[RN] Run ReactCommon C++ unit tests on GitHub CI

.github/actions/run-cxx-tests/action.yml

@@ -0,0 +1,74 @@
+name: Run C++ Tests
+description: Builds and runs the ReactCommon C++ (gtest) unit-test harness on Linux
+inputs:
+  gradle-cache-encryption-key:
+    description: 'The encryption key needed to store the Gradle Configuration cache'
+runs:
+  using: composite
+  steps:
+    - name: Install dependencies
+      shell: bash
+      run: |
+        sudo apt update
+        sudo apt install -y git cmake openssl libssl-dev clang
+    - name: Setup git safe folders
+      shell: bash
+      run: git config --global --add safe.directory '*'
+    - name: Setup node.js
+      uses: ./.github/actions/setup-node
+    - name: Install node dependencies
+      uses: ./.github/actions/yarn-install
+    - name: Setup gradle
+      uses: ./.github/actions/setup-gradle
+      with:
+        cache-read-only: 'false'
+        cache-encryption-key: ${{ inputs.gradle-cache-encryption-key }}
+    - name: Restore C++ tests ccache
+      uses: actions/cache/restore@v5
+      with:
+        path: /github/home/.cache/ccache
+        key:
+          v1-ccache-cxx-tests-${{ github.job }}-${{ github.ref }}-${{ hashFiles(
+          'packages/react-native/ReactCommon/**/*.cpp',
+          'packages/react-native/ReactCommon/**/*.h',
+          'packages/react-native/ReactCommon/**/CMakeLists.txt'
+          ) }}
+        restore-keys: |
+          v1-ccache-cxx-tests-${{ github.job }}-${{ github.ref }}-
+          v1-ccache-cxx-tests-${{ github.job }}-
+          v1-ccache-cxx-tests-
+    - name: Show ccache stats (before)
+      shell: bash
+      run: ccache -s -v
+    - name: Build and run C++ tests
+      shell: bash
+      run: yarn workspace @react-native/tests-cxx test
+      env:
+        CC: clang
+        CXX: clang++
+        # libhermesvm.so is a shared library; gtest_discover_tests runs the
+        # binaries at build time and CTest runs them again, so it must be
+        # loadable in both phases.
+        LD_LIBRARY_PATH: ${{ github.workspace }}/packages/react-native/ReactAndroid/hermes-engine/build/hermes/lib
+    - name: Save C++ tests ccache
+      if: ${{ github.ref == 'refs/heads/main' || contains(github.ref, '-stable') }}
+      uses: actions/cache/save@v5
+      with:
+        path: /github/home/.cache/ccache
+        key:
+          v1-ccache-cxx-tests-${{ github.job }}-${{ github.ref }}-${{ hashFiles(
+          'packages/react-native/ReactCommon/**/*.cpp',
+          'packages/react-native/ReactCommon/**/*.h',
+          'packages/react-native/ReactCommon/**/CMakeLists.txt'
+          ) }}
+    - name: Show ccache stats (after)
+      shell: bash
+      run: ccache -s -v
+    - name: Upload test results
+      if: ${{ always() }}
+      uses: actions/upload-artifact@v6
+      with:
+        name: run-cxx-tests-results
+        compression-level: 1
+        path: |
+          private/react-native-tests/build/reports

.github/workflows/test-all.yml

@@ -300,6 +300,31 @@ jobs:
       fail-on-error: true
     secrets: inherit
 
+  test_cxx:
+    runs-on: 8-core-ubuntu
+    needs: [check_code_changes, lint]
+    if: needs.check_code_changes.outputs.any_code_change == 'true'
+    # Advisory during rollout: a harness issue must not block merges. Once this
+    # job is consistently green, drop continue-on-error and add it to branch
+    # protection so it becomes a required merge gate.
+    continue-on-error: true
+    container:
+      image: reactnativecommunity/react-native-android:latest
+      env:
+        # Set the encoding to resolve a known character encoding issue with decompressing tar.gz files in containers
+        # via Gradle: https://github.com/gradle/gradle/issues/23391#issuecomment-1878979127
+        LC_ALL: C.UTF8
+        TERM: 'dumb'
+        GRADLE_OPTS: '-Dorg.gradle.daemon=false'
+        REACT_NATIVE_DOWNLOADS_DIR: /opt/react-native-downloads
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Run C++ Tests
+        uses: ./.github/actions/run-cxx-tests
+        with:
+          gradle-cache-encryption-key: ${{ secrets.GRADLE_CACHE_ENCRYPTION_KEY }}
+
   build_android:
     runs-on: 8-core-ubuntu
     needs: [set_release_type, check_code_changes]

