Abstract

This PEP proposes the creation of a new platform tag for Python package built distributions, such as wheels, called manylinux1_{x86_64,i686} with external dependencies limited to a standardized, restricted subset of the Linux kernel and core userspace ABI. It proposes that PyPI support uploading and distributing wheels with this platform tag, and that pip support downloading and installing these packages on compatible platforms.

Rationale

Currently, distribution of binary Python extensions for Windows and OS X is straightforward. Developers and packagers build wheels [1] [2], which are assigned platform tags such as win32 or macosx_10_6_intel, and upload these wheels to PyPI. Users can download and install these wheels using tools such as pip.

For Linux, the situation is much more delicate. In general, compiled Python extension modules built on one Linux distribution will not work on other Linux distributions, or even on different machines running the same Linux distribution with different system libraries installed.

Build tools using PEP 425 platform tags [3] do not track information about the particular Linux distribution or installed system libraries, and instead assign all wheels the too-vague linux_i686 or linux_x86_64 tags. Because of this ambiguity, there is no expectation that linux-tagged built distributions compiled on one machine will work properly on another, and for this reason, PyPI has not permitted the uploading of wheels for Linux.

It would be ideal if wheel packages could be compiled that would work on any linux system. But, because of the incredible diversity of Linux systems -- from PCs to Android to embedded systems with custom libcs -- this cannot be guaranteed in general.

Instead, we define a standard subset of the kernel+core userspace ABI that, in practice, is compatible enough that packages conforming to this standard will work on many linux systems, including essentially all of the desktop and server distributions in common use. We know this because there are companies who have been distributing such widely-portable pre-compiled Python extension modules for Linux -- e.g. Enthought with Canopy [4] and Continuum Analytics with Anaconda [5].

Building on the compability lessons learned from these companies, we thus define a baseline manylinux1 platform tag for use by binary Python wheels, and introduce the implementation of preliminary tools to aid in the construction of these manylinux1 wheels.

Key Causes of Inter-Linux Binary Incompatibility

To properly define a standard that will guarantee that wheel packages meeting this specification will operate on many linux platforms, it is necessary to understand the root causes which often prevent portability of pre-compiled binaries on Linux. The two key causes are dependencies on shared libraries which are not present on users' systems, and dependencies on particular versions of certain core libraries like glibc.

The manylinux1 policy

For these reasons, to achieve broad portability, Python wheels

• should depend only on an extremely limited set of external shared libraries; and
• should depend only on "old" symbol versions in those external shared libraries; and
• should depend only on a widely-compatible kernel ABI.

To be eligible for the manylinux1 platform tag, a Python wheel must therefore both (a) contain binary executables and compiled code that links only to libraries with SONAMEs included in the following list:

libpanelw.so.5
libncursesw.so.5
libgcc_s.so.1
libstdc++.so.6
libm.so.6
libdl.so.2
librt.so.1
libcrypt.so.1
libc.so.6
libnsl.so.1
libutil.so.1
libpthread.so.0
libX11.so.6
libXext.so.6
libXrender.so.1
libICE.so.6
libSM.so.6
libGL.so.1
libgobject-2.0.so.0
libgthread-2.0.so.0
libglib-2.0.so.0

and, (b) work on a stock CentOS 5.11 [6] system that contains the system package manager's provided versions of these libraries.

Because CentOS 5 is only available for x86_64 and i686 architectures, these are the only architectures currently supported by the manylinux1 policy.

On Debian-based systems, these libraries are provided by the packages

libncurses5 libgcc1 libstdc++6 libc6 libx11-6 libxext6
libxrender1 libice6 libsm6 libgl1-mesa-glx libglib2.0-0

On RPM-based systems, these libraries are provided by the packages

ncurses libgcc libstdc++ glibc libXext libXrender
libICE libSM mesa-libGL glib2

This list was compiled by checking the external shared library dependencies of the Canopy [4] and Anaconda [5] distributions, which both include a wide array of the most popular Python modules and have been confirmed in practice to work across a wide swath of Linux systems in the wild.

Many of the permitted system libraries listed above use symbol versioning schemes for backward compatibility. The latest symbol versions provided with the CentOS 5.11 versions of these libraries are:

GLIBC_2.5
CXXABI_3.4.8
GLIBCXX_3.4.9
GCC_4.2.0

Therefore, as a consequence of requirement (b), any wheel that depends on versioned symbols from the above shared libraries may depend only on symbols with the following versions:

GLIBC <= 2.5
CXXABI <= 3.4.8
GLIBCXX <= 3.4.9
GCC <= 4.2.0

These recommendations are the outcome of the relevant discussions in January 2016 [7], [8].

