[Python-Dev] Support of the Android platform

[Xavier de Gaye]

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
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