private/react-native-tests/README.md

@@ -0,0 +1,79 @@
+# @react-native/tests-cxx
+
+A Linux + CMake harness that builds and runs a **representative subset** of
+ReactCommon's C++ (gtest) unit tests on GitHub CI.
+
+## Why this exists
+
+ReactCommon's C++ test sources (`<subsystem>/tests/*.cpp`) are part of the
+repository, but the build rules that compile and run them are Meta-internal and
+are not part of the open-source tree. As a result these unit tests did not run
+on GitHub CI at all — the renderer, layout, and runtime C++ core was only
+covered by higher-level integration tests (Fantom) and by platform builds.
+
+This harness closes that gap cheaply: C++ unit tests build and run on standard
+(free) Linux runners, so GitHub CI can gate merges on them.
+
+The goal is a **representative**, high-signal subset — not full coverage. Suites
+are added deliberately, weighted by how often they catch regressions and by
+build cost.
+
+## How it works
+
+The harness reuses the exact desktop C++ toolchain already proven by the Fantom
+tester (`private/react-native-fantom`):
+
+- **Gradle** (`build.gradle.kts`) stages the third-party dependencies (folly,
+  boost, glog, double-conversion, fast_float, fmt, gflags) by depending on the
+  Fantom tester's `prepareNative3pDependencies`, then invokes CMake.
+- **CMake** (`cxx/CMakeLists.txt`) compiles each in-scope ReactCommon subsystem
+  into its library and links its `tests/*.cpp` into a per-subsystem gtest binary,
+  registered with CTest. GoogleTest itself is fetched via `FetchContent`.
+
+## Coverage
+
+30 suites (~900 gtest cases) run today, spanning the C++ core:
+
+- **Primitives / parsing / serialization:** graphics, css, utils, mapbuffer,
+  timing, featureflags
+- **Fabric renderer:** renderer/core, mounting, components/{view, text,
+  scrollview, image, root}, attributedstring, textlayoutmanager, element,
+  componentregistry (via deps), imagemanager, renderer/debug
+- **Scheduling / runtime:** runtimescheduler, scheduler, uimanager,
+  uimanager/consistency, performance/timeline, animated, animations
+- **Debugger / infra:** debug/redbox, reactperflogger/fusebox, jserrorhandler,
+  telemetry, cxxreact
+
+Suites for `renderer/core`, `runtimescheduler`, `scheduler` and `animated`
+create a JS runtime and link the Hermes VM.
+
+Not yet included: `jsinspector-modern` and `jsinspector-modern/tracing` (their
+tests depend on a specific GoogleTest version / `std::source_location` support
+that this harness's GoogleTest doesn't match), and `react/bridging` (its test
+helper header uses a Buck header-namespace that doesn't map to CMake).
+
+## Running locally (Meta-internal)
+
+Requires the Android SDK's CMake and the same environment used to build the
+Fantom tester.
+
+```sh
+yarn workspace @react-native/tests-cxx test   # build + run (ctest)
+yarn workspace @react-native/tests-cxx build  # build only
+```
+
+## Adding a subsystem
+
+1. Add the subsystem and any missing dependencies to the
+   `add_react_common_subdir(...)` list in `cxx/CMakeLists.txt`.
+2. Add a `react_native_add_cxx_test_suite(...)` call listing the subsystem's
+   full library closure (subsystem lib + its ReactCommon deps + third-party).
+   ReactCommon libraries are CMake `OBJECT` libraries, so the closure must be
+   listed explicitly rather than relying on transitive linking.
+3. Confirm the new suite builds and passes in the `test_cxx` CI job.
+
+## Rollout
+
+The `test_cxx` GitHub Actions job starts **advisory** (`continue-on-error`) so a
+harness issue cannot block merges. Once it is consistently green it should be
+made a **required** check via branch protection.

