CST-agent/docs/android.md

CST-agent on Android

The engine runs on the phone as a plain bionic binary -- no Termux, no proot, no second process for the UI to talk to. It is the same C core the CLI, the TUI and the WebUI use, cross-compiled with the NDK; on the phone the Qt front end (app/) will link libcstcore.a in-process and register its own platform tools under the owner app:android.

Two scripts cover the engine side:

script	what it does
scripts/build_third_party_android.sh ABI [all|mbedtls|curl]	static mbedTLS + libcurl into third_party/prefix-android-ABI
scripts/build_android.sh [ABI] [--push] [--run] [--run-tests]	libcstcore.a, cst-agent, the test binaries; optional push and on-device run

Toolchain

NixOS: nixpkgs carries the NDK (the Android SDK license makes it unfree, hence the flag; the packaged NDK has a patched interpreter, so its clang runs here):

export ANDROID_NDK_HOME=$(NIXPKGS_ALLOW_UNFREE=1 nix build --impure --no-link \
  --print-out-paths 'nixpkgs#androidenv.androidPkgs.ndk-bundle')/libexec/android-sdk/ndk-bundle

adb comes from nix-shell -p android-tools; the builds need cmake, ninja and curl. End to end:

nix-shell -p cmake ninja curl android-tools --run '
  JOBS=16 ./scripts/build_third_party_android.sh arm64-v8a all'
nix-shell -p cmake ninja curl android-tools --run '
  JOBS=16 ./scripts/build_android.sh arm64-v8a --push --run'

--run finishes with the two smoke checks on the device (cst-agent --version and tests/test_embed, the in-process embedding contract); --run-tests builds the whole tests/ suite, pushes it and runs every binary on the phone, printing a pass/fail line per test.

The APK (Qt front end)

scripts/build_apk_android.sh [--install] [--logs] [--clean] configures with the Qt for Android kit, builds the apk target (Gradle, no port opened) and optionally installs it. Bits it needs, all overridable through the environment:

variable	what it points at
QT_ANDROID	Qt for Android 6.11.2 (aqt install-qt all_os android 6.11.2 android_arm64_v8a -O ~/Qt --archives qtbase qtdeclarative qtquickcontrols2)
QT_HOST	the host Qt of the same version (aqt install-qt linux desktop 6.11.2 linux_gcc_64 -O ~/Qt --archives qtbase qtdeclarative qtsvg)
ANDROID_SDK_ROOT	SDK with platforms/android-<n> and build-tools/<v>
ANDROID_NDK_HOME	the NDK the engine was built with
JAVA_HOME	JDK 17+ for Gradle

The SDK without sdkmanager

nixpkgs's full androidsdk pulls the emulator and system images (gigabytes); the two components actually needed are cheap. The platform comes from the nix store (unfree, so the license flag is required) and build-tools from Google's own zip, laid out as the SDK sdkmanager would:

PLATFORM=$(NIXPKGS_ALLOW_UNFREE=1 NIXPKGS_ACCEPT_ANDROID_SDK_LICENSE=1 nix build \
  --impure --no-link --print-out-paths --expr \
  'builtins.elemAt (import <nixpkgs> {}).androidenv.androidPkgs.platforms 2')   # android-35
mkdir -p ~/android-sdk/{build-tools,platforms}
ln -s $PLATFORM/libexec/android-sdk/platforms/android-35 ~/android-sdk/platforms/

androiddeployqt then picks build-tools and platform-tools from there. For build-tools, dl.google.com is the only source: from here it hands out ~100 KB/s per connection, so the script downloads the zip in 16 ranged requests (≈25x faster) and fixes the exec bits Python's zipfile drops.

Host Qt on NixOS

The host Qt is built for a regular distribution: its tools (qmlimportscanner first of all) look for libgssapi_krb5.so.2, libglib-2.0.so.0, libz.so.1, libfontconfig, libX11, libEGL/libGL, libzstd, libbrotlidec, libdbus-1, libfreetype, libstdc++, none of which exist in NixOS's search paths. The tools carry RUNPATH=$ORIGIN/../lib, so symlinking those libraries from the nix store into <QT_HOST>/lib/ is enough:

nix build --no-link --print-out-paths nixpkgs#krb5.lib nixpkgs#dbus.lib \
  nixpkgs#libglvnd nixpkgs#fontconfig.lib nixpkgs#freetype nixpkgs#glib.lib \
  nixpkgs#libx11 nixpkgs#libxkbcommon nixpkgs#zlib nixpkgs#zstd \
  nixpkgs#brotli.lib nixpkgs#stdenv.cc.cc.lib      # then ln -s <lib> ~/Qt/6.11.2/gcc_64/lib/

