Ensure libpython's standard lib compatibility

[aaaaalbert]

We currently build libpython separately and out-of-app through Kivy (see relevant commit 9322af3), extract the shared object, and include that in ./distribution/python27/lib/armeabi-v7a (and the Python headers (also from the Kivy build dir) in ./distribution/python27/include) for the rest of our build process.

At some point in time, it would be nice to be able to build libpython directly from our main Gradle conf; more importantly, we need to ensure today that libpython links against a compatible version of the standard libraries.

The issue is that Android doesn't seem to version its system libraries, but their scope changes significantly between versions. A problem I ran into on Lollipop that involved forkpty seemingly has its roots in this mess --- updating to Nougat "fixed" the problem, in that Nougat's libc provides the call that the pre-built libpython2.7.so that I got from this repo requires.

Thus, when rebuilding libpython, we must test on several Android versions! I don't know if emulation works/makes sense/is efficient in this case, or we just grab another couple of phones that we flash to the various versions. (Dual-boot is not really an option. Third-party images can do it, but require root etc., so we'd be looking at non-stock and thus non-representative firmware afterwards.)

---

For easier reference, I'll reproduce the error message and a few sources that led me to my conclusions below too. It's well possible that other combinations of build and runtime versions give errors for different standard library calls.

W/linker  ( 7159): libpython2.7.so: unused DT entry: type 0x6ffffffe arg 0x1143c
W/linker  ( 7159): libpython2.7.so: unused DT entry: type 0x6fffffff arg 0x3
E/art     ( 7159): dlopen("/data/app/com.sensibility_testbed-1/lib/arm/libpython2.7.so", RTLD_LAZY) failed: dlopen failed: cannot locate symbol "forkpty" referenced by "libpython2.7.so"...
D/AndroidRuntime( 7159): Shutting down VM
E/AndroidRuntime( 7159): FATAL EXCEPTION: main
E/AndroidRuntime( 7159): Process: com.sensibility_testbed, PID: 7159
E/AndroidRuntime( 7159): java.lang.UnsatisfiedLinkError: dlopen failed: cannot locate symbol "forkpty" referenced by "libpython2.7.so"...
E/AndroidRuntime( 7159): 	at java.lang.Runtime.loadLibrary(Runtime.java:371)
E/AndroidRuntime( 7159): 	at java.lang.System.loadLibrary(System.java:988)
E/AndroidRuntime( 7159): 	at com.snakei.PythonInterpreterService.<clinit>(PythonInterpreterService.java:72)
E/AndroidRuntime( 7159): 	at com.sensibility_testbed.SensibilityActivity$2.onClick(SensibilityActivity.java:203)
E/AndroidRuntime( 7159): 	at android.view.View.performClick(View.java:4780)
E/AndroidRuntime( 7159): 	at android.view.View$PerformClick.run(View.java:19866)
E/AndroidRuntime( 7159): 	at android.os.Handler.handleCallback(Handler.java:739)
E/AndroidRuntime( 7159): 	at android.os.Handler.dispatchMessage(Handler.java:95)
E/AndroidRuntime( 7159): 	at android.os.Looper.loop(Looper.java:135)
E/AndroidRuntime( 7159): 	at android.app.ActivityThread.main(ActivityThread.java:5254)
E/AndroidRuntime( 7159): 	at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime( 7159): 	at java.lang.reflect.Method.invoke(Method.java:372)
E/AndroidRuntime( 7159): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
E/AndroidRuntime( 7159): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
W/ActivityManager(  813):   Force finishing activity 1 com.sensibility_testbed/.SensibilityActivity

Hints taken from:

• Experimental gradle plugin with NDK library version mismatches
• Might need to add libutil to linker flags
• Android L removed MD5 functions from libc

[aaaaalbert]

For the forkpty call in libc in particular, API level 23 exposes it, whereas 22 doesn't. I haven't (yet?) found another library on 22 that implements forkpty.

[aaaaalbert]

I've rebuilt libpython2.7.so for API level 21, downgraded the Gradle config too, removed SensibilityActivity.java/onRequestPermissionsResult which isn't compatible with that API level, rebuilt the project, and behold, it works on my LG H420 running Android 5.0.1!

I guess that's a good sign!

Build command (see my Python build instructions):

p4a apk --arch=armeabi-v7a --private=/home/lukas/kivy-app --package=org.example.myapp --name "My app" --version 0.1 --requirements=python2,libffi --sdk_dir=$HOME/Android/Sdk --ndk_dir=$HOME/android-ndk-r13b --android_api=21 --ndk_ver=r13b

TODO: Document that we want backwards compatibility, and figure out how far backwards we can sanely go. Currently, the NDK supports API levels >=21, which covers around 60% of the world's Android population. Other API level pitfalls may lurk about.