[Python-Dev] Support of the Android platform

Mon Jan 22 12:28:22 EST 2018

Hi,

I'm still talking with Paul Peny (pmpp on IRC) who is trying to build
the master branch of Python on Android, using cross-compilation or
directly on an Android device. I started to took notes since Android
is a complex platforms and it's not easy for me to remember
everything.

http://vstinner.readthedocs.io/python_android.html

Paul would like to support Android 4.4 Kitkat (API 19) just because
it's possible to find cheap devices running Android, but usually only
with old Android versions. Technically, it doesn't see difficult to
support API 19+ (instead of 21+), a few tiny patches are needed. But I
don't want to have a "full support" of API 19+, only basic support
like "make sure that the compilation doesn't fail", not "all tests
must pass".

It seems like sys.platform == 'android' would be more appropriate
since Android is not Linux: different libc, different filesystems,
etc.

While Xavier promotes cross-compilation, Paul would like to build
Python directly on Android to get pip and wheels.

Honestly, I have no strong opinion, since I don't know well Android.
I'm trying to help everybody working on the Android support. IMHO it's
fine to support multiple ways to build Python for Android. It's not
like there is very obvious option which has no drawback... Cross
compilation is complex, getting a C compiler on Android also seems to
be complex. From my point of view, compared to a common Fedora Linux,
Android doesn't seem easy to use to develop on CPython...

