Updating Native Shims

Native shims are Kotlin/Native cinterop wrappers around third-party C/C++ libraries (gRPC core and protobuf wire). They live in native-deps/shims/ as a standalone Gradle build (separate from the main kotlinx-rpc project).

How to Execute

Use the running_gradle_builds skill for all Gradle build/publish tasks and the running_gradle_tests skill for all Gradle test tasks. Do NOT run ./gradlew directly via shell.

Because the shims build is a standalone Gradle project, you must set projectRoot to the absolute path of native-deps/shims/ when invoking Gradle skills for shim tasks. For verification tasks that run in the main kotlinx-rpc project, use the default projectRoot (omit it or point to the repo root).

Module Layout

native-deps/shims/
  grpc/                          # :kotlinx-rpc-grpc-core-shim
    src/cpp/kgrpc.cpp            # C shim wrapper source
    src/nativeInterop/cinterop/
      grpcCoreInterop.def        # CInterop definition (template — staticLibraries injected at build)
    include/kgrpc.h              # Shim header
    BUILD.bazel                  # Bazel target :grpc_shim
    MODULE.bazel                 # Bazel module (GRPC_VERSION synced from version catalog)
    overlap-archive-excludes.txt # Archives excluded because protobuf-shim already provides them
    tools/analyze_overlap.py     # Symbol overlap analysis helper
    build.gradle.kts

  protobuf/                      # :kotlinx-rpc-protobuf-shim
    src/cpp/protowire.cpp        # C shim wrapper source
    src/nativeInterop/cinterop/
      libprotowire.def           # CInterop definition (per-target static libs listed inline)
    include/protowire.h          # Shim header
    BUILD.bazel                  # Bazel target :protowire_fat
    MODULE.bazel                 # Bazel module (PROTOBUF_VERSION synced from version catalog)
    build.gradle.kts

  annotation/                    # :kotlinx-rpc-native-shims-annotation
    src/commonMain/kotlin/
      kotlinx/rpc/grpc/internal/shim/InternalNativeRpcApi.kt
      kotlinx/rpc/protobuf/internal/shim/InternalNativeProtobufApi.kt
    build.gradle.kts

  settings.gradle.kts
  gradle.properties

native-deps/scripts/
  build_target.sh                # Runs Bazel for one target, copies static library out

The build helper build_target.sh lives in the sibling native-deps/scripts/ directory (alongside the other native-deps utility scripts). It is still invoked from each shim module directory (which owns the Bazel workspace) by the registerNativeShimBazelBuildTask Gradle helper.

The native-deps/grpc-c-prebuilt/ sibling directory publishes raw .a static archives and headers consumed by the gRPC shim. The protobuf shim builds its own fat archive via Bazel directly.

Build Pipeline (per shim, per native target)

1. Bazel builds a target-specific static library (build_target.sh wraps bazel build).
2. Gradle unpacks prebuilt archives (gRPC) or uses the Bazel output directly (protobuf).
3. CInterop generates Kotlin bindings from the .def file + headers + static libs.
4. The generated KLIB is published alongside the annotation artifact.

Versioning

Versions live in versions-root/libs.versions.toml:

When making shim code changes (not upstream bumps), bump only the shim revision (the part after the dash). When bumping the upstream library version, update the base version and reset the shim revision to 1.

The base version is automatically synced to the corresponding MODULE.bazel file (GRPC_VERSION or PROTOBUF_VERSION variable) by a Gradle task at build time.

Making Changes

Changing C/C++ shim code or headers

The shim C/C++ source lives alongside its Bazel build:

• gRPC: grpc/src/cpp/kgrpc.cpp + grpc/include/kgrpc.h
• Protobuf: protobuf/src/cpp/protowire.cpp + protobuf/include/protowire.h

After editing:

1. If you added/removed/renamed C functions, update the corresponding .def file to reflect the new API surface. The .def file controls what CInterop exposes to Kotlin.
2. Bump the shim revision in versions-root/libs.versions.toml.
3. Build and verify (see sections below).

Changing CInterop definitions

• gRPC: grpc/src/nativeInterop/cinterop/grpcCoreInterop.def — this is a template. The __STATIC_LIBRARIES__ placeholder is replaced at build time with the full archive list from the unpacked gRPC bundle. Edit filter rules, header includes, strict enums, and noStringConversion entries here.
• Protobuf: protobuf/src/nativeInterop/cinterop/libprotowire.def — this file lists per-target staticLibraries inline (one staticLibraries.{target} entry per native target). Edit header includes and filter rules here.

Changing the annotation module

Annotations live in annotation/src/commonMain/kotlin/. Both are @RequiresOptIn(level = ERROR). Changes here are rare — typically only when adding a new shim family.

Managing archive overlaps (KRPC-540)

gRPC and protobuf both bundle native Abseil/protobuf archives. When a final binary links both shims, duplicate symbols can cause linker failures (especially on Linux).

