ci: add Android cross-compilation CI job

Author: jaoleal

.github/workflows/ci.yml

@@ -211,6 +211,26 @@ jobs:
     - name: Build and test
       run: cargo test -vv
 
+
+  android-cross-compile:
+    name: Cross-Compile for Android (${{ matrix.package }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        package:
+          - android-aarch64
+          - android-armv7
+          - android-x86_64
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Install Nix
+      uses: cachix/install-nix-action@v31
+
+    - name: Build
+      run: nix build .#${{ matrix.package }} -L
+
   fuzz-corpus:
     name: Verify Fuzz Corpus
     runs-on: ubuntu-latest

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Added Android cross-compilation support (`aarch64-linux-android`, `armv7-linux-androideabi`, `x86_64-linux-android`) via Nix package outputs (`nix build .#android-aarch64`, etc.).
+- Added Nix package outputs for Android with bundled NDK r27, Rust toolchains, Boost, and cmake.
 - Added `BlockTreeEntry::ancestor` to look up an ancestor block at a given height. Returns `None` if the height is out of range. This operation is O(log N).
 - Added `Transaction::locktime()` to retrieve a transaction's `nLockTime` value as a `u32`.
 - Added `TxIn::sequence()` to retrieve an input's `nSequence` value as a `u32`.

README.md

@@ -34,6 +34,22 @@ dependencies. Once setup, run:
 cargo b
 ```
 
+### Android Cross-Compilation
+
+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
+needed on rust-bitcoinkernel side.
+
+```bash
+nix build .#android-aarch64
+nix build .#android-armv7
+nix build .#android-x86_64
+```
+
+The resulting libraries are placed in `result/lib/`.
+
 ## MSRV (Minimum Supported Rust Version)
 
 The minimum supported Rust version is 1.71. Users on rustc older than