Victor

2017-12-10 15:19 GMT+01:00 Xavier de Gaye <xdegaye at gmail.com>:
> The following note is a proposal to add the support of the Android platform.
>
> The note is easier to read with clickable links at
> https://github.com/xdegaye/cagibi/blob/master/doc/android_support.rst
>
> Motivations
> ===========
>
> * Android is ubiquitous.
> * This would be the first platform supported by Python that is
> cross-compiled,
>   thanks to many contributors.
> * Although the Android operating system is linux, it is different from most
>   linux platforms, for example it does not use GNU libc and runs SELinux in
>   enforcing mode. Therefore supporting this platform would make Python more
>   robust and also would allow testing it on arm 64-bit processors.
> * Python running on Android is also a handheld calculator, a successor of
> the
>   slide rule and the `HP 41`_.
>
> Current status
> ==============
>
> * The Python test suite succeeds when run on Android emulators using
> buildbot
>   strenuous settings with the following architectures on API 24: x86,
> x86_64,
>   armv7 and arm64.
> * The `Android build system`_ is described in another section.
> * The `buildmaster-config PR 26`_ proposes to update ``master.cfg`` to
> enable
>   buildbots to run a given Android API and architecture on the emulators.
> * The Android emulator is actually ``qemu``, so the test suites for x86 and
>   x86_64 last about the same time as the test suite run natively when the
>   processor of the build system is of the x86 family. The test suites for
> the
>   arm architectures last much longer: about 8 hours for arm64 and 10 hours
> for
>   armv7 on a four years old laptop.
> * The changes that have been made to achieve this status are listed in
>   `bpo-26865`_, the Android meta-issue.
> * Given the cpu resources required to run the test suite on the arm
> emulators,
>   it may be difficult to find a contributed buildbot worker. So it remains
> to
>   find the hardware to run these buildbots.
>
> Proposal
> ========
>
> Support the Android platform on API 24 [1]_ for the x86_64, armv7 and arm64
> architectures built with NDK 14b.
>
> *API 24*
>   * API 21 is the first version to provide usable support for wide
> characters
>     and where SELinux is run in enforcing mode.
>
>   * API 22 introduces an annoying bug on the linker that prints something
> like
>     this when python is started::
>
>       ``WARNING: linker: libpython3.6m.so.1.0: unused DT entry: type
> 0x6ffffffe arg 0x14554``.
>
>     The `termux`_ Android terminal emulator describes this problem at the
> end
>     of its `termux-packages`_ gitlab page and has implemented a
>     ``termux-elf-cleaner`` tool to strip the useless entries from the ELF
>     header of executables.
>
>   * API 24 is the first version where the `adb`_ shell is run on the
> emulator
>     as a ``shell`` user instead of the ``root`` user previously, and the
> first
>     version that supports arm64.
>
> *x86_64*
>   It seems that no handheld device exists using that architecture. It is
>   supported because the x86_64 Android emulator runs fast and therefore is a
>   good candidate as a buildbot worker.
>
> *NDK 14b*
>   This release of the NDK is the first one to use `Unified headers`_ fixing
>   numerous problems that had been fixed by updating the Python configure
> script
>   until now (those changes have been reverted by now).
>
> Android idiosyncrasies
> ======================
>
> * The default shell is ``/system/bin/sh``.
> * The file system layout is not a traditional unix layout, there is no
>   ``/tmp`` for example. Most directories have user restricted access,
>   ``/sdcard`` is mounted as ``noexec`` for example.
> * The (java) applications are allocated a unix user id and a subdirectory on
>   ``/data/data``.
> * SELinux is run in enforcing mode.
> * Shared memory and semaphores are not supported.
> * The default encoding is UTF-8.
>
> Android build system
> ====================
>
> The Android build system is implemented at `bpo-30386`_ with `PR 1629`_ and
> is documented by its `README`_. It provides the following features:
>
> * To build a distribution for a device or an emulator with a given API level
>   and a given architecture.
> * To start the emulator and
>     + install the distribution
>     + start a remote interactive shell
>     + or run remotely a python command
>     + or run remotely the buildbottest
> * Run gdb on the python process that is running on the emulator with python
>   pretty-printing.
>
> The build system adds the ``Android/`` directory and the
> ``configure-android``
> script to the root of the Python source directory on the master branch
> without
> modifying any other file. The build system can be installed, upgraded (i.e.
> the
> SDK and NDK) and run remotely, through ssh for example.
>
> The following external libraries, when they are configured in the build
> system,
> are downloaded from the internet and cross-compiled (only once, on the first
> run of the build system) before the cross-compilation of the extension
> modules:
>
> * ``ncurses``
> * ``readline``
> * ``sqlite``
> * ``libffi``
> * ``openssl``, the cross-compilation of openssl fails on x86_64 and arm64
> and
>   this step is skipped on those architectures.
>
> The following extension modules are disabled by adding them to the
> ``*disabled*`` section of ``Modules/Setup``:
>
> * ``_uuid``, Android has no uuid/uuid.h header.
> * ``grp`` some grp.h functions are not declared.
> * ``_crypt``, Android does not have crypt.h.
> * ``_ctypes`` on x86_64 where all long double tests fail (`bpo-32202`_) and
> on
>   arm64 (see `bpo-32203`_).
>
> .. [1] On Wikipedia `Android version history`_ lists the correspondence
> between
>    API level, commercial name and version for each release. It also provides
>    information on the global Android version distribution, see the two
> charts
>    on top.
>
> .. _`README`:
> https://github.com/xdegaye/cpython/blob/bpo-30386/Android/README.rst
> .. _`bpo-26865`: https://bugs.python.org/issue26865
> .. _`bpo-30386`: https://bugs.python.org/issue30386
> .. _`bpo-32202`: https://bugs.python.org/issue32202
> .. _`bpo-32203`: https://bugs.python.org/issue32203
> .. _`PR 1629`: https://github.com/python/cpython/pull/1629
> .. _`buildmaster-config PR 26`:
> https://github.com/python/buildmaster-config/pull/26
> .. _`Android version history`:
> https://en.wikipedia.org/wiki/Android_version_history
> .. _`termux`: https://termux.com/
> .. _`termux-packages`: https://gitlab.com/jbwhips883/termux-packages
> .. _`adb`: https://developer.android.com/studio/command-line/adb.html
> .. _`Unified headers`:
> https://android.googlesource.com/platform/ndk.git/+/ndk-r14-release/docs/UnifiedHeaders.md
> .. _`HP 41`: https://en.wikipedia.org/wiki/HP-41C
> .. vim:filetype=rst:tw=78:ts=8:sts=2:sw=2:et:
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