(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` (`<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 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.