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+.. _mozbuild-files:
+
+===============
+moz.build Files
+===============
+
+``moz.build`` files are the mechanism by which tree metadata (notably
+the build configuration) is defined.
+
+Directories in the tree contain ``moz.build`` files which declare
+functionality for their respective part of the tree. This includes
+things such as the list of C++ files to compile, where to find tests,
+etc.
+
+``moz.build`` files are actually Python scripts. However, their
+execution is governed by special rules. This is explained below.
+
+moz.build Python Sandbox
+========================
+
+As mentioned above, ``moz.build`` files are Python scripts. However,
+they are executed in a special Python *sandbox* that significantly
+changes and limits the execution environment. The environment is so
+different, it's doubtful most ``moz.build`` files would execute without
+error if executed by a vanilla Python interpreter (e.g. ``python
+moz.build``.
+
+The following properties make execution of ``moz.build`` files special:
+
+1. The execution environment exposes a limited subset of Python.
+2. There is a special set of global symbols and an enforced naming
+   convention of symbols.
+3. Some symbols are inherited from previously-executed ``moz.build``
+   files.
+
+The limited subset of Python is actually an extremely limited subset.
+Only a few symbols from ``__builtins__`` are exposed. These include
+``True``, ``False``, and ``None``. Global functions like ``import``,
+``print``, and ``open`` aren't available. Without these, ``moz.build``
+files can do very little. *This is by design*.
+
+The execution sandbox treats all ``UPPERCASE`` variables specially. Any
+``UPPERCASE`` variable must be known to the sandbox before the script
+executes. Any attempt to read or write to an unknown ``UPPERCASE``
+variable will result in an exception being raised. Furthermore, the
+types of all ``UPPERCASE`` variables is strictly enforced. Attempts to
+assign an incompatible type to an ``UPPERCASE`` variable will result in
+an exception being raised.
+
+The strictness of behavior with ``UPPERCASE`` variables is a very
+intentional design decision. By ensuring strict behavior, any operation
+involving an ``UPPERCASE`` variable is guaranteed to have well-defined
+side-effects. Previously, when the build configuration was defined in
+``Makefiles``, assignments to variables that did nothing would go
+unnoticed. ``moz.build`` files fix this problem by eliminating the
+potential for false promises.
+
+After a ``moz.build`` file has completed execution, only the
+``UPPERCASE`` variables are used to retrieve state.
+
+The set of variables and functions available to the Python sandbox is
+defined by the :py:mod:`mozbuild.frontend.context` module. The
+data structures in this module are consumed by the
+:py:class:`mozbuild.frontend.reader.MozbuildSandbox` class to construct
+the sandbox. There are tests to ensure that the set of symbols exposed
+to an empty sandbox are all defined in the ``context`` module.
+This module also contains documentation for each symbol, so nothing can
+sneak into the sandbox without being explicitly defined and documented.
+
+Reading and Traversing moz.build Files
+======================================
+
+The process for reading ``moz.build`` files roughly consists of:
+
+1. Start at the root ``moz.build`` (``<topsrcdir>/moz.build``).
+2. Evaluate the ``moz.build`` file in a new sandbox.
+3. Emit the main *context* and any *sub-contexts* from the executed
+   sandbox.
+4. Extract a set of ``moz.build`` files to execute next.
+5. For each additional ``moz.build`` file, goto #2 and repeat until all
+   referenced files have executed.
+
+From the perspective of the consumer, the output of reading is a stream
+of :py:class:`mozbuild.frontend.reader.context.Context` instances. Each
+``Context`` defines a particular aspect of data. Consumers iterate over
+these objects and do something with the data inside. Each object is
+essentially a dictionary of all the ``UPPERCASE`` variables populated
+during its execution.
+
+.. note::
+
+   Historically, there was only one ``context`` per ``moz.build`` file.
+   As the number of things tracked by ``moz.build`` files grew and more
+   and more complex processing was desired, it was necessary to split these
+   contexts into multiple logical parts. It is now common to emit
+   multiple contexts per ``moz.build`` file.
+
+Build System Reading Mode
+-------------------------
+
+The traditional mode of evaluation of ``moz.build`` files is what's
+called *build system traversal mode.* In this mode, the ``CONFIG``
+variable in each ``moz.build`` sandbox is populated from data coming
+from ``config.status``, which is produced by ``configure``.
+
+During evaluation, ``moz.build`` files often make decisions conditional
+on the state of the build configuration. e.g. *only compile foo.cpp if
+feature X is enabled*.
+
+In this mode, traversal of ``moz.build`` files is governed by variables
+like ``DIRS`` and ``TEST_DIRS``. For example, to execute a child
+directory, ``foo``, you would add ``DIRS += ['foo']`` to a ``moz.build``
+file and ``foo/moz.build`` would be evaluated.
+
+.. _mozbuild_fs_reading_mode:
+
+Filesystem Reading Mode
+-----------------------
+
+There is an alternative reading mode that doesn't involve the build
+system and doesn't use ``DIRS`` variables to control traversal into
+child directories. This mode is called *filesystem reading mode*.
+
+In this reading mode, the ``CONFIG`` variable is a dummy, mostly empty
+object. Accessing all but a few special variables will return an empty
+value. This means that nearly all ``if CONFIG['FOO']:`` branches will
+not be taken.
+
+Instead of using content from within the evaluated ``moz.build``
+file to drive traversal into subsequent ``moz.build`` files, the set
+of files to evaluate is controlled by the thing doing the reading.
+
+A single ``moz.build`` file is not guaranteed to be executable in
+isolation. Instead, we must evaluate all *parent* ``moz.build`` files
+first. For example, in order to evaluate ``/foo/moz.build``, one must
+execute ``/moz.build`` and have its state influence the execution of
+``/foo/moz.build``.
+
+Filesystem reading mode is utilized to power the
+:ref:`mozbuild_files_metadata` feature.
+
+Technical Details
+-----------------
+
+The code for reading ``moz.build`` files lives in
+:py:mod:`mozbuild.frontend.reader`. The Python sandboxes evaluation results
+(:py:class:`mozbuild.frontend.context.Context`) are passed into
+:py:mod:`mozbuild.frontend.emitter`, which converts them to classes defined
+in :py:mod:`mozbuild.frontend.data`. Each class in this module defines a
+domain-specific component of tree metdata. e.g. there will be separate
+classes that represent a JavaScript file vs a compiled C++ file or test
+manifests. This means downstream consumers of this data can filter on class
+types to only consume what they are interested in.
+
+There is no well-defined mapping between ``moz.build`` file instances
+and the number of :py:mod:`mozbuild.frontend.data` classes derived from
+each. Depending on the content of the ``moz.build`` file, there may be 1
+object derived or 100.
+
+The purpose of the ``emitter`` layer between low-level sandbox execution
+and metadata representation is to facilitate a unified normalization and
+verification step. There are multiple downstream consumers of the
+``moz.build``-derived data and many will perform the same actions. This
+logic can be complicated, so we have a component dedicated to it.
+
+:py:class:`mozbuild.frontend.reader.BuildReader`` and
+:py:class:`mozbuild.frontend.reader.TreeMetadataEmitter`` have a
+stream-based API courtesy of generators. When you hook them up properly,
+the :py:mod:`mozbuild.frontend.data` classes are emitted before all
+``moz.build`` files have been read. This means that downstream errors
+are raised soon after sandbox execution.
+
+Lots of the code for evaluating Python sandboxes is applicable to
+non-Mozilla systems. In theory, it could be extracted into a standalone
+and generic package. However, until there is a need, there will
+likely be some tightly coupled bits.