feat: android support

Add Android cross-compilation support to the build system. Detects
Android targets, configures cmake with the NDK toolchain, and links
the NDK C++ static libraries.

Author: jaoleal

libbitcoinkernel-sys/CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Added build support for android
 - New `btck_ConsensusParams` opaque type for holding consensus parameters
 - New `btck_chain_parameters_get_consensus_params` for extracting consensus params from `btck_ChainParameters` (lifetime-bound to the chain parameters object)
 - New `btck_block_check` for context-free block validation (size limits, coinbase structure, sigop limits, with optional POW and merkle-root checks via `btck_BlockCheckFlags`)

libbitcoinkernel-sys/build.rs

@@ -4,6 +4,35 @@ use std::path::Path;
 use std::path::PathBuf;
 use std::process::Command;
 
+/// Maps Rust target triples to Android ABI names used by the NDK.
+fn android_abi(target: &str) -> Option<&'static str> {
+    match target {
+        t if t.contains("aarch64") => Some("arm64-v8a"),
+        t if t.contains("armv7") => Some("armeabi-v7a"),
+        t if t.contains("x86_64") => Some("x86_64"),
+        t if t.contains("i686") => Some("x86"),
+        _ => None,
+    }
+}
+
+/// Maps Rust target triples to the NDK sysroot directory names.
+/// These differ from Rust triples for armv7 targets.
+fn android_sysroot_triple(target: &str) -> &str {
+    if target.starts_with("armv7") {
+        "arm-linux-androideabi"
+    } else {
+        target
+    }
+}
+
+/// Detect whether we are cross-compiling for Android and return the NDK home path if so.
+fn android_ndk_home() -> Option<String> {
+    env::var("ANDROID_NDK_HOME")
+        .or_else(|_| env::var("ANDROID_NDK_ROOT"))
+        .or_else(|_| env::var("NDK_HOME"))
+        .ok()
+}
+
 fn main() {
     let bitcoin_dir = Path::new("bitcoin");
     let out_dir = env::var("OUT_DIR").unwrap();
@@ -17,7 +46,8 @@ fn main() {
 
     let build_config = "RelWithDebInfo";
 
-    Command::new("cmake")
+    let mut cmake_configure = Command::new("cmake");
+    cmake_configure
         .arg("-B")
         .arg(&build_dir)
         .arg("-S")
@@ -40,7 +70,42 @@ fn main() {
         .arg("-DBUILD_SHARED_LIBS=OFF")
         .arg("-DCMAKE_INSTALL_LIBDIR=lib")
         .arg("-DENABLE_IPC=OFF")
-        .arg(format!("-DCMAKE_INSTALL_PREFIX={}", install_dir.display()))
+        .arg(format!("-DCMAKE_INSTALL_PREFIX={}", install_dir.display()));
+
+    let target = env::var("TARGET").unwrap();
+    let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap();
+
+    if target_os == "android" {
+        let ndk =
+            android_ndk_home().expect("Android target detected but ANDROID_NDK_HOME is not set");
+        let toolchain_file = format!("{ndk}/build/cmake/android.toolchain.cmake");
+        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"
+        );
+        let abi =
+            android_abi(&target).unwrap_or_else(|| panic!("unsupported Android target: {target}"));
+
+        // API level 24+ is required because Bitcoin Core uses getifaddrs
+        // which was introduced in Android API 24 (Nougat).
+        let api_level = env::var("ANDROID_API_LEVEL").unwrap_or_else(|_| "24".to_string());
+
+        cmake_configure
+            .arg(format!("-DCMAKE_TOOLCHAIN_FILE={toolchain_file}"))
+            .arg(format!("-DANDROID_ABI={abi}"))
+            .arg(format!("-DANDROID_PLATFORM=android-{api_level}"))
+            .arg("-DCMAKE_SYSTEM_NAME=Android")
+            .arg(format!("-DCMAKE_ANDROID_ARCH_ABI={abi}"))
+            .arg(format!("-DCMAKE_SYSTEM_VERSION={api_level}"))
+            .arg(format!("-DCMAKE_ANDROID_NDK={ndk}"))
+            // The Android NDK toolchain sets CMAKE_FIND_ROOT_PATH_MODE_PACKAGE
+            // to ONLY, which prevents cmake from finding host packages via
+            // CMAKE_PREFIX_PATH. Override it so Boost headers can be located.
+            .arg("-DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=BOTH");
+    }
+
+    cmake_configure
         .status()
         .expect("cmake should be installed and available in PATH");
 
@@ -97,14 +162,27 @@ fn main() {
         .expect("Couldn't write bindings!");
 
     let compiler = cc::Build::new().get_compiler();
-    let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap();
 
     if target_os == "windows" {
         println!("cargo:rustc-link-lib=bcrypt");
         println!("cargo:rustc-link-lib=shell32");
-    }
-
-    if compiler.is_like_clang() {
+    } else if target_os == "android" {
+        // Android NDK ships libc++_static.a and libc++abi.a in the
+        // per-architecture sysroot directory (not the API-level subdirectory).
+        if let Some(ndk) = android_ndk_home() {
+            let ndk_triple = android_sysroot_triple(&target);
+            let host_tag = if cfg!(target_os = "macos") {
+                "darwin-x86_64"
+            } else {
+                "linux-x86_64"
+            };
+            let ndk_lib_dir =
+                format!("{ndk}/toolchains/llvm/prebuilt/{host_tag}/sysroot/usr/lib/{ndk_triple}");
+            println!("cargo:rustc-link-search=native={ndk_lib_dir}");
+        }
+        println!("cargo:rustc-link-lib=static=c++_static");
+        println!("cargo:rustc-link-lib=static=c++abi");
+    } else if compiler.is_like_clang() {
         if target_os == "macos" {
             println!("cargo:rustc-link-lib=dylib=c++");
         } else {