Two more things the script does on the way:

• GLESv2/EGL: Qt's FindGLESv2.cmake only searches CMAKE_FIND_ROOT_PATH prefixes (<sysroot>/include), but the NDK keeps the headers in <sysroot>/usr/include -- without -DGLESv2_INCLUDE_DIR/-DEGL_INCLUDE_DIR find_package(Qt6 Gui) fails with "dependency GLESv2 could not be found".
• -L$PREFIX/lib on every link type: the APK's payload is a shared module, so CMAKE_SHARED_LINKER_FLAGS/CMAKE_MODULE_LINKER_FLAGS need the prefix too, not just the EXE flags.

Gradle

The generated project resolves plugins and dependencies from google() and mavenCentral(), plus a Gradle distribution from services.gradle.org. From here all three are slow (dl.google.com ~100 KB/s), so two local tweaks:

• ~/.gradle/init.d/*.gradle (machine-local, not in the repo) prepends the Aliyun/Tencent mirrors for pluginManagement, dependencyResolutionManagement and every buildscript.
• the wrapper in <QT_ANDROID>/src/3rdparty/gradle/gradle/wrapper/ points at the Tencent Gradle mirror (51 MB/s vs 100 KB/s).

scripts/gradle-android-init.gradle (installed into ~/.gradle/init.d/ by the build script) switches off release lint: lintVitalAnalyzeRelease resolves its own tool jars straight from dl.google.com, bypassing every repository the build configures, and fails the build after all the real work is done.

Signing

androiddeployqt copies the unsigned Gradle output and signs it itself (QT_ANDROID_SIGN_APK + the QT_ANDROID_KEYSTORE_* variables; a signed Gradle output makes it fail with "Could not copy unsigned APK"). The script exports those variables and creates the standard Android debug keystore (~/.android/debug.keystore, alias androiddebugkey) on first use; CST_ANDROID_KEYSTORE and CST_ANDROID_KEY_ALIAS/_PASS override it. apksigner is a bash script with a #!/bin/bash shebang, which NixOS does not have, so the script rewrites the shebang of the build-tools wrappers.

What is Android-specific

• API level 24 (API_LEVEL=24): bionic provides threads, exec and /system/etc/security/cacerts, which is where libcurl is pointed (CURL_CA_PATH). Bump the level if a newer syscall or CA layout is needed.
• No TUI, no WebUI: -DCST_BUILD_TUI=OFF -DCST_BUILD_WEBUI=OFF -- there is no ncurses on Android, and the phone front end talks to the engine in-process instead of over HTTP. src/webui_stub.c compiles in place of the server.
• Sandbox: src/sandbox_exec.c only enables the seccomp/Landlock paths under defined(__linux__) && !defined(__ANDROID__), so the phone build gets the plain exec path -- the same rule the Termux build follows. The seccomp supervisor used to compile in and the probe reported "seccomp", but on the device a write outside the workspace was not denied (tests/test_sandbox.c:308), so the phone build now reports no sandbox instead of one that does not hold.
• sashidown is built per ABI (vendor/sashidown/libsashidown-<abi>.a) with the NDK's aarch64-linux-android24-clang wrapper: CMake's compiler is plain clang and keeps --target/--sysroot in its own variables, which the vendored makefile cannot see. The host archive stays host-built.
• Static libcurl needs -L$PREFIX/lib on the link line, because curl's own CMake config lists its mbedTLS dependencies as -l flags rather than paths.

Status

• arm64-v8a, Android 16 (NX789J): cst-agent runs, tests/test_embed passes.
• The whole suite runs on the device (scripts/build_android.sh arm64-v8a --run-tests): 36/36 after the diff test learned to tell toybox's diff from GNU diffutils.
• The APK builds, installs and runs: the Qt front end links the engine in-process (no port), main.cpp puts the workspace, config and tmp under /data/user/0/xyz.whcself.cstagent/files/, and a turn does a real HTTPS request from inside the ART process (with no API key the provider answers 401 -- the engine, curl and TLS parts of the round trip are what that shows). Rotation keeps the session; the sidebar appears in landscape.
• Still to come: the Android platform tools behind app:android (notification_send, clipboard, share sheet, SAF file picker), the back gesture, and the icon glyphs -- ThemeIcon finds no icon files on the phone and draws empty boxes where the desktop shows Breeze icons.