feat: create transforms for adding dependencies

These transforms will implement part of the work that the multi_dep
loader is doing in other projects.

Issue: taskcluster#227

docs/reference/index.rst

@@ -6,6 +6,7 @@ Reference
 
    cli
    parameters
+   transforms/index
    optimization-strategies
    source/modules
    migrations

docs/reference/source/taskgraph.transforms.rst

@@ -20,6 +20,14 @@ taskgraph.transforms.base module
    :undoc-members:
    :show-inheritance:
 
+taskgraph.transforms.from\_deps module
+--------------------------------------
+
+.. automodule:: taskgraph.transforms.from_deps
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
 taskgraph.transforms.cached\_tasks module
 -----------------------------------------
 

docs/reference/transforms/from_deps.rst

@@ -0,0 +1,168 @@
+.. _from_deps:
+
+From Dependencies
+=================
+
+The :mod:`taskgraph.transforms.from_deps` transforms can be used to create
+tasks based on the kind dependencies, filtering on common attributes like the
+``build-type``.
+
+These transforms are useful when you want to create follow-up tasks for some
+indeterminate subset of existing tasks. For example, maybe you want to run
+a signing task after each build task.
+
+
+Usage
+-----
+
+Add the transform to the ``transforms`` key in your ``kind.yml`` file:
+
+.. code-block:: yaml
+
+   transforms:
+     - taskgraph.transforms.from_deps
+     # ...
+
+Then create a ``from-deps`` section in your task definition, e.g:
+
+.. code-block:: yaml
+
+   kind-dependencies:
+     - build
+     - toolchain
+
+   tasks:
+     signing:
+       from-deps:
+         kinds: [build]
+
+This example will split the ``signing`` task into many, one for each build. If
+``kinds`` is unspecified, then it defaults to all kinds listed in the
+``kind-dependencies`` key. So the following is valid:
+
+.. code-block:: yaml
+
+   kind-dependencies:
+     - build
+     - toolchain
+
+   tasks:
+     signing:
+       from-deps: {}
+
+In this example, a task will be created for each build *and* each toolchain.
+
+Limiting Dependencies by Attribute
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It may not be desirable to create a new task for *all* tasks in the given kind.
+It's possible to limit the tasks given some attribute:
+
+.. code-block:: yaml
+
+   kind-dependencies:
+     - build
+
+   tasks:
+     signing:
+       from-deps:
+         with-attributes:
+           platform: linux
+
+In the above example, follow-up tasks will only be created for builds whose
+``platform`` attribute equals "linux". Multiple attributes can be specified,
+these are resolved using the :func:`~taskgraph.util.attributes.attrmatch`
+utility function.
+
+Grouping Dependencies
+~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes, it may be desirable to run a task after a group of related tasks
+rather than an individual task. One example of this, is a task to notify when a
+release is finished for each platform we're shipping. We want it to depend on
+every task from that particular release phase.
+
+To accomplish this, we specify the ``group-by`` key:
+
+.. code-block:: yaml
+
+   kind-dependencies:
+     - build
+     - signing
+     - publish
+
+   tasks:
+     notify:
+       from-deps:
+         group-by:
+           attribute: platform
+
+In this example, tasks across the ``build``, ``signing`` and ``publish`` kinds will
+be scanned for an attribute called "platform" and sorted into corresponding groups.
+Assuming we're shipping on Windows, Mac and Linux, it might create the following
+groups:
+
+.. code-block::
+
+   - build-windows, signing-windows, publish-windows
+   - build-mac, signing-mac, publish-mac
+   - build-linux, signing-linux, publish-linux
+
+Then the ``notify`` task will be duplicated into three, one for each group. The
+notify tasks will depend on each task in its associated group.
+
+Custom Grouping
+~~~~~~~~~~~~~~~
+
+Only the default ``single`` and the ``attribute`` group-by functions are
+built-in. But if more complex grouping is needed, custom functions can be
+implemented as well:
+
+.. code-block:: python
+
+   from typing import List
+
+   from taskgraph.task import Task
+   from taskgraph.transforms.from_deps import group_by
+   from taskgraph.transforms.base import TransformConfig
+
+   @group_by("custom-name")
+   def group_by(config: TransformConfig, tasks: List[Task]) -> List[List[Task]]:
+      pass
+
+It's also possible to specify a schema for your custom group-by function, which
+allows tasks to pass down additional context (such as with the built-in
+``attribute`` function):
+
+.. code-block:: python
+
+   from typing import List
+
+   from taskgraph.task import Task
+   from taskgraph.transforms.from_deps import group_by
+   from taskgraph.transforms.base import TransformConfig
+   from taskgraph.util.schema import Schema
+
+   @group_by("custom-name", schema=Schema(str))
+   def group_by(config: TransformConfig, tasks: List[Task], ctx: str) -> List[List[Task]]:
+      pass
+
+Primary Kind
+~~~~~~~~~~~~
+
+Each task has a ``primary-kind``. This is the kind dependency that comes first
+in the list of supported kinds (either via the ``kind-dependencies`` in the
+``kind.yml`` file, or via the ``from-deps.kinds`` key).
+
+The primary kind is used to derive the task's label, as well as copy attributes if the
+``copy-attributes`` key is set to ``True`` (see next section).
+
+Each task created by the ``from_deps`` transforms, will have a
+``primary-kind-dependency`` attribute set.
+
+Copying Attributes
+~~~~~~~~~~~~~~~~~~
+
+It's often useful to copy attributes from a dependency. When this key is set to ``True``,
+all attributes from the ``primary-kind`` (see above) will be copied over to the task. If
+the task contain pre-existing attributes, they will not be overwritten.

docs/reference/transforms/index.rst

@@ -0,0 +1,11 @@
+Transforms
+==========
+
+Taskgraph includes several transforms out of the box. These can be used to
+accomplish common tasks such as adding dependencies programmatically or setting
+up notifications. This section contains a reference of the transforms Taskgraph
+provides, read below to learn how to use them.
+
+.. toctree::
+
+   from_deps