Respect ANDROID_SDK_ROOT and ANDROID_NDK_ROOT env vars to override the Android SDK and/or NDK locations

[jakepetroules]

Currently Swift Build discovers the Android SDK and NDK under the default paths on each platform:

• Windows: %LOCALAPPDATA%\Android\Sdk
• macOS: ~/Library/Android/sdk
• Linux: ~/Android/Sdk

If ANDROID_SDK_ROOT and/or ANDROID_NDK_ROOT are set in the environment, those should override the default SDK and NDK location lookup, respectively.

[marcprux]

Note that ANDROID_NDK_ROOT is currently consulted, but in a way that breaks Android builds, at least on macOS and Linux. We currently need to explicitly un-set the environment in order to build for Android.

If we could solve the long-standing swiftrt.o issue, then ideally we could eliminate the "ndk-root" link script that we're using (see swiftlang/swift-docker#467 (comment)) and just default the sdkRootPath (which probably ought to be optional, but isn't) to use the ANDROID_NDK_ROOT environment, and everything would just work.

[jakepetroules]

Note that ANDROID_NDK_ROOT is currently consulted, but in a way that swiftlang/swift-driver#1879, at least on macOS and Linux. We currently need to explicitly un-set the environment in order to build for Android.

You're talking about swift-driver, but this issue is about Swift Build. We still need to consult ANDROID_NDK_ROOT here at this level in order to look up various locations correctly. One reason is because Swift Build will explicitly pass --sysroot to the Swift driver regardless of whether ANDROID_NDK_ROOT is set.

It sounds like your #1879 PR will fix the swift-driver issue though, right? Shall we move that out of draft and get it landed?

[marcprux]

I've moved swiftlang/swift-driver#1879 out of draft

[marcprux]

Aside from the unimplemented desire to consult ANDROID_NDK_ROOT, should this be expected to be currently working with the auto-discovered NDK location logic?

swift-build/Sources/SWBAndroidPlatform/AndroidSDK.swift

Line 128 in f49864e

public static func findInstallations(host: OperatingSystem, fs: any FSProxy) async throws -> [AndroidSDK] {

Because it currently is failing on macOS with the latest Android SDK 6.2 snapshot release at https://github.com/swift-android-sdk/swift-android-sdk/releases:

% ~/Library/Developer/Toolchains/swift-6.2-DEVELOPMENT-SNAPSHOT-2025-07-17-a.xctoolchain/usr/bin/swift build --swift-sdk aarch64-unknown-linux-android28 --build-system swiftbuild -v
warning: multiple Swift SDKs match target triple `aarch64-unknown-linux-android28` and host triple arm64-apple-macosx15.0, selected one at /Users/marc/Library/org.swift.swiftpm/swift-sdks/swift-6.2-DEVELOPMENT-SNAPSHOT-2025-07-17-a-android-0.1.artifactbundle/swift-android
info: PIF: Building PIF project for package 'android-fetchurl' (1 products, 1 modules)
info: Compute target dependency graph
info: Building targets in dependency order
info: Target dependency graph (3 targets)
info: Target 'AllExcludingTests' in project 'Aggregate'
info: ➜ Explicit dependency on target 'android-fetchurl-product' in project 'android-fetchurl'
info: ➜ Explicit dependency on target 'android-fetchurl' in project 'android-fetchurl'
info: Target 'android-fetchurl' in project 'android-fetchurl' (no dependencies)
info: Target 'android-fetchurl-product' in project 'android-fetchurl' (no dependencies)
info: Gather provisioning inputs
info: Create build description
error: unable to resolve run destination SDK: 'android'
error: unable to find sdk 'android'
error: unable to resolve run destination SDK: 'android'
warning: There are no architectures to compile for because the VALID_ARCHS build setting is an empty list.
error: unable to find sdk 'android'
error: unable to resolve run destination SDK: 'android'
error: unable to find sdk 'android'
error: unable to resolve run destination SDK: 'android'
warning: There are no architectures to compile for because the VALID_ARCHS build setting is an empty list.
error: Build failed

My Android SDK is in the expected location:

% ls -1 ~/Library/Android/sdk/ndk
26.3.11579264
27.0.12077973

[jakepetroules]

Works for me, though I have NDKr28. Can you build a local swiftpm+swiftbuild 6.2 and see if the call to cachedAndroidSDKInstallations in AndroidSDKRegistryExtension.additionalSDKs throws an error? I have a feeling the abis.json or platforms.json schema could've possibly changed since NDKr26 or NDKr27.

cachedAndroidSDKInstallations is called with an optional-try today because we don't want failures for Android to impact building for other platforms, but I think we should probably have that at least emit a build warning so that it's obvious if the SDK discovery process fails.

[jakepetroules]

Related: #659