Using wheels to distribute Python packages

Wheels

If your SPK uses Python packages you can use the 'wheel' format to distribute the packages together with your SPK. Read up on the format here.

Pure-python, cross-compiling using python crossenv or api/abi limited

Generally speaking, there are tree types of Python packages:

1. Pure-python packages. Wheels are platform independent and self-contained for the most part in terms of dependencies;
2. Packages with (optional) C-extensions. These packages have to be compiled with GCC, and require a (cross-)compiled Python to be available along with setting up a Python crossenv being setup;
3. Packages with enforced limited api/abi where you may need to limit API compatibility to Python 3.x (cp3x) and ABI to Python 3 (abi3). Other than that theses are similar in every way to (cross-)compiled wheels using Python crossenv.

For spksrc specifically, we also define a fourth type of package that meat any of the following two types:

• Packages with C-extensions, which depend on other cross-packages at build time;
• Packages that need patches to be applied in order to create a working wheel.

This type of package generally requires a new spksrc cross package to be created. Generally speaking, these packages will also require a (cross-)compiled Python to be available.

How does spksrc handle wheels?

By default spksrc consider all packages to be of Python crossenv (cross-)compiled type thus allowing you to use any given name for the requirement file (although Python default normally is requirements.txt).

Otherwise, spksrc uses 3x distinct requirement files to handle wheels:

• requirements-crossenv.txt is used for cross-compiled packages using python crossenv
• requirements-pure.txt is user for pure-python packages
• requirements-abi3.txt is for api/abi limited packages

The package requirements-crossenv|pure|abi3.txt filename needs to be added to the WHEELS variable in the SPKs Makefile. In order to create reproducible builds, all the required packages should be frozen to a specific version (e.g. docutils==0.17.1).

The general building steps are the following:

1. spksrc will store a requirements-crossenv|pure|abi3.txt in $(WORK_DIR)/wheelhouse.
2. From there it will compile each of the needed types of wheels and then store the original in $(WORK_DIR)/wheelhouse.
3. Finally, spksrc it will rename the wheel so it always matches the uname -m of the target DSM and copy to $(INSTALL_DIR)/$(INSTALL_PREFIX)/share/wheelhouse for later packaging along with creating a consolidated requirements.txt that will include all wheels of any given type. This is mainly useful for arm arches as they need to exactly refer to the device machine hardware name (armv5tel, armv7, armv7l, etc.)

Pure-Python packages

Rule of thumb is that most packages will only require (cross-)compiled wheels. Although when building a noarch package or if a wheel doesn't compile properly in crossenv we then fallback to pure-python wheels. These are managed exactly the same way as packages with C-extensions below with the exception of using a requirements-pure.txt file. This default behavior of (cross-)compiling using Python crossenv can be changed by setting WHEEL_DEFAULT_PREFIX=pure as needed.

Using the requirements-pure.txt file, spksrc will create a pure-python wheel by clearing all the build flags and using the native/python310 host Python interpreter. The resulting wheels and requirements-pure.txt are stored in $(WORK_DIR)/wheelhouse for later processing and packaging.

For noarch packages (such as beets for example) it needs the following:

1. It mandatory requires to add BUILD_DEPENDS += native/python310 to the SPKs Makefile. This ensures having a host native Python and that other requirements are met.
2. Define the location of pip to point towards native Python such as:

PIP = $(WORK_DIR)/../../../native/python310/work-native/install/usr/local/bin/pip

Packages with C-extensions

1. It mandatory requires to add BUILD_DEPENDS += cross/python310 to the SPKs Makefile. This ensures that Python is (cross-)compiled and other requirements are setup correctly to create a crossenv that includes pip, wheel, setuptools, cffi and cryptography and poetry. This crossenv is key to generate (cross-)compiled wheels later-on in the build process;
2. Add the requirement filename to the WHEELS variable in spk/Makefile. We suggest using the default requirements.txt filename when there only are cross-compiled wheels OR always use requirements-crossenv.txt when there also is pure|abi3 wheels to be created. In order to create reproducible builds, all the required packages are frozen to a specific version (e.g. mercurial==4.0.1);
3. spksrc will build a cross-compiled wheel using pip by including all the build flags and using the crossenv python interpreter along with the TC_ARCH toolchain;
4. The resulting wheels and requirements-crossenv.txt are stored in $(WORK_DIR)/wheelhouse for later processing and packaging;

Packages with limited API/ABI

There are managed exactly the same way as packages with C-extensions above with the exception of using a requirements-abi3.txt file. The API/ABI limitation can be set with the PYTHON_LIMITED_API variable such as PYTHON_LIMITED_API = cp35 (which will fallback to Python 3.5 API).

Python packages using a cross package

Usage of cross/ python wheels can often be circumvented by:

1. Adding the needed DEPENDS += to the SPKs Makefile (ex: DEPENDS += cross/c-ares);
2. Including in the shell environment the package needed arguments (ex: ENV += PYCARES_USE_SYSTEM_LIB=1);
3. Adding the required packages frozen to a specific version (e.g. pycares==4.1.2)

When above procedure is insufficient we must then fallback to using a cross package:

1. Create a new package in cross/ with the correct details. Add an include to spksrc.python-wheel.mk in the Makefile so spksrc knows how to build the package;
2. Add BUILD_DEPENDS = cross/python to the SPKs Makefile. This ensures that Python is cross-compiled and that the crossenv requirements are setup correctly to create cross-compiled wheels;
3. Add the new cross package to DEPENDS in the SPKs Makefile;
4. In contrast to the other two types, this type of package should normally not be included in any requirements-crossenv|pure|abi3.txt.

