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diff --git a/python/docs/index.rst b/python/docs/index.rst new file mode 100644 index 0000000000..0c97270bbc --- /dev/null +++ b/python/docs/index.rst @@ -0,0 +1,228 @@ +================================= +Using third-party Python packages +================================= + +Mach and its associated commands have a variety of 3rd-party Python dependencies. Many of these +are vendored in ``third_party/python``, while others are installed at runtime via ``pip``. + +The dependencies of Mach itself can be found at ``python/sites/mach.txt``. Mach commands +may have additional dependencies which are specified at ``python/sites/<site>.txt``. + +For example, the following Mach command would have its 3rd-party dependencies declared at +``python/sites/foo.txt``. + +.. code:: python + + @Command( + "foo-it", + virtualenv_name="foo", + ) + # ... + def foo_it_command(): + import specific_dependency + +The format of ``<site>.txt`` files are documented further in the +:py:class:`~mach.requirements.MachEnvRequirements` class. + +Adding a Python package +======================= + +There's two ways of using 3rd-party Python dependencies: + +* :ref:`pip install the packages <python-pip-install>`. Python dependencies with native code must + be installed using ``pip``. This is the recommended technique for adding new Python dependencies. +* :ref:`Vendor the source of the Python package in-tree <python-vendor>`. Dependencies of the Mach + core logic or of building Firefox itself must be vendored. + +.. note:: + + For dependencies that meet both restrictions (dependency of Mach/build, *and* has + native code), see the :ref:`mach-and-build-native-dependencies` section below. + +.. _python-pip-install: + +``pip install`` the package +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +To add a ``pip install``-d package dependency, add it to your site's +``python/sites/<site>.txt`` manifest file: + +.. code:: text + + ... + pypi:new-package==<version> + ... + +.. note:: + + Some tasks are not permitted to use external resources, and for those we can + publish packages to an internal PyPI mirror. + See `how to upload to internal PyPI <https://wiki.mozilla.org/ReleaseEngineering/How_To/Upload_to_internal_Pypi>`_ + for more details. + +.. _python-vendor: + +Vendoring Python packages +~~~~~~~~~~~~~~~~~~~~~~~~~ + +To vendor a Python package, add it to ``third_party/python/requirements.in`` +and then run ``mach vendor python``. This will update the tree of pinned +dependencies in ``third_party/python/requirements.txt`` and download them all +into the ``third_party/python`` directory. + +Next, add that package and any new transitive dependencies (you'll see them added in +``third_party/python/requirements.txt``) to the associated site's dependency manifest in +``python/sites/<site>.txt``: + +.. code:: text + + ... + vendored:third_party/python/new-package + vendored:third_party/python/new-package-dependency-foo + vendored:third_party/python/new-package-dependency-bar + ... + +.. note:: + + The following policy applies to **ALL** vendored packages: + + * Vendored PyPI libraries **MUST NOT** be modified + * Vendored libraries **SHOULD** be released copies of libraries available on + PyPI. + + * When considering manually vendoring a package, discuss the situation with + the ``#build`` team to ensure that other, more maintainable options are exhausted. + +.. note:: + + We require that it is possible to build Firefox using only a checkout of the source, + without depending on a package index. This ensures that building Firefox is + deterministic and dependable, avoids packages from changing out from under us, + and means we’re not affected when 3rd party services are offline. We don't want a + DoS against PyPI or a random package maintainer removing an old tarball to delay + a Firefox chemspill. Therefore, packages required by Mach core logic or for building + Firefox itself must be vendored. + +.. _mach-and-build-native-dependencies: + +Mach/Build Native 3rd-party Dependencies +======================================== + +There are cases where Firefox is built without being able to ``pip install``, but where +native 3rd party Python dependencies enable optional functionality. This can't be solved +by vendoring the platform-specific libraries, as then each one would have to be stored +multiple times in-tree according to how many platforms we wish to support. + +Instead, this is solved by pre-installing such native packages onto the host system +in advance, then having Mach attempt to use such packages directly from the system. +This feature is only viable in very specific environments, as the system Python packages +have to be compatible with Mach's vendored packages. + +.. note: + + All of these native build-specific dependencies **MUST** be optional requirements + as to support the "no strings