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117 lines
5.4 KiB
ReStructuredText
117 lines
5.4 KiB
ReStructuredText
.. _mozbuild-files:
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===============
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moz.build Files
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===============
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``moz.build`` files are the mechanism by which tree metadata (notably
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the build configuration) is defined.
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Directories in the tree contain ``moz.build`` files which declare
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functionality for their respective part of the tree. This includes
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things such as the list of C++ files to compile, where to find tests,
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etc.
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``moz.build`` files are actually Python scripts. However, their
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execution is governed by special rules. This is explained below.
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moz.build Python Sandbox
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========================
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As mentioned above, ``moz.build`` files are Python scripts. However,
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they are executed in a special Python *sandbox* that significantly
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changes and limits the execution environment. The environment is so
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different, it's doubtful most ``moz.build`` files would execute without
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error if executed by a vanilla Python interpreter (e.g. ``python
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moz.build``.
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The following properties make execution of ``moz.build`` files special:
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1. The execution environment exposes a limited subset of Python.
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2. There is a special set of global symbols and an enforced naming
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convention of symbols.
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The limited subset of Python is actually an extremely limited subset.
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Only a few symbols from ``__builtins__`` are exposed. These include
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``True``, ``False``, and ``None``. Global functions like ``import``,
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``print``, and ``open`` aren't available. Without these, ``moz.build``
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files can do very little. *This is by design*.
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The execution sandbox treats all ``UPPERCASE`` variables specially. Any
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``UPPERCASE`` variable must be known to the sandbox before the script
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executes. Any attempt to read or write to an unknown ``UPPERCASE``
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variable will result in an exception being raised. Furthermore, the
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types of all ``UPPERCASE`` variables is strictly enforced. Attempts to
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assign an incompatible type to an ``UPPERCASE`` variable will result in
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an exception being raised.
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The strictness of behavior with ``UPPERCASE`` variables is a very
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intentional design decision. By ensuring strict behavior, any operation
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involving an ``UPPERCASE`` variable is guaranteed to have well-defined
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side-effects. Previously, when the build configuration was defined in
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``Makefiles``, assignments to variables that did nothing would go
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unnoticed. ``moz.build`` files fix this problem by eliminating the
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potential for false promises.
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In the sandbox, all ``UPPERCASE`` variables are globals and all
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non-``UPPERCASE`` variables are locals. After a ``moz.build`` file has
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completed execution, only the globals are used to retrieve state.
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The set of variables and functions available to the Python sandbox is
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defined by the :py:mod:`mozbuild.frontend.sandbox_symbols` module. The
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data structures in this module are consumed by the
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:py:class:`mozbuild.frontend.reader.MozbuildSandbox` class to construct
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the sandbox. There are tests to ensure that the set of symbols exposed
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to an empty sandbox are all defined in the ``sandbox_symbols`` module.
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This module also contains documentation for each symbol, so nothing can
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sneak into the sandbox without being explicitly defined and documented.
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Reading and Traversing moz.build Files
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======================================
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The process responsible for reading ``moz.build`` files simply starts at
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a root ``moz.build`` file, processes it, emits the globals namespace to
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a consumer, and then proceeds to process additional referenced
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``moz.build`` files from the original file. The consumer then examines
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the globals/``UPPERCASE`` variables set as part of execution and then
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converts the data therein to Python class instances.
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The executed Python sandbox is essentially represented as a dictionary
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of all the special ``UPPERCASE`` variables populated during its
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execution.
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The code for reading ``moz.build`` files lives in
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:py:mod:`mozbuild.frontend.reader`. The evaluated Python sandboxes are
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passed into :py:mod:`mozbuild.frontend.emitter`, which converts them to
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classes defined in :py:mod:`mozbuild.frontend.data`. Each class in this
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module define a domain-specific component of tree metdata. e.g. there
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will be separate classes that represent a JavaScript file vs a compiled
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C++ file or test manifests. This means downstream consumers of this data
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can filter on class types to only consume what they are interested in.
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There is no well-defined mapping between ``moz.build`` file instances
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and the number of :py:mod:`mozbuild.frontend.data` classes derived from
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each. Depending on the content of the ``moz.build`` file, there may be 1
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object derived or 100.
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The purpose of the ``emitter`` layer between low-level sandbox execution
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and metadata representation is to facilitate a unified normalization and
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verification step. There are multiple downstream consumers of the
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``moz.build``-derived data and many will perform the same actions. This
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logic can be complicated, so we a component dedicated to it.
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Other Notes
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===========
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:py:class:`mozbuild.frontend.reader.BuildReader`` and
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:py:class:`mozbuild.frontend.reader.TreeMetadataEmitter`` have a
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stream-based API courtesy of generators. When you hook them up properly,
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the :py:mod:`mozbuild.frontend.data` classes are emitted before all
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``moz.build`` files have been read. This means that downstream errors
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are raised soon after sandbox execution.
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Lots of the code for evaluating Python sandboxes is applicable to
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non-Mozilla systems. In theory, it could be extracted into a standalone
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and generic package. However, until there is a need, there will
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likely be some tightly coupled bits.
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