(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 `__builtin__` are exposed. These include `True`, `False`, `None`, `sorted`, `int`, and `set`. 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` (`/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 metadata. 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.