Enable Android build (#384)

* Support build for target_os android

The target_os `android` is quite similar to `linux` but must be
mentioned in the guards explicitly. Tested for target
`aarch64-linux-android`.

* Add target aarch64-linux-android to ci

Use `cross` for building target `aarch64-linux-android`. In the `ci`
workflow the matrix branch of `os` `ubuntu-latest` is used because the
matrix organization is host os centric instead of target.

Releases are build with `cross` for target `aarch64-linux-android`.

Install `cross` if needed for a run. At the time of writing the latest
release of `cross` can't build binaries for target
`aarch64-linux-android` and it's common sense that the latest git tag or
`master` shall be used [1].

[1] https://github.com/cross-rs/cross/issues/1222

* Add android related readme section

.github/workflows/ci.yaml

@@ -47,13 +47,25 @@ jobs:
           toolchain: ${{ matrix.rust }}
           components: rustfmt, clippy
 
+      - name: Install Cross on Ubuntu
+        if: matrix.os == 'ubuntu-latest'
+        # The latest realese of `cross` is not able to build/link for `aarch64-linux-android`
+        # See: https://github.com/cross-rs/cross/issues/1222
+        # This is fixed on `main` but not yet released. To avoid a breakage somewhen in the future
+        # pin the cross revision used to the latest HEAD at 04/2024. 
+        # Remove the git source and revision once cross 0.3 is released.
+        run: cargo install --git https://github.com/cross-rs/cross.git --rev 085092c cross
+
       - name: Check formatting
         run: cargo fmt --all -- --check
 
       - name: Build
-        id: build
         run: cargo build --verbose
 
+      - name: Build target aarch64-linux-android
+        if: matrix.os == 'ubuntu-latest'
+        run: cross build --target aarch64-linux-android --verbose
+
       # This is useful for debugging problems when the expected build artifacts
       # (like shell completions and man pages) aren't generated.
       - name: Show build.rs stderr
@@ -103,6 +115,13 @@ jobs:
           name: ${{ matrix.os }}-${{ matrix.rust }}-failed_snapshots
           path: '**/*.snap.new'
 
+      - name: Upload android binary
+        if: ${{ matrix.os == 'ubuntu-latest' && ( success() || steps.build.outcome == 'success' ) }}
+        uses: actions/upload-artifact@v3
+        with:
+          name: aarch64-linux-android-${{ matrix.rust }}
+          path: target/aarch64-linux-android/debug/bandwhich
+
       - name: Upload unix binary
         if: ${{ matrix.os != 'windows-latest' && ( success() || steps.build.outcome == 'success' ) }}
         uses: actions/upload-artifact@v3

.github/workflows/release.yaml

@@ -52,11 +52,17 @@ jobs:
     strategy:
       matrix:
         build:
+          - aarch64-linux-android
           - linux-x64-gnu
           - linux-x64-musl
           - macos-x64
           - windows-x64-msvc
         include:
+          - cargo: cargo # default; overwrite with `cross` if necessary
+          - build: aarch64-linux-android
+            os: ubuntu-latest
+            target: aarch64-linux-android
+            cargo: cross
           - build: linux-x64-gnu
             os: ubuntu-latest
             target: x86_64-unknown-linux-gnu
@@ -84,13 +90,17 @@ jobs:
         if: matrix.target == 'x86_64-unknown-linux-musl'
         run: sudo apt-get install -y --no-install-recommends musl-tools
 
+      - name: Install cross
+        if: matrix.cargo == 'cross'
+        run: cargo install --git https://github.com/cross-rs/cross.git --rev 085092c cross
+
       - name: Build release binary
         shell: bash
         env:
           RUSTFLAGS: "-C strip=symbols"
         run: |
           mkdir -p "$BANDWHICH_GEN_DIR"
-          cargo build --verbose --release --target ${{ matrix.target }}
+          ${{ matrix.cargo }} build --verbose --release --target ${{ matrix.target }}
 
       - name: Collect build artifacts
         shell: bash