Overlap excludes file: grpc/overlap-archive-excludes.txt

• Format: all:<archive> (every target) or <target>:<archive> (target-specific)
• Archives can be named in bundle-manifest form (lib/foo/bar.a) or flattened interop form (foo+__bar.a)
• Currently excludes utf8_range from gRPC shim since protobuf-shim provides it

Symbol rewrite workaround (Linux only, temporary — remove when protobuf becomes Kotlin-only): The protobuf shim build rewrites AbslInternalGetFileMappingHint to KrpcProtobuf_AbslInternalGetFileMappingHint using llvm-objcopy --redefine-sym. This lives in protobuf/build.gradle.kts.

To investigate new overlaps:

cd native-deps/shims
python3 grpc/tools/analyze_overlap.py \
  --grpc-klib <path-to-grpc-cinterop-klib> \
  --protobuf-klib <path-to-protobuf-cinterop-klib> \
  --target linux_x64

Use --write-excludes <file> --exclude-scope <target> to auto-append entries.

Bumping an upstream version

1. Update the base version in versions-root/libs.versions.toml (e.g., change internal-native-grpc-shim from 1.80.0-1 to 1.81.0-1).
2. The MODULE.bazel version variable is synced automatically by the syncGrpcVersionToBazelModule / syncProtobufVersionToBazelModule Gradle task.
3. For gRPC: also update the grpc-c-prebuilt version if the prebuilt archive version needs to match.
4. Build, verify, and check for new symbol overlaps.

Publishing Locally

All tasks in this section target the shims standalone build (projectRoot: native-deps/shims/).

Prerequisite: Bazel (or Bazelisk) must be on PATH. On macOS, Xcode (not just CommandLineTools) should be installed for cross-compilation to iOS/tvOS/watchOS.

Publish to the local build repo

• gRPC shim + annotation: task :kotlinx-rpc-grpc-core-shim:publishAllPublicationsToNativeDepsBuildRepoRepository :kotlinx-rpc-native-shims-annotation:publishAllPublicationsToNativeDepsBuildRepoRepository
• Protobuf shim + annotation: task :kotlinx-rpc-protobuf-shim:publishAllPublicationsToNativeDepsBuildRepoRepository :kotlinx-rpc-native-shims-annotation:publishAllPublicationsToNativeDepsBuildRepoRepository

Published artifacts land in native-deps/build/repo/. The main kotlinx-rpc project is configured to resolve from this path during development.

Publish both shims together

Task: :kotlinx-rpc-grpc-core-shim:publishAllPublicationsToNativeDepsBuildRepoRepository :kotlinx-rpc-protobuf-shim:publishAllPublicationsToNativeDepsBuildRepoRepository :kotlinx-rpc-native-shims-annotation:publishAllPublicationsToNativeDepsBuildRepoRepository

Build without publishing (sanity check)

• gRPC shim: task :kotlinx-rpc-grpc-core-shim:assemble
• Protobuf shim: task :kotlinx-rpc-protobuf-shim:assemble

Verification

1. Verify in the main project

After publishing locally, build the shim consumers in the main project (default projectRoot) via running_gradle_builds:

• Protobuf shim consumer: task :protobuf:protobuf-lite:compileKotlinMacosArm64
• gRPC shim consumer: task :grpc:grpc-core:compileKotlinMacosArm64

Use whichever native target matches your host. If both compile cleanly, the shim changes integrate correctly with the rest of the project.

2. Check for symbol overlaps (when changing archives)

If you modified archive contents, overlap excludes, or bumped an upstream version:

cd native-deps/shims
python3 grpc/tools/analyze_overlap.py \
  --grpc-klib build/classes/kotlin/macosArm64/main/cinterop/cinterop-grpcCoreInterop.klib \
  --protobuf-klib build/classes/kotlin/macosArm64/main/cinterop/cinterop-libprotowire.klib \
  --target macos_arm64

Check Linux targets separately (they have the KRPC-540 symbol rewrite):

python3 grpc/tools/analyze_overlap.py \
  --grpc-klib build/classes/kotlin/linuxX64/main/cinterop/cinterop-grpcCoreInterop.klib \
  --protobuf-klib build/classes/kotlin/linuxX64/main/cinterop/cinterop-libprotowire.klib \
  --target linux_x64

Troubleshooting

Bazel not found: Install Bazelisk (brew install bazelisk on macOS) — it manages Bazel versions automatically.

Xcode issues on macOS: The build needs full Xcode for Apple platform SDKs. If DEVELOPER_DIR is not set, the build script defaults to /Applications/Xcode.app/Contents/Developer.

KONAN_HOME resolution: The build downloads Kotlin/Native automatically via the :core:downloadKotlinNativeDistribution task from the main project. If this fails, check that the main project builds first.

Linux symbol collision at link time: If you see duplicate AbslInternalGetFileMappingHint errors, the KRPC-540 workaround in protobuf/build.gradle.kts may need updating. Check that llvm-objcopy is available (the build searches Konan dependencies, then PATH).

Clean rebuild (projectRoot: native-deps/shims/):

1. Run clean task
2. Optionally run bazel clean --expunge via shell for Bazel cache
3. Run :kotlinx-rpc-grpc-core-shim:assemble task