Feat: Android support by jaoleal · Pull Request #158 · sedited/rust-bitcoinkernel

jaoleal · 2026-04-10T17:49:36Z

This pr contain 3 commits:

an old version of Add MSRV Cargo-minimal.lock and Cargo-recent.lock files #150 to make CI work again flawlessly.
A patch to the projects flake adding a nix-shell that give us the necessary setup for the android build, which includes the android toolchain and some environment variables. The second commit also introduces a new job on the CI that executes the building for android.
Android building instructions for build.rs

This is just a PoC, the main commit is the third one and the CI job containing the android building at that point should run fine.
For more context read #156

sedited · 2026-04-23T08:02:59Z

This looks nice, can you rebase this?

jaoleal · 2026-04-30T20:33:49Z

Changelog:

Removed some PoC code
Included android native building in ci (whitout nix).
A fix regarding building on macos was included to.
Rebased on top of Chore: remove bindgen and replace with hand-rolled FFI bindings #163 and included a fix regarding pointer sizes.

jaoleal · 2026-05-06T20:08:49Z

I reduced a lot the code and i think this is the concise version to support android

alexanderwiederin · 2026-05-07T09:53:42Z

~~I don't think android is a supported target for libbitcoinkernel. Doesn't this require upstream work on bitcoin core first?~~

alexanderwiederin

From what I understand the ubuntu boost packages don't ship with the cmake config that core needs when cross-compiling.

Am I right that you got it working on nix locally? If so, can we stick with with only supporting the nix build for android?

jaoleal · 2026-05-07T20:55:29Z

Am I right that you got it working on nix locally? If so, can we stick with with only supporting the nix build for android?

Yes, i worked all this on nix but Im not sure about leaving android building exclusively for nix, this would make it a direct dependency of the project and its not something really special that only nix can do. Besides the dependency problem, nix can drift versions because of nixpkgs releases and that can raise problems only by itself.

~~Therefore, i still would like for CI to keep the runner as ubuntu. Its easier for the general developer audience to replicate the build on their machine if they need~~

Edit: see the comment below

jaoleal · 2026-05-07T22:11:57Z

Correction, I changed my mind... The idea is that the build.rs file is already bloated because of the android support additions, adding support for ubuntu bloated it even more... It looks like leaving to nix to cross-compile to android is actually a good choice, the changes stay minimal on build.rs side and it have broader support running on linux and macos machines... Ill make sure to document that somewhere and state how to make the android build.

This commit presents the minimal work needed for this crate to be able to be compiled for android, this is minimal to avoid bloating too much the build process, which would make it hard to review and maintain. The rest of the work will be done on nix side, which provides better ergonomics for such job.

This commit presents the rest of the work to provide android builds, the packaging and compiling is done trough nix for simplicity

jaoleal · 2026-05-08T00:37:47Z

Okay the android build being exclusive for nix made things easier for this PR, the changes on build.rs are minimal... I included some inline doc comments so the reviewer can follow why each line were added.

I added a section on the readme explaining the android build but its not that explanatory in a level that one can learn how to build this lib to android, do you guys think thats needed ?

I can extend the docs to include instructions for one to reproduce it on a downstream build.rs, just tell me where to put it.
AFAIK is trivial for those using Kotlin, as mandacaru is using, to pull an already compiled shared object during build time. Thats what ill do on floresta-nix for mandacaru, ideally the CI would also offer the compiled objects too for those who might need it.

alexanderwiederin

Great to see this working.

If we land the hand-written bindings PR (#163) first, the changes in build.rs should become simpler. I would suggest we wait for it.

alexanderwiederin · 2026-05-12T09:09:23Z

+        assert!(
+            Path::new(&toolchain_file).exists(),
+            "Android NDK toolchain file not found at {toolchain_file}. \
+             Check that ANDROID_NDK_HOME points to a valid NDK installation"
+        );


I think asserts are for faults in logic, not environment variables. I think a !path.exists() { panic!(...) } or .expect() would read more naturally here.

alexanderwiederin · 2026-05-12T09:16:02Z

+            .arg(format!("-DCMAKE_ANDROID_ARCH_ABI={abi}"))
+            .arg(format!("-DCMAKE_SYSTEM_VERSION={api_level}"))


