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short-description: Contributing to Meson
...

# Contributing to Meson

A large fraction of Meson is contributed by people outside the core
team. This documentation explains some of the design rationales of
Meson as well as how to create and submit your patches for inclusion
to Meson.

Thank you for your interest in participating to the development.

## Submitting patches

All changes must be submitted as [pull requests to
Github](https://github.com/mesonbuild/meson/pulls). This causes them
to be run through the CI system. All submissions must pass a full CI
test run before they are even considered for submission.

## Tests

All new features must come with automatic tests that throughly prove
that the feature is working as expected. Similarly bug fixes must come
with a unit test that demonstrates the bug, proves that it has been
fixed and prevents the feature from breaking in the future.

Sometimes it is difficult to create a unit test for a given bug. If
this is the case, note this in your pull request. We may permit bug
fix merge requests in these cases. This is done on a case by case
basis. Sometimes it may be easier to write the test than convince the
maintainers that one is not needed. Exercise judgment and ask for help
in problematic cases.

The tests are split into two different parts: unit tests and full
project tests. To run all tests, execute `./run_tests.py`. Unit tests
can be run with `./run_unittests.py` and project tests with
`./run_project_tests.py`.

Each project test is a standalone project that can be compiled on its
own. They are all in `test cases` subdirectory. The simplest way to
run a single project test is to do something like `./meson.py test\
cases/common/1\ trivial builddir`. The one exception to this is `test
cases/unit` directory discussed below.

The test cases in the `common` subdirectory are meant to be run always
for all backends. They should only depend on C and C++, without any
external dependencies such as libraries. Tests that require those are
in the `test cases/frameworks` directory. If there is a need for an
external program in the common directory, such as a code generator, it
should be implemented as a Python script. The goal of test projects is
also to provide sample projects that end users can use as a base for
their own projects.

All project tests follow the same pattern: they are compiled, tests
are run and finally install is run. Passing means that building and
tests succeed and installed files match the `installed_files.txt` file
in the test's source root. Any tests that require more thorough
analysis, such as checking that certain compiler arguments can be
found in the command line or that the generated pkg-config files
actually work should be done with a unit test.

Projects needed by unit tests are in the `test cases/unit`
subdirectory. They are not run as part of `./run_project_tests.py`.

## Documentation

The `docs` directory contains the full documentation that will be used
to generate [the Meson web site](http://mesonbuild.com). Every change
in functionality must change the documentation pages. In most cases
this means updating the reference documentation page but bigger
changes might need changes in other documentation, too.

All new functionality needs to have a mention in the release
notes. These features should be written in standalone files in the
`docs/markdown/snippets` directory. The release manager will combine
them into one page when doing the release.

## Python Coding style

Meson follows the basic Python coding style. Additional rules are the
following:

- indent 4 spaces, no tabs ever
- indent meson.build files with two spaces
- try to keep the code as simple as possible
- contact the mailing list before embarking on large scale projects
  to avoid wasted effort

Meson uses Flake8 for style guide enforcement. The Flake8 options for
the project are contained in setup.cfg.

To run Flake8 on your local clone of Meson:

```console
$ python3 -m pip install flake8
$ cd meson
$ flake8
```

## C/C++ coding style

Meson has a bunch of test code in several languages. The rules for
those are simple.

- indent 4 spaces, no tabs ever
- brace always on the same line as if/for/else/function definition


## External dependencies

The goal of Meson is to be as easily usable as possible. The user
experience should be "get Python3 and Ninja, run", even on
Windows. Unfortunately this means that we can't have dependencies on
projects outside of Python's standard library. This applies only to
core functionality, though. For additional helper programs etc the use
of external dependencies may be ok. If you feel that you are dealing
with this kind of case, please contact the developers first with your
use case.

## Turing completeness

The main design principle of Meson is that the definition language is
not Turing complete. Any change that would make Meson Turing complete
is automatically rejected. In practice this means that defining your
own functions inside `meson.build` files and generalised loops will
not be added to the language.

## Do I need to sign a CLA in order to contribute?

No you don't. All contributions are welcome.

## No lingering state

Meson operates in much the same way as functional programming
languages. It has inputs, which include `meson.build` files, values of
options, compilers and so on. These are passed to a function, which
generates output build definition. This function is pure, which means that:

 - for any given input the output is always the same
 - running Meson twice in a row _always_ produce the same output in both runs

The latter one is important, because it enforces that there is no way
for "secret state" to pass between consecutive invocations of
Meson. This is the reason why, for example, there is no `set_option`
function even though there is a `get_option` one.

If this were not the case, we could never know if the build output is
"stable". For example suppose there were a `set_option` function and a
boolean variable `flipflop`. Then you could do this:

```meson
set_option('flipflop', not get_option('flipflop'))
```

This piece of code would never converge. Every Meson run would change
the value of the option and thus the output you get out of this build
definition would be random.

Meson does not permit this by forbidding these sorts of covert channels.

There is one exception to this rule. Users can call into external
commands with `run_command`. If the output of that command does not
behave like a pure function, this problem arises. Meson does not try
to guard against this case, it is the responsibility of the user to
make sure the commands they run behave like pure functions.

## Environment variables

Environment variables are like global variables, except that they are
also hidden by default. Envvars should be avoided whenever possible,
all functionality should be exposed in better ways such as command
line switches.