rsend: push phone folders to a home SSH host over rsync

A small Android app for one-way folder backup, a KISS alternative to
Syncthing. It bundles rsync (built from pinned source via the NDK) and
a pure-Go SSH transport, both shipped in the APK as lib*.so and run from
the native library directory.

rsend pins the host key, stores the ed25519 identity Keystore-encrypted,
pushes each folder additively or as a mirror, and runs on demand or on a
WiFi-only schedule. The build is self-contained and reproducible: make
setup provisions the toolchain, make builds rsync, rsh, and the APK.

1 files changed, 49 insertions, 0 deletions

diff --git a/rsh/build.sh b/rsh/build.sh
new file mode 100755
index 0000000..88cdf9c
--- /dev/null
+++ b/rsh/build.sh
@@ -0,0 +1,49 @@
+#!/bin/sh
+# Build the rsh SSH transport for each Android ABI as a Bionic PIE executable
+# named lib*.so (packaged inside the APK and exec'd from nativeLibraryDir),
+# using the Android NDK. Also build a plain host binary for tests and manual
+# debugging.
+#
+# Android needs PIE and the Bionic dynamic linker (/system/bin/linker64), so
+# the on-device build is GOOS=android with cgo and the NDK clang. A GOOS=linux
+# build targets glibc's linker and will not exec on a phone.
+set -eu
+
+root=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+. "$root/versions"
+
+# Locate the NDK toolchain.
+ndk=${ANDROID_NDK_HOME:-${ANDROID_NDK_ROOT:-}}
+if [ -z "$ndk" ] && [ -n "${ANDROID_HOME:-}" ]; then
+	ndk="$ANDROID_HOME/ndk/$ANDROID_NDK"
+fi
+if [ -z "$ndk" ] || [ ! -d "$ndk" ]; then
+	echo "rsh: Android NDK not found; set ANDROID_NDK_HOME" >&2
+	exit 1
+fi
+tc="$ndk/toolchains/llvm/prebuilt/linux-x86_64/bin"
+
+ldflags="-s -w -buildid="
+
+cd "$root/rsh"
+
+for abi in $ABIS; do
+	case "$abi" in
+	arm64-v8a)   goarch=arm64 goarm= cc="aarch64-linux-android${ANDROID_MIN_SDK}-clang" ;;
+	armeabi-v7a) goarch=arm   goarm=7 cc="armv7a-linux-androideabi${ANDROID_MIN_SDK}-clang" ;;
+	x86_64)      goarch=amd64 goarm= cc="x86_64-linux-android${ANDROID_MIN_SDK}-clang" ;;
+	x86)         goarch=386   goarm= cc="i686-linux-android${ANDROID_MIN_SDK}-clang" ;;
+	*) echo "rsh: unknown ABI $abi" >&2; exit 1 ;;
+	esac
+	dest="$root/app/src/main/jniLibs/$abi"
+	mkdir -p "$dest"
+	echo "rsh: building $abi (android/$goarch)"
+	CGO_ENABLED=1 GOOS=android GOARCH="$goarch" GOARM="$goarm" \
+		CC="$tc/$cc" GOFLAGS=-trimpath CGO_CFLAGS="-ffile-prefix-map=$root=." \
+		go build -buildmode=pie -ldflags "$ldflags" -o "$dest/libxrsh.so" .
+done
+
+# Host binary (no NDK, cgo off) for tests and manual debugging.
+mkdir -p "$root/out"
+echo "rsh: building host binary -> out/rsh"
+CGO_ENABLED=0 GOFLAGS=-trimpath go build -ldflags "$ldflags" -o "$root/out/rsh" .