "any" Possible future work -------------------- Nested structures ~~~~~~~~~~~~~~~~~ Counterintuitively, only top-level keys are considered while merging workflow template and user data: lists and dictionaries are not merged, and if a list or a dictionary exists in both the workflow template and the user data, the version in the user data is ignored. Most of our current workflows have been designed to take this into account, but that requires a flat structure for workflow parameters that can be a little overwhelming. However, changing this would have other implications. ``dput-ng`` profiles have similar nesting limitations, which caused us to unnest Debusine-specific entries there in :mr:`1769`, and adding nesting within ``debusine_workflow_data`` would reintroduce that difficulty. Also, the current flat structure suggests the possibility of a web UI to start a workflow that offers each of the available parameters as options and introspects the corresponding Pydantic models to pick the appropriate widgets; it becomes less obvious how to do this if we increase nesting. Additional variable templating ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Some workflow task data entries are naturally constructed from other entries: for example, collection names can be constructed based on vendor/codename. A possible direction would be to add a template language that can build template data based on some kind of variable expansion pass, in order that only a minimal set of variables needs to be presented as options to the user. However, adding templates to generate templates carries intrinsic risk of confusion, and many of the same benefits can be gained by more thoughtful workflow design: workflow orchestrators are free to do their own computations when building values for child work requests, and they can do so in ways that would be difficult to express in a template language, such as looking up state in the database. For now, it seems best to defer this until (if ever) we can design a domain-specific language powerful enough to express everything we do in workflow orchestrators, at which point something like this might be a good fit for it. autopkgtest parameter choice restrictions ----------------------------------------- A workspace administrator might want to allow setting ``autopkgtest_backend`` to ``unshare`` or ``incus-lxc`` but not to ``incus-vm`` or ``qemu``, for resource consumption reasons. However, this particular case cannot be handled solely at the workflow template level. ``autopkgtest_backend: auto`` is currently synonymous with ``autopkgtest_backend: unshare``, but for :issue:`803` it would be useful to have it be sensitive to whether the autopkgtest in question declares the ``isolation-container`` or ``isolation-machine`` restriction and use the simplest possible backend that will work. To make that work, the task itself would need to be responsible for deciding what ``auto`` means. Merging template and user task data happens before the task runs, so neither new ``WorkflowTemplate`` fields nor the current :ref:`task configuration ` mechanism can easily express this restriction. A new ``allow_backends`` parameter seems tempting, but there are some problems. The configuration context for the :task:`Autopkgtest` task is currently just the codename, so we can't use that for architecture-specific configuration and would need to complicate the task with explicit by-architecture parameters. This may be a reason to add multiple configuration contexts. We also need to handle this sort of backend selection at the workflow level rather than the task level, since the decision of which worker to dispatch the task to may depend on which backend is selected. That introduces the difficulty that the workflow doesn't currently have access to the ``Restrictions`` declared by a given source package's tests, and `that information isn't in the .dsc `__; it would have to unpack the source package to find those, which would have to be done on a worker since running ``dpkg-source -x`` on untrusted source packages isn't safe, and then cache the result somewhere.