Fix LiteRT Kotlin Litert.aar Compile Issue

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LiteRT Next Kotlin litert.aar Compile Issue: A Deep Dive and Solution

Hey everyone! Today, we're diving into a tricky issue encountered while trying to compile the litert.aar file using Bazel for a Kotlin-based Android project. Specifically, the error arises due to conflicts between the Android and Linux build environments within the Bazel build process. Let's break down the problem, understand the context, and then explore a robust solution to get you back on track.

Understanding the Problem: Bazel Conflicts

The core issue, as highlighted in the provided image, stems from Bazel's attempt to reconcile build configurations meant for two distinct target platforms: Android (arm64 architecture in this case) and Linux. Bazel, being a powerful and versatile build tool, allows you to define different build configurations for various target environments. However, when these configurations clash or aren't properly segregated, you can run into errors like the one observed.

Why does this happen?

  1. Configuration Overlap: Bazel uses configuration files (often specified using --config flags) to define compiler settings, target architectures, and dependencies. If your Android and Linux configurations share common elements or aren't explicitly separated, Bazel might get confused about which settings to apply during the build process.

  2. Native Dependencies: litert.aar likely depends on native libraries (written in C/C++) that need to be compiled specifically for the target architecture. If Bazel tries to use Linux-compiled native libraries in an Android build (or vice versa), it will lead to incompatibility errors.

  3. Toolchain Issues: The Android NDK (Native Development Kit) provides toolchains (compilers, linkers, etc.) specifically designed for building native code for Android. If Bazel isn't correctly configured to use the Android NDK toolchain when building for Android, it might default to using the system's native Linux toolchain, resulting in incompatible binaries.

  4. Incorrect Build Flags: Sometimes, seemingly innocuous build flags can inadvertently cause conflicts. For instance, flags related to optimization levels, debugging symbols, or target SDK versions can have different meanings or effects on Android and Linux, leading to build errors.

In essence, the error message indicates that Bazel is struggling to create a consistent build environment that satisfies both the Android and Linux requirements. This typically arises from misconfigured build settings or a lack of clear separation between the Android and Linux build targets.

Replicating the Scenario

The user attempted to compile the litert.aar file using the following Bazel command within a Docker container:

bazel build --config=android_arm64 //litert/kotlin:litert

This command instructs Bazel to build the target //litert/kotlin:litert (presumably a Kotlin library) using the android_arm64 configuration. The assumption here is that android_arm64 is a predefined Bazel configuration that sets up the build environment for Android arm64 architecture.

The fact that the user successfully compiled Capi (presumably a C/C++ library) suggests that the Android NDK and related toolchains are correctly installed and configured within the Docker environment. However, the subsequent failure to build litert.aar indicates that the issue lies specifically in how the Kotlin library and its dependencies are being handled within the Bazel build process.

Solution: Resolving the Bazel Conflict

To resolve this Bazel conflict, we need to ensure that the Android and Linux build environments are properly isolated and that Bazel correctly uses the Android NDK toolchain when building for Android. Here's a step-by-step approach to tackle this:

  1. Isolate Configurations: The most crucial step is to ensure that your Android and Linux build configurations are completely isolated. This means creating separate configuration files or defining distinct configuration blocks within your BUILD files.

    • android.config: This file should contain all the settings specific to the Android build, including the target architecture (arm64-v8a in this case), the Android NDK toolchain path, and any Android-specific compiler flags.

    • linux.config: This file should contain the settings for the Linux build, including the target architecture (e.g., x86_64), the system's default compiler, and any Linux-specific compiler flags.

  2. Define Toolchains: Explicitly define the Android NDK toolchain in your BUILD files. This ensures that Bazel uses the correct compiler and linker when building native code for Android.

    android_ndk_repository(
    name = "androidndk",
    path = "/path/to/your/android/ndk",  # Replace with the actual path to your NDK
    api_level = 29,  # Replace with your target API level
)

    Make sure to replace /path/to/your/android/ndk with the actual path to your Android NDK installation. The api_level should match the target Android API level for your project.

  3. Use select Statements: Employ Bazel's select statement to conditionally apply different settings based on the target platform. This allows you to use the same BUILD file for both Android and Linux builds while ensuring that the correct settings are applied in each case.

    cc_library(
    name = "mylibrary",
    srcs = ["mylibrary.cc"],
    copts = select({
        "//:android": ["-DANDROID"],
        "//conditions:default": ["-DLINUX"],
    }),
    toolchains = select({
        "//:android": ["@androidndk//:toolchain"],
        "//conditions:default": ["@bazel_tools//tools/cpp:toolchain"],
    }),
)

    In this example, the copts (compiler options) and toolchains attributes are conditionally set based on whether the target platform is Android or Linux. The //:android target would need to be defined to trigger the android configuration.

  4. Configure BUILD files: Add BUILD file to the root directory with the following content.

    config_setting(
    name = "android",
    constraint_values = [
        "@platforms//os:android",
        "@platforms//cpu:arm64",
    ],
)

  5. Modify Bazel Command: Refine the Bazel command to explicitly specify the target platform. This helps Bazel to correctly select the appropriate build configuration.

bazel build --platforms=@platforms//os:android --config=android_arm64 //litert/kotlin:litert ```

  1. Clean and Rebuild: After making these changes, it's essential to clean the Bazel cache and rebuild the project from scratch. This ensures that Bazel picks up the new configurations and avoids using any cached artifacts from previous builds.

bazel clean --expunge bazel build --platforms=@platforms//os:android --config=android_arm64 //litert/kotlin:litert ```

  1. Docker Considerations: Ensure that your Docker environment is properly configured to support Android development. This includes installing the Android SDK, NDK, and any necessary dependencies. Also, verify that the environment variables required by the Android NDK are correctly set within the Docker container.

Example Scenario

Let's imagine you have a project structure like this:

myproject/
โ”œโ”€โ”€ BUILD
โ”œโ”€โ”€ android.config
โ”œโ”€โ”€ linux.config
โ”œโ”€โ”€ litert/
โ”‚   โ”œโ”€โ”€ kotlin/
โ”‚   โ”‚   โ”œโ”€โ”€ BUILD
โ”‚   โ”‚   โ”œโ”€โ”€ litert.kt
โ”‚   โ”‚   โ””โ”€โ”€ ...
โ”‚   โ””โ”€โ”€ ...
โ””โ”€โ”€ ...

Your root BUILD file might contain the android_ndk_repository definition and the config_setting for Android. The android.config and linux.config files would contain the specific compiler flags and toolchain settings for each platform. The litert/kotlin/BUILD file would use select statements to conditionally apply settings based on the target platform.

Key Takeaways

  • Configuration is Key: The root cause of this issue is often related to misconfigured or conflicting build settings. Carefully review your Bazel configuration files and ensure that Android and Linux settings are properly isolated.
  • Toolchain Matters: Explicitly define the Android NDK toolchain in your BUILD files to ensure that Bazel uses the correct compiler and linker for Android builds.
  • select is Your Friend: Use Bazel's select statement to conditionally apply different settings based on the target platform, allowing you to maintain a single set of BUILD files for both Android and Linux builds.
  • Cleanliness Counts: Always clean the Bazel cache and rebuild the project from scratch after making significant configuration changes.

By following these steps, you should be able to resolve the Bazel conflict and successfully compile the litert.aar file for your Kotlin-based Android project. Remember to adapt the specific settings and paths to match your project's configuration. Good luck, and happy building!