attached" builds that only use vendored packages. + +To control this behaviour, the ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` environment +variable can be used: + +.. list-table:: ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` + :header-rows: 1 + + * - ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` + - Behaviour + * - ``"pip"`` + - Mach will ``pip install`` all needed dependencies from PyPI at runtime into a Python + virtual environment that's reused in future Mach invocations. + * - ``"none"`` + - Mach will perform the build using only vendored packages. No Python virtual environment + will be created for Mach. + * - ``"system"`` + - Mach will use the host system's Python packages as part of doing the build. This option + allows the usage of native Python packages without leaning on a ``pip install`` at + build-time. This is generally slower because the system Python packages have to + be asserted to be compatible with Mach. Additionally, dependency lockfiles are ignored, + so there's higher risk of breakage. Finally, as with ``"none"``, no Python virtualenv + environment is created for Mach. + * - ``<unset>`` + - Same behaviour as ``"pip"`` if ``MOZ_AUTOMATION`` isn't set. Otherwise, uses + the same behaviour as ``"none"``. + +There's a couple restrictions here: + +* ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` only applies to the top-level ``"mach"`` site, + the ``"common"`` site and the ``"build"`` site. All other sites will use ``pip install`` at + run-time as needed. + +* ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="system"`` is not allowed when using any site other + than ``"mach"``, ``"common"`` or ``"build"``, because: + + * As described in :ref:`package-compatibility` below, packages used by Mach are still + in scope when commands are run, and + * The host system is practically guaranteed to be incompatible with commands' dependency + lockfiles. + +The ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` environment variable fits into the following use +cases: + +Mozilla CI Builds +~~~~~~~~~~~~~~~~~ + +We need access to the native packages of ``zstandard`` and ``psutil`` to extract archives and +get OS information respectively. Use ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="system"``. + +Mozilla CI non-Build Tasks +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +We generally don't want to create a Mach virtual environment to avoid redundant processing, +but it's ok to ``pip install`` for specific command sites as needed, so leave +``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` unset (``MOZ_AUTOMATION`` implies the default +behaviour of ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none"``). + +In cases where native packages *are* needed by Mach, use +``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="pip"``. + +Downstream CI Builds +~~~~~~~~~~~~~~~~~~~~ + +Sometimes these builds happen in sandboxed, network-less environments, and usually these builds +don't need any of the behaviour enabled by installing native Python dependencies. +Use ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none"``. + +Gentoo Builds +~~~~~~~~~~~~~ + +When installing Firefox via the package manager, Gentoo generally builds it from source rather than +distributing a compiled binary artifact. Accordingly, users doing a build of Firefox in this +context don't want stray files created in ``~/.mozbuild`` or unnecessary ``pip install`` calls. +Use ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none"``. + +Firefox Developers +~~~~~~~~~~~~~~~~~~ + +Leave ``MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE`` unset so that all Mach commands can be run, +Python dependency lockfiles are respected, and optional behaviour is enabled by installing +native packages. + +.. _package-compatibility: + +Package compatibility +===================== + +Mach requires that all commands' package requirements be compatible with those of Mach itself. +(This is because functions and state created by Mach are still usable from within the commands, and +they may still need access to their associated 3rd-party modules). + +However, it is OK for Mach commands to have package requirements which are incompatible with each +other. This allows the flexibility for some Mach commands to depend on modern dependencies while +other, more mature commands may still only be compatible with a much older version. + +.. note:: + + Only one version of a package may be vendored at any given time. If two Mach commands need to + have conflicting packages, then at least one of them must ``pip install`` the package instead + of vendoring. + + If a Mach command's dependency conflicts with a vendored package, and that vendored package + isn't needed by Mach itself, then that vendored dependency should be moved from + ``python/sites/mach.txt`` to its associated environment. |