The building steps are such as:

1. spksrc will (cross-)compile Python, then process python-cc.mk. Due to include ../../mk/spksrc.python-wheel.mk, spksrc creates a (cross-)compiled wheel. Wheel building process is invoked using python -c "import setuptools;... call instead of using pip like it does for usual crossenv wheels.
2. The resulting wheels and requirements-cross.txt are stored in $(WORK_DIR)/wheelhouse for later processing and packaging;

Next steps

After the above, spksrc will resume its normal activities to build the SPK.

• To include $(INSTALL_DIR)/$(INSTALL_PREFIX)/share/wheelhouse in the SPK itself, add rsc:share/wheelhouse to the SPKs PLIST.
• In addition, the installer should contain a line to install the wheels in a Python virtualenv. The generic format to create a virtualenv and install wheels for SynoCommunity packages is:

# Create a Python virtualenv
    ${VIRTUALENV} --system-site-packages ${INSTALL_DIR}/env > /dev/null
# Install the wheels
    ${INSTALL_DIR}/env/bin/pip install --no-deps --no-index -U --force-reinstall -f ${INSTALL_DIR}/share/wheelhouse ${INSTALL_DIR}/share/wheelhouse/*.whl > /dev/null 2>&1

Example: Let's look at the Mercurial SPK

Add python wheels

The Mercurial SPK contains two Python packages: Mercurial itself, and Docutils, which is a dependency of Mercurial.

Mercurial needs cross-compiling because it contains C-extensions. In addition, it also has to be patched to ensure a working wheel is created. That means it's a package of the third type as previously described. Docutils on the other hand is a pure-python package.

Starting off with Docutils: Mercurials Makefile sets WHEELS = src/requirements.txt. This requirements file contains docutils==0.17.1 as its only entry. As this only package builds properly using cross-compiling set by default the requirement filename is using default requirements.txt. This is all that needs to be done to create a Docutils wheel.

For Mercurial itself, a bit more is needed:

1. spksrc/cross/mercurial/Makefile is created with the correct content. There's no need for dependencies in this case, as docutils is handled via the requirements file.
2. The Makefile's include is set to create cross-compiled wheels: include ../../mk/spksrc.python-wheel.mk.
3. The appropriate patches for Mercurial are added to the patches directory.
4. A digests file should be created, to ensure the file download is not corrupted.
5. The SPKs Makefile then needs the following: BUILD_DEPENDS = cross/python to cross-compile Mercurial. BUILD_DEPENDS also contains cross/mercurial (although it could also be added to DEPENDS as there's nothing in the PLIST)
6. The last step is to add rsc:share/wheelhouse to the SPKs PLIST.

Building the SPK via make arch-$(ARCH) should now result in two wheels in $(WORK_DIR)/wheelhouse. The wheels are also stored in $(INSTALL_DIR)/$(INSTALL_PREFIX)/share/wheelhouse, but with a uname -m DSM architecture matching naming format to ensure the wheels are recognized as valid on the target device.

During the processing of the SPKs PLIST, the wheelhouse directory is copied to $(STAGING_DIR)/share/wheelhouse.

Installing the wheels on the target device

Once the Python packages are successfully created and included in the package, you'll need to make sure the wheels are installed.

1. In Mercurial installer, include the generic command to first create a Python virtual environment: ${VIRTUALENV} --system-site-packages ${INSTALL_DIR}/env > /dev/null. Note that in some cases you might not want to use --system-site-packages.
2. Install all available wheels into the virtual environment as follows: ${INSTALL_DIR}/env/bin/pip install --no-deps --no-index -U --force-reinstall -f ${INSTALL_DIR}/share/wheelhouse ${INSTALL_DIR}/share/wheelhouse/*.whl > /dev/null 2>&1

Tips and tricks

• Generally speaking, you should start with the assumption that all the required Python packages are pure-python. When building a pure-python wheel fails, the build process will halt with an error, after which you can decide what to do. A good next step is to assume that one or more packages should be cross-compiled, which means adding BUILD_DEPENDS = cross/python and see if that works better.
• To identify if a package pure-python or not, in most cases the wheels name can tell you. If the wheel package is uploaded on PyPI you can search for the package. The following also applies to wheels created by spksrc:
  • Pure-python packages generally end with a format like py2-none-any.whl or py2.py3-none-any.whl e.g. chardet.
  • Packages with C-extensions might end with cp34-cp34m-manylinux1_x86_64.whl e.g. lxml.
• While debugging and finding the best configuration it is possible to use a unique requirements.txt file and append a pure:, cross: or abi3: prefix to the needed wheels. From there using make spkclean will clean-up the wheelhouse in order for it to be regenerated at make time. Note that a final package must not use any prefixes.
• It is required to pin packages to a specific version. Example: mercurial==4.0.1. Other version specifiers are not allowed.
• It is required to add all requirements to the package. Upstream maintainers sometimes only list so-called top-level requirements for their packages, and rely on pip to process dependencies during installation. This can cause issues during installation of SynoCommunity packages. To make sure all the requirements are included in your final requirements.txt, run pip install -r requirements.txt (without specifying --no-deps) in a separate virtualenv. After pip has processed all the requirements, run pip freeze, and use that output as starting point for the final requirements.txt.
• References to setuptools, pip or wheel should not be included in requirements.txt or be commented out.
• Python packages that are processed as DEPENDS, or cross packages, should not be included in requirements.txt or be commented out.
• Errors such as command 'gcc' failed with exit status 1 means cross-compiling is required.
• In some cases, wheels appear to build successfully as a pure-python wheel, but fail to install or work correctly on the target. Make sure to test the package, and if you run into issues, try to cross-compile the wheel instead.

Notices

• Some native wheel code may not compile without CFLAGS=-Wno-error=format-security
• Some wheel code archive like gevent does not include generated C code. Prefer to download wheel source archives PyPi.org from https://pypi.org/project/gevent/1.4.0/#files