Note that in our recommendations below, we do not suggest that pip or PyPI should attempt to check for and enforce the details of this policy (just as they don't check for and enforce the details of existing platform tags like win32). The text above is provided (a) as advice to package builders, and (b) as a method for allocating blame if a given wheel doesn't work on some system: if it satisfies the policy above, then this is a bug in the spec or the installation tool; if it does not satisfy the policy above, then it's a bug in the wheel. One useful consequence of this approach is that it leaves open the possibility of further updates and tweaks as we gain more experience, e.g., we could have a "manylinux 1.1" policy which targets the same systems and uses the same manylinux1 platform tag (and thus requires no further changes to pip or PyPI), but that adjusts the list above to remove libraries that have turned out to be problematic or add libraries that have turned out to be safe.

PKG-VERSION-cp27-cp27mu-manylinux1_x86_64.whl
                 ^^^^^^ Good!

PKG-VERSION-cp27-cp27m-manylinux1_x86_64.whl
                 ^^^^^ Okay!

PKG-VERSION-cp27-none-manylinux1_x86_64.whl
                 ^^^^ BAD! Don't do this!

Compilation of Compliant Wheels

The way glibc, libgcc, and libstdc++ manage their symbol versioning means that in practice, the compiler toolchains that most developers use to do their daily work are incapable of building manylinux1-compliant wheels. Therefore we do not attempt to change the default behavior of pip wheel / bdist_wheel: they will continue to generate regular linux_* platform tags, and developers who wish to use them to generate manylinux1-tagged wheels will have to change the tag as a second post-processing step.

To support the compilation of wheels meeting the manylinux1 standard, we provide initial drafts of two tools.

Bundled Wheels on Linux

While we acknowledge many approaches for dealing with third-party library dependencies within manylinux1 wheels, we recognize that the manylinux1 policy encourages bundling external dependencies, a practice which runs counter to the package management policies of many linux distributions' system package managers [14], [15]. The primary purpose of this is cross-distro compatibility. Furthermore, manylinux1 wheels on PyPI occupy a different niche than the Python packages available through the system package manager.

The decision in this PEP to encourage departure from general Linux distribution unbundling policies is informed by the following concerns:

1. In these days of automated continuous integration and deployment pipelines, publishing new versions and updating dependencies is easier than it was when those policies were defined.
2. pip users remain free to use the "--no-binary" option if they want to force local builds rather than using pre-built wheel files.
3. The popularity of modern container based deployment and "immutable infrastructure" models involve substantial bundling at the application layer anyway.
4. Distribution of bundled wheels through PyPI is currently the norm for Windows and OS X.
5. This PEP doesn't rule out the idea of offering more targeted binaries for particular Linux distributions in the future.

The model described in this PEP is most ideally suited for cross-platform Python packages, because it means they can reuse much of the work that they're already doing to make static Windows and OS X wheels. We recognize that it is less optimal for Linux-specific packages that might prefer to interact more closely with Linux's unique package management functionality and only care about targeting a small set of particular distos.

Platform Detection for Installers

Above, we defined what it means for a wheel to be manylinux1-compatible. Here we discuss what it means for a Python installation to be manylinux1-compatible. In particular, this is important for tools like pip to know when deciding whether or not they should consider manylinux1-tagged wheels for installation.

Because the manylinux1 profile is already known to work for the many thousands of users of popular commercial Python distributions, we suggest that installation tools should error on the side of assuming that a system is compatible, unless there is specific reason to think otherwise.

We know of four main sources of potential incompatibility that are likely to arise in practice:

• Eventually, in the future, there may exist distributions that break compatibility with this profile (e.g., if one of the libraries in the profile changes its ABI in a backwards-incompatible way)
• A linux distribution that is too old (e.g. RHEL 4)
• A linux distribution that does not use glibc (e.g. Alpine Linux, which is based on musl libc, or Android)

To address these we propose a two-pronged approach. To handle potential future incompatibilities, we standardize a mechanism for a Python distributor to signal that a particular Python install definitely is or is not compatible with manylinux1: this is done by installing a module named _manylinux, and setting its manylinux1_compatible attribute. We do not propose adding any such module to the standard library -- this is merely a well-known name by which distributors and installation tools can rendezvous. However, if a distributor does add this module, they should add it to the standard library rather than to a site-packages/ directory, because the standard library is inherited by virtualenvs (which we want), and site-packages/ in general is not.