Do we really have to pass these in a second time after lines 96 and 97?

alexanderwiederin · 2026-05-12T09:23:39Z

+						};
+						androidComposition = androidPkgs.androidenv.composeAndroidPackages {
+							platformVersions = [ "34" ];
+							ndkVersions = [ "27.2.12479018" ];


27.2.12479018 is duplicated on line 94. I would suggest we make it a variable.

alexanderwiederin · 2026-05-12T09:28:26Z

+										name = rustVersion;
+										sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";


The sha256 here is already defined at the top of the flake as part of rustToolchain. Extract it into a rustSha256 variable alongside rustVersion and reference it in both places. Otherwise a toolchain bump requires updating two identical hashes.

alexanderwiederin · 2026-05-12T09:31:55Z

+							config.allowUnfree = true;
+						};
+						androidComposition = androidPkgs.androidenv.composeAndroidPackages {
+							platformVersions = [ "34" ];


Can we add a comment above this?

# platformVersions is the SDK tooling version, not the minimum API level. # The NDK target floor is set via ANDROID_API_LEVEL in build.rs (default 24).

alexanderwiederin · 2026-05-12T09:34:05Z

+						android-aarch64 = mkAndroidPackage "aarch64-linux-android";
+						android-armv7 = mkAndroidPackage "armv7-linux-androideabi";
+						android-x86_64 = mkAndroidPackage "x86_64-linux-android";


in build.rs, the android_abi() function handles four targets, but here only three are exposed. Is that intentional?

alexanderwiederin · 2026-05-12T09:38:03Z

+    name: Cross-Compile for Android (${{ matrix.package }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false


alexanderwiederin · 2026-05-12T09:38:52Z

+Android cross-compilation requires [Nix](https://nixos.org/).
+
+Nix package outputs bundle the exact NDK version, Rust toolchains with Android
+targets, Boost, and cmake in a single reproducible build drying out the support


what do you mean "drying" out the support?

alexanderwiederin · 2026-05-12T09:41:39Z

 cargo b
 ```

+### Android Cross-Compilation


Worth adding a note that the output targets Android API 24+ (Nougat) minimum, so people know the minimum Android version their app must support.

jaoleal mentioned this pull request Apr 10, 2026

Android Support #156

Open

jaoleal mentioned this pull request Apr 30, 2026

Chore: remove bindgen and replace with hand-rolled FFI bindings #163

Open

jaoleal force-pushed the android_support branch from 55272e4 to 378a3bc Compare April 30, 2026 20:31

jaoleal mentioned this pull request Apr 30, 2026

Android cross-compilation blocked by aws-lc-rs and bitcoinkernel getfloresta/Floresta#943

Open

jaoleal force-pushed the android_support branch 6 times, most recently from a2c959f to 0f3252f Compare May 6, 2026 20:07

jaoleal changed the title ~~POC: Android support~~ Feat: Android support May 6, 2026

jaoleal marked this pull request as ready for review May 6, 2026 20:08

alexanderwiederin reviewed May 7, 2026

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Comment thread flake.nix Outdated

alexanderwiederin reviewed May 7, 2026

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Comment thread flake.nix Outdated

jaoleal force-pushed the android_support branch from 0f3252f to 49309b8 Compare May 7, 2026 23:43

jaoleal force-pushed the android_support branch 3 times, most recently from e9e6806 to 479401e Compare May 8, 2026 00:21

jaoleal added 2 commits May 7, 2026 21:22

feat(nix): add android cross-compilation packages

179271f

This commit presents the rest of the work to provide android builds, the packaging and compiling is done trough nix for simplicity

ci: add Android cross-compilation CI job

30f44a8

jaoleal force-pushed the android_support branch from 479401e to 30f44a8 Compare May 8, 2026 00:24

alexanderwiederin reviewed May 12, 2026

View reviewed changes

		.arg(format!("-DCMAKE_ANDROID_ARCH_ABI={abi}"))
		.arg(format!("-DCMAKE_SYSTEM_VERSION={api_level}"))

		name = rustVersion;
		sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";

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