Implement kotlinx-datetime for the Android native targets

[dkhalanskyjb]

To test, run an Android emulator, and then, in the command line, ./gradlew androidNativeArm64TestBinaries && adb push core/build/bin/androidNativeArm64/debugTest/test.kexe /data/local/tmp/ && adb shell /data/local/tmp/test.kexe

Change Arm64 to another platfom (X86, X64, or Arm32) as needed.

Fixes #311

[ilya-g]

I wonder would it be possible to restructure native sources later, so that the source tree resembled source set tree more?
E.g.
native
-- tzfile
---- tzdbOnFilesystem
------ darwin
------ linux
---- androidNative
-- windows

[dkhalanskyjb]

So, native/ would be

src/
test/
tzfile/
windows/

? Looks a bit strange to me. But I agree that the directory structure is too cumbersome now.

[natario1]

Shouldn't the immutable fallback be last in list? @dkhalanskyjb

[dkhalanskyjb]

@natario1, why? On line 42, the last put to execute will determine what gets used in the end. So, for Europe/London, for example, first, the map will receive the value from the fallback, but then, the value will be overwritten by the up-to-date timezone database. No?

[natario1]

I see, sorry for bothering then! It just felt wrong, I'm used to see 'fallbacks' at the end of lists. Thanks for working on this 🙏

[hfhbd]

Unrelated to the actual changes but there is also a new API to run android emulator tests by using Gradle: https://developer.android.com/studio/test/gradle-managed-devices
See https://github.com/hfhbd/kotlinx-uuid/blob/main/kotlinx-uuid-core/build.gradle.kts#L63 for a sample. Do you want to migrate to this API? If so, I can provide a PR.

[dkhalanskyjb]

@hfhbd, it would be very nice to have tests for the Android native targets, but aren't these Android tests for the JVM version?

[hfhbd]

@dkhalanskyjb Honestly, I don't know! They do work for the JVM version (of course) and you can bundle your c/native code in a .aar file to call it from Java too.
But the docs (and many information on the internet too) about android native are very thin compared to the Java ones, so asking someone from Google/create an issue would be helpful.

[dkhalanskyjb]

@hfhbd, if you want to dive deep into this and discover a way to test native Android code on the emulator, we'll gladly accept your PR. I'd advise not spending time on this, though. If my intuitive understanding is correct, this would be a lot of work. The way Kotlin/Native builds tests is by compiling them into a separate executable, and you'd need to write some JVM wrapper to call that executable and check the test results.

[natario1]

In case it helps, I built https://github.com/deepmedia/multiplatform-testing a few years ago, when trying to push for androidNative support in kotlinx repos (lack of testing was said to be one of the blockers).

It can download Android tools, create and spin up matching emulators, and run Android Native tests as a linux executable. At the time, I found a couple of bugs in atomicfu and k/n runtime thanks to it.