How Tos

=======

All of this equipment is here to help you get your work done more efficiently.

However, learning how task-graphs are generated is probably not the work you

are interested in doing.  This section should help you accomplish some of the

more common changes to the task graph with minimal fuss.

.. important::

    If you cannot accomplish what you need with the information provided here,

    please consider whether you can achieve your goal in a different way.

    Perhaps something simpler would cost a bit more in compute time, but save

    the much more expensive resource of developers' mental bandwidth.

    Task-graph generation is already complex enough!

    If you want to proceed, you may need to delve into the implementation of

    task-graph generation.  The documentation and code are designed to help, as

    are the authors - ``hg blame`` may help track down helpful people.

    As you write your new transform or add a new kind, please consider the next

    developer.  Where possible, make your change data-driven and general, so

    that others can make a much smaller change.  Document the semantics of what

    you are changing clearly, especially if it involves modifying a transform

    schema.  And if you are adding complexity temporarily while making a

    gradual transition, please open a new bug to remind yourself to remove the

    complexity when the transition is complete.

Hacking Task Graphs

-------------------

The recommended process for changing task graphs is this:

1. Run one of the ``mach taskgraph`` subcommands (see :doc:`mach`) to

   generate a baseline against which to measure your changes.

   .. code-block:: none

       ./mach taskgraph tasks --json > old-tasks.json

2. Make your modifications under ``taskcluster/``.

3. Run the same ``mach taskgraph`` command, sending the output to a new file,

   and use ``diff`` to compare the old and new files.  Make sure your changes

   have the desired effect and no undesirable side-effects.  A plain unified

   diff should be useful for most changes, but in some cases it may be helpful

   to post-process the JSON to strip distracting changes.

4. When you are satisfied with the changes, push them to try to ensure that the

   modified tasks work as expected.

Hacking Actions

...............

If you are working on an action task and wish to test it out locally, use the

``./mach taskgraph test-action-callback`` command:

   .. code-block:: none

        ./mach taskgraph test-action-callback \

            --task-id I4gu9KDmSZWu3KHx6ba6tw --task-group-id sMO4ybV9Qb2tmcI1sDHClQ \

            --input input.yml hello_world_action

This invocation will run the hello world callback with the given inputs and

print any created tasks to stdout, rather than actually creating them.

Common Changes

--------------

Changing Test Characteristics

.............................

First, find the test description.  This will be in

``taskcluster/ci/*/tests.yml``, for the appropriate kind (consult

:doc:`kinds`).  You will find a YAML stanza for each test suite, and each

stanza defines the test's characteristics.  For example, the ``chunks``

property gives the number of chunks to run.  This can be specified as a simple

integer if all platforms have the same chunk count, or it can be keyed by test

platform.  For example:

.. code-block:: yaml

    chunks:

        by-test-platform:

            linux64/debug: 10

            default: 8

The full set of available properties is in

``taskcluster/taskgraph/transform/tests/test_description.py``.  Some other

commonly-modified properties are ``max-run-time`` (useful if tests are being

killed for exceeding maxRunTime) and ``treeherder-symbol``.

.. note::

    Android tests are also chunked at the mozharness level, so you will need to

    modify the relevant mozharness config, as well.

Adding a Test Suite

...................

To add a new test suite, you will need to know the proper mozharness invocation

for that suite, and which kind it fits into (consult :doc:`kinds`).

Add a new stanza to ``taskcluster/ci/<kind>/tests.yml``, copying from the other

stanzas in that file.  The meanings should be clear, but authoritative

documentation is in

``taskcluster/taskgraph/transform/tests/test_description.py`` should you need

it.  The stanza name is the name by which the test will be referenced in try

syntax.

Add your new test to a test set in ``test-sets.yml`` in the same directory.  If

the test should only run on a limited set of platforms, you may need to define

a new test set and reference that from the appropriate platforms in

``test-platforms.yml``.  If you do so, include some helpful comments in

``test-sets.yml`` for the next person.

Greening Up a New Test

......................