Then, to handle the last two cases for existing Python distributions, we suggest a simple and reliable method to check for the presence and version of glibc (basically using it as a "clock" for the overall age of the distribution).

Specifically, the algorithm we propose is:

def is_manylinux1_compatible():
    # Only Linux, and only x86-64 / i686
    from distutils.util import get_platform
    if get_platform() not in ["linux-x86_64", "linux-i686"]:
        return False

    # Check for presence of _manylinux module
    try:
        import _manylinux
        return bool(_manylinux.manylinux1_compatible)
    except (ImportError, AttributeError):
        # Fall through to heuristic check below
        pass

    # Check glibc version. CentOS 5 uses glibc 2.5.
    return have_compatible_glibc(2, 5)

def have_compatible_glibc(major, minimum_minor):
    import ctypes

    process_namespace = ctypes.CDLL(None)
    try:
        gnu_get_libc_version = process_namespace.gnu_get_libc_version
    except AttributeError:
        # Symbol doesn't exist -> therefore, we are not linked to
        # glibc.
        return False

    # Call gnu_get_libc_version, which returns a string like "2.5".
    gnu_get_libc_version.restype = ctypes.c_char_p
    version_str = gnu_get_libc_version()
    # py2 / py3 compatibility:
    if not isinstance(version_str, str):
        version_str = version_str.decode("ascii")

    # Parse string and check against requested version.
    version = [int(piece) for piece in version_str.split(".")]
    assert len(version) == 2
    if major != version[0]:
        return False
    if minimum_minor > version[1]:
        return False
    return True

Rejected alternatives: We also considered using a configuration file, e.g. /etc/python/compatibility.cfg. The problem with this is that a single filesystem might contain many different interpreter environments, each with their own ABI profile -- the manylinux1 compatibility of a system-installed x86_64 CPython might not tell us much about the manylinux1 compatibility of a user-installed i686 PyPy. Locating this configuration information within the Python environment itself ensures that it remains attached to the correct binary, and dramatically simplifies lookup code.

We also considered using a more elaborate structure, like a list of all platform tags that should be considered compatible, together with their preference ordering, for example: _binary_compat.compatible = ["manylinux1_x86_64", "centos5_x86_64", "linux_x86_64"]. However, this introduces several complications. For example, we want to be able to distinguish between the state of "doesn't support manylinux1" (or eventually manylinux2, etc.) versus "doesn't specify either way whether it supports manylinux1", which is not entirely obvious in the above representation; and, it's not at all clear what features are really needed vis a vis preference ordering given that right now the only possible platform tags are manylinux1 and linux. So we're deferring a more complete solution here for a separate PEP, when / if Linux gets more platform tags.

For the library compatibility check, we also considered much more elaborate checks (e.g. checking the kernel version, searching for and checking the versions of all the individual libraries listed in the manylinux1 profile, etc.), but ultimately decided that this would be more likely to introduce confusing bugs than actually help the user. (For example: different distributions vary in where they actually put these libraries, and if our checking code failed to use the correct path search then it could easily return incorrect answers.)

PyPI Support

PyPI should permit wheels containing the manylinux1 platform tag to be uploaded. PyPI should not attempt to formally verify that wheels containing the manylinux1 platform tag adhere to the manylinux1 policy described in this document. This verification tasks should be left to other tools, like auditwheel, that are developed separately.

Rejected Alternatives

One alternative would be to provide separate platform tags for each Linux distribution (and each version thereof), e.g. RHEL6, ubuntu14_10, debian_jessie, etc. Nothing in this proposal rules out the possibility of adding such platform tags in the future, or of further extensions to wheel metadata that would allow wheels to declare dependencies on external system-installed packages. However, such extensions would require substantially more work than this proposal, and still might not be appreciated by package developers who would prefer not to have to maintain multiple build environments and build multiple wheels in order to cover all the common Linux distributions. Therefore we consider such proposals to be out-of-scope for this PEP.

Future updates

We anticipate that at some point in the future there will be a manylinux2 specifying a more modern baseline environment (perhaps based on CentOS 6), and someday a manylinux3 and so forth, but we defer specifying these until we have more experience with the initial manylinux1 proposal.

References

Copyright

This document has been placed into the public domain.