Understanding Artifact Builds
=============================

Firefox for Desktop and Android supports a **fast build mode** called
*artifact mode*. The resulting builds are called *artifact builds*.
Artifact mode downloads pre-built C++ components rather than building them
locally, trading bandwidth for time.

Artifact builds will be useful to many developers who are not working
with compiled code (see "Restrictions" below). Artifacts are typically
fetched from `mozilla-central <https://hg.mozilla.org/mozilla-central/>`__.

To automatically download and use pre-built binary artifacts, add the
following lines into your
`mozconfig <https://developer.mozilla.org/docs/Mozilla/Developer_guide/Build_Instructions/Configuring_Build_Options#Using_a_.mozconfig_Configuration_File>`__
file:

.. code-block:: shell

   # Automatically download and use compiled C++ components:
   ac_add_options --enable-artifact-builds

   # Write build artifacts to:
   mk_add_options MOZ_OBJDIR=./objdir-frontend

To automatically download and use the debug version of the pre-built
binary artifact (currently supported for Linux, OSX and Windows
artifacts), add ``ac_add_options --enable-debug`` to your mozconfig file
(with artifact builds option already enabled):

.. code-block:: shell

   # Enable debug versions of the pre-built binary artifacts:
   ac_add_options --enable-debug

   # Automatically download and use compiled C++ components:
   ac_add_options --enable-artifact-builds

   # Write build artifacts to:
   mk_add_options MOZ_OBJDIR=./objdir-frontend-debug-artifact


Prerequisites
-------------

Artifact builds are supported for users of Mercurial and Git. Git
artifact builds require a mozilla-central clone made with the help of
`git-cinnabar <https://github.com/glandium/git-cinnabar>`__. Please
follow the instructions on the git-cinnabar project page to install
git-cinnabar. Further information about using git-cinnabar to interact
with Mozilla repositories can be found on `the project
wiki <https://github.com/glandium/git-cinnabar/wiki/Mozilla:-A-git-workflow-for-Gecko-development>`__.

Building
--------

If you've added ``--enable-artifact-builds`` to your ``mozconfig``, each
time you run ``mach build`` and ``mach build path/to/subdirectory`` the
build system will determine what the best pre-built binary artifacts
available are, download them, and put them in place for you. The
computations are cached, so the additional calculations should be very
fast after the up-to-date artifacts are downloaded -- just a second or
two on modern hardware. Most Desktop developers should find that

.. code-block:: shell

   ./mach build
   ./mach run

just works.

To only rebuild local changes (to avoid re-checking for pushes and/or
unzipping the downloaded cached artifacts after local commits), you can
use:

.. code-block:: shell

   ./mach build faster

which only "builds" local JS, CSS and packaged (e.g. images and other
asset) files.

Most Firefox for Android developers should find that

.. code-block:: shell

   ./mach build
   ./mach package
   ./mach install

just works.

Pulling artifacts from a try build
----------------------------------

To only accept artifacts from a specific revision (such as a try build),
set ``MOZ_ARTIFACT_REVISION`` in your environment to the value of the
revision that is at the head of the desired push. Note that this will
override the default behavior of finding a recent candidate build with
the required artifacts, and will cause builds to fail if the specified
revision does not contain the required artifacts.

Restrictions
------------

Oh, so many. Artifact builds are rather delicate: any mismatch between
your local source directory and the downloaded binary artifacts can
result in difficult to diagnose incompatibilities, including unexplained
crashes and catastrophic XPCOM initialization and registration
failures. These are rare, but do happen.

Things that are supported
-------------------------

-  Modifying JavaScript, (X)HTML, and CSS resources; and string
   properties and DTD and FTL files.
-  Modifying Android Java code, resources, and strings.
-  Running mochitests and xpcshell tests.
-  Modifying ``Scalars.yaml`` to add Scalar Telemetry (since {{
   Bug("1425909") }}, except artifact builds on try).
-  Modifying ``Events.yaml`` to add Event Telemetry (since {{
   Bug("1448945") }}, except artifact builds on try).

Essentially everything updated by ``mach build faster`` should work with
artifact builds.

Things that are not supported
-----------------------------

-  Products other than Firefox for Desktop and Firefox for Android are
   not supported and are unlikely to ever be supported.
-  You cannot modify C, C++, or Rust source code anywhere in the tree.
   If it’s compiled to machine code, it can't be changed.
-  You cannot modify ``histograms.json`` to add Telemetry histogram
   definitions.(But see `Bug 1206117 <https://bugzilla.mozilla.org/show_bug.cgi?id=1206117>`__).
-  Modifying build system configuration and definitions does not work in
   all situations.

Things that are not **yet** supported
-------------------------------------

-  Tests other than mochitests, xpcshell, and Marionette-based tests.
   There aren’t inherent barriers here, but these are not known to work.
-  Modifying WebIDL definitions, even ones implemented in JavaScript.

Troubleshooting
---------------

There are two parts to artifact mode:
the ``--disable-compile-environment`` option, and the ``mach artifact``
command that implements the downloading and caching. Start by running

.. code-block:: shell

   ./mach artifact install --verbose

to see what the build system is trying to do. There is some support for
querying and printing the cache; run ``mach artifact`` to see
information about those commands.

Downloaded artifacts are stored in
``$MOZBUILD_STATE_PATH/package-frontend``, which is almost always
``~/.mozbuild/package-frontend``.

Discussion is best started on the `dev-builds mailing
list <https://lists.mozilla.org/listinfo/dev-builds>`__. Questions are
best raised in `#build <https://chat.mozilla.org/#/room/#build:mozilla.org>`__ on `Matrix <https://chat.mozilla.org/>`__. Please
file bugs in *Firefox Build System :: General*, blocking  `Bug 901840 <https://bugzilla.mozilla.org/show_bug.cgi?id=901840>`__