When a test is not yet reliably green, configuration for that test should not

be landed on integration branches.  Of course, you can control where the

configuration is landed!  For many cases, it is easiest to green up a test in

try: push the configuration to run the test to try along with your work to fix

the remaining test failures.

When working with a group, check out a "twig" repository to share among your

group, and land the test configuration in that repository.  Once the test is

green, merge to an integration branch and the test will begin running there as

well.

Adding a New Task

.................

If you are adding a new task that is not a test suite, there are a number of

options.  A few questions to consider:

 * Is this a new build platform or variant that will produce an artifact to

   be run through the usual test suites?

 * Does this task depend on other tasks?  Do other tasks depend on it?

 * Is this one of a few related tasks, or will you need to generate a large

   set of tasks using some programmatic means (for example, chunking)?

 * How is the task actually excuted?  Mozharness?  Mach?

 * What kind of environment does the task require?

Armed with that information, you can choose among a few options for

implementing this new task.  Try to choose the simplest solution that will

satisfy your near-term needs.  Since this is all implemented in-tree, it

is not difficult to refactor later when you need more generality.

Existing Kind

`````````````

The simplest option is to add your task to an existing kind.  This is most

practical when the task "makes sense" as part of that kind -- for example, if

your task is building an installer for a new platform using mozharness scripts

similar to the existing build tasks, it makes most sense to add your task to

the ``build`` kind.  If you need some additional functionality in the kind,

it's OK to modify the implementation as necessary, as long as the modification

is complete and useful to the next developer to come along.

Tasks in the ``build`` kind generate Firefox installers, and the ``test`` kind

will add a full set of Firefox tests for each ``build`` task.

New Kind

````````

The next option to consider is adding a new kind.  A distinct kind gives you

some isolation from other task types, which can be nice if you are adding an

experimental kind of task.

Kinds can range in complexity.  The simplest sort of kind uses the transform

loader to read a list of jobs from the ``jobs`` key, and applies the standard

``job`` and ``task`` transforms:

.. code-block:: yaml

    implementation: taskgraph.task.transform:TransformTask

    transforms:

       - taskgraph.transforms.job:transforms

       - taskgraph.transforms.task:transforms

    jobs:

       - ..your job description here..

Job descriptions are defined and documented in

``taskcluster/taskgraph/transforms/job/__init__.py``.

Custom Kind Loader

``````````````````

If your task depends on other tasks, then the decision of which tasks to create

may require some code.  For example, the ``test`` kind iterates over

the builds in the graph, generating a full set of test tasks for each one.  This specific

post-build behavior is implemented as a loader defined in ``taskcluster/taskgraph/loader/test.py``.

A custom loader is useful when the set of tasks you want to create is not

static but based on something else (such as the available builds) or when the

dependency relationships for your tasks are complex.

Custom Transforms

`````````````````

Most loaders apply a series of ":doc:`transforms <transforms>`" that start with

an initial human-friendly description of a task and end with a task definition

suitable for insertion into a Taskcluster queue.

Custom transforms can be useful to apply defaults, simplifying the YAML files

in your kind. They can also apply business logic that is more easily expressed

in code than in YAML.

Transforms need not be one-to-one: a transform can produce zero or more outputs

for each input. For example, the test transforms perform chunking by producing

an output for each chunk of a given input.

Ideally those transforms will produce job descriptions, so you can use the

existing ``job`` and ``task`` transforms:

.. code-block:: yaml

    transforms:

       - taskgraph.transforms.my_stuff:transforms

       - taskgraph.transforms.job:transforms

       - taskgraph.transforms.task:transforms

Try to keep transforms simple, single-purpose and well-documented!

Custom Run-Using

````````````````

If the way your task is executed is unique (so, not a mach command or

mozharness invocation), you can add a new implementation of the job

description's "run" section.  Before you do this, consider that it might be a

better investment to modify your task to support invocation via mozharness or

mach, instead.  If this is not possible, then adding a new file in

``taskcluster/taskgraph/transforms/jobs`` with a structure similar to its peers

will make the new run-using option available for job descriptions.

Something Else?

...............

If you make another change not described here that turns out to be simple or

common, please include an update to this file in your patch.