feat(nix): nix-shell with android setup and ci checks for it

Part of an implementation example for android support, this offers a nix shell that helps with the setup and exports some checks that try to build the project for android, these checks will fail until the next commit adds the build.rs changes.

Author: jaoleal

.github/workflows/ci.yml

@@ -211,6 +211,28 @@ jobs:
     - name: Build and test
       run: cargo test -vv
 
+
+  android-nix-cross-check:
+    name: Cross-Compile (${{ matrix.check }}) building using nix
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        check:
+          - android-aarch64
+          - android-armv7
+          - android-x86_64
+    steps:
+    - uses: actions/checkout@v4
+
+    - uses: cachix/install-nix-action@v27
+      with:
+        extra_nix_config: |
+          accept-flake-config = true
+
+    - name: Build check
+      run: nix build .#checks.x86_64-linux.${{ matrix.check }} -L
+
   fuzz-corpus:
     name: Verify Fuzz Corpus
     runs-on: ubuntu-latest

flake.nix

@@ -15,6 +15,8 @@
 			let
 				pkgs = import nixpkgs {
 					inherit system;
+					config.android_sdk.accept_license = true;
+					config.allowUnfree = true;
 				};
 
 				rustVersion = "1.71.0";
@@ -35,6 +37,98 @@
 					cargo = rustBuildToolchainNightly;
 					rustc = rustBuildToolchainNightly;
 				};
+				androidComposition = pkgs.androidenv.composeAndroidPackages {
+					platformVersions = [ "34" ];
+					ndkVersions = [ "27.2.12479018" ];
+					includeNDK = true;
+				};
+				androidSdk = androidComposition.androidsdk;
+				androidNdk = "${androidSdk}/libexec/android-sdk/ndk/27.2.12479018";
+
+				rustAndroidToolchain = fenix.packages.${system}.combine [
+					rustToolchain.rustc
+					rustToolchain.cargo
+					rustToolchain.rust-src
+					rustToolchain.rust-std
+					(fenix.packages.${system}.targets.aarch64-linux-android.fromToolchainName {
+						name = rustVersion;
+						sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";
+					}).rust-std
+					(fenix.packages.${system}.targets.armv7-linux-androideabi.fromToolchainName {
+						name = rustVersion;
+						sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";
+					}).rust-std
+					(fenix.packages.${system}.targets.x86_64-linux-android.fromToolchainName {
+						name = rustVersion;
+						sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";
+					}).rust-std
+				];
+
+				# -- Source filtering --
+				src = pkgs.lib.cleanSourceWith {
+					src = self;
+					filter = path: type:
+						!(builtins.any (suffix: pkgs.lib.hasSuffix suffix path) [
+							"/target"
+							"/.git"
+							"/result"
+						]);
+				};
+
+				# -- Stable Rust for checks (MSRV is too old for some vendored crate manifests) --
+				rustStable = fenix.packages.${system}.stable;
+				mkAndroidCheck = { name, rustTarget }:
+					let
+						androidStableToolchain = fenix.packages.${system}.combine [
+							rustStable.rustc
+							rustStable.cargo
+							rustStable.rust-src
+							rustStable.rust-std
+							fenix.packages.${system}.targets.${rustTarget}.stable.rust-std
+						];
+						androidRustPlatform = pkgs.makeRustPlatform {
+							cargo = androidStableToolchain;
+							rustc = androidStableToolchain;
+						};
+					in
+					androidRustPlatform.buildRustPackage {
+						pname = "rust-bitcoinkernel-android-${name}";
+						version = "0.2.0";
+						inherit src;
+
+						cargoLock.lockFile = self + "/Cargo-recent.lock";
+
+						nativeBuildInputs = [
+							pkgs.cmake
+							pkgs.pkg-config
+							pkgs.python3
+							androidSdk
+						];
+
+						buildInputs = [
+							pkgs.boost.dev
+						];
+
+						LIBCLANG_PATH = "${pkgs.llvmPackages.clang-unwrapped.lib}/lib/";
+						ANDROID_HOME = "${androidSdk}/libexec/android-sdk";
+						ANDROID_NDK_HOME = androidNdk;
+						ANDROID_NDK_ROOT = androidNdk;
+						CARGO_BUILD_TARGET = rustTarget;
+
+						buildPhase = ''
+							runHook preBuild
+							cargo build --target ${rustTarget} --release -p libbitcoinkernel-sys
+							runHook postBuild
+						'';
+						doCheck = false;
+
+						installPhase = ''
+							touch $out
+						'';
+
+						dontFixup = true;
+					};
+
 				rustfilt = rustPlatformNightly.buildRustPackage rec {
 					pname = "rustfilt";
 					version = "0.2.1";
@@ -77,6 +171,47 @@
 						pkgs.gcc.cc.lib
 					];
 				};
+
+				# Android development shell. Enter with:
+				#   nix develop .#android
+				# This gives you a shell with the Android NDK, Rust toolchains
+				# (with Android targets), cmake, and boost — everything needed to
+				# run: cargo build --target aarch64-linux-android -p libbitcoinkernel-sys
+				devShells.android = pkgs.mkShell {
+					packages = [
+						rustAndroidToolchain
+
+						pkgs.cmake
+						pkgs.boost.dev
+						androidSdk
+					];
+
+					LIBCLANG_PATH = "${pkgs.llvmPackages.clang-unwrapped.lib}/lib/";
+					ANDROID_HOME = "${androidSdk}/libexec/android-sdk";
+					ANDROID_NDK_HOME = androidNdk;
+					ANDROID_NDK_ROOT = androidNdk;
+				};
+
+				# Verify Android cross-compilation. Run with:
+				#   nix build .#checks.x86_64-linux.android-aarch64 -L
+				#   nix build .#checks.x86_64-linux.android-armv7 -L
+				#   nix build .#checks.x86_64-linux.android-x86_64 -L
+				checks = {
+					android-aarch64 = mkAndroidCheck {
+						name = "aarch64";
+						rustTarget = "aarch64-linux-android";
+					};
+
+					android-armv7 = mkAndroidCheck {
+						name = "armv7";
+						rustTarget = "armv7-linux-androideabi";
+					};
+
+					android-x86_64 = mkAndroidCheck {
+						name = "x86_64";
+						rustTarget = "x86_64-linux-android";
+					};
+				};
 			}
 		);
 }