.. _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. 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. In the sandbox, all ``UPPERCASE`` variables are globals and all non-``UPPERCASE`` variables are locals. After a ``moz.build`` file has completed execution, only the globals are used to retrieve state. The set of variables and functions available to the Python sandbox is defined by the :py:mod:`mozbuild.frontend.sandbox_symbols` 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 ``sandbox_symbols`` 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 responsible for reading ``moz.build`` files simply starts at a root ``moz.build`` file, processes it, emits the globals namespace to a consumer, and then proceeds to process additional referenced ``moz.build`` files from the original file. The consumer then examines the globals/``UPPERCASE`` variables set as part of execution and then converts the data therein to Python class instances. The executed Python sandbox is essentially represented as a dictionary of all the special ``UPPERCASE`` variables populated during its execution. The code for reading ``moz.build`` files lives in :py:mod:`mozbuild.frontend.reader`. The evaluated Python sandboxes 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 define 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 a component dedicated to it. Other Notes =========== :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.