private/react-native-tests/build.gradle.kts

@@ -0,0 +1,139 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import com.facebook.react.tasks.internal.*
+import com.facebook.react.tasks.internal.utils.*
+
+plugins { id("com.facebook.react") }
+
+// CMake shipped with the Android SDK (same version the Fantom tester uses). If
+// missing it is downloaded automatically by the Android SDK manager.
+val cmakeVersion = System.getenv("CMAKE_VERSION") ?: "3.30.5"
+val cmakePath = "${getSDKPath()}/cmake/$cmakeVersion"
+val cmakeBinaryPath = "${cmakePath}/bin/cmake"
+val ctestBinaryPath = "${cmakePath}/bin/ctest"
+val buildJobs = Runtime.getRuntime().availableProcessors().toString()
+
+fun getSDKPath(): String {
+  val androidSdkRoot = System.getenv("ANDROID_SDK_ROOT")
+  val androidHome = System.getenv("ANDROID_HOME")
+  return when {
+    !androidSdkRoot.isNullOrBlank() -> androidSdkRoot
+    !androidHome.isNullOrBlank() -> androidHome
+    else -> throw IllegalStateException("Neither ANDROID_SDK_ROOT nor ANDROID_HOME is set.")
+  }
+}
+
+val buildDir = project.layout.buildDirectory.get().asFile
+val reportsDir = File("$buildDir/reports")
+val reactNativeRootDir = projectDir.parentFile.parentFile
+val reactNativeDir = File("$reactNativeRootDir/packages/react-native")
+val reactAndroidDir = File("$reactNativeDir/ReactAndroid")
+val reactAndroidBuildDir = File("$reactAndroidDir/build")
+
+// The C++ test harness reuses the third-party dependencies staged by the Fantom
+// tester build: folly and gflags land in <fantom>/build/third-party; glog,
+// double-conversion, fast_float, fmt and boost land in
+// ReactAndroid/build/third-party-ndk. Depending on Fantom's
+// prepareNative3pDependencies guarantees all of them are prepared.
+val fantomDir = File("$reactNativeRootDir/private/react-native-fantom")
+val stagedThirdPartyDir = File("$fantomDir/build/third-party")
+val testerThirdPartySrcDir = File("$fantomDir/tester/third-party")
+
+val cxxDir = File("$projectDir/cxx")
+val cxxBuildDir = File("$buildDir/cxx")
+val cxxBuildOutputFileTree =
+    fileTree(cxxBuildDir.toString())
+        .include("**/*.cmake", "**/*.marks", "**/compiler_depends.ts", "**/Makefile", "**/link.txt")
+
+val createReportsDir by tasks.registering { reportsDir.mkdirs() }
+
+// Generated codegen sources (FBReactNativeSpec) that Fabric component tests
+// depend on. `generateCodegenArtifactsFromSchema` produces them under
+// ReactAndroid/build/generated; stage them next to codegen/CMakeLists.txt.
+val codegenSrcDir = File("$reactAndroidBuildDir/generated/source/codegen/jni")
+val codegenOutDir = File("$buildDir/codegen")
+val prepareRNCodegen by
+    tasks.registering(Copy::class) {
+      dependsOn(":packages:react-native:ReactAndroid:generateCodegenArtifactsFromSchema")
+      from(codegenSrcDir)
+      from("codegen")
+      include("react/**/*.h", "react/**/*.cpp", "CMakeLists.txt")
+      includeEmptyDirs = false
+      duplicatesStrategy = DuplicatesStrategy.INCLUDE
+      into(codegenOutDir)
+    }
+
+// Hermes VM + prefab headers, for suites whose tests create a JS runtime
+// (hermes::makeHermesRuntime). Mirrors the Fantom tester's Hermes setup.
+val enableHermesBuild by tasks.registering {
+  project(":packages:react-native:ReactAndroid:hermes-engine") {
+    tasks.configureEach { enabled = true }
+  }
+}
+val prepareHermesDependencies by tasks.registering {
+  dependsOn(
+      enableHermesBuild,
+      ":packages:react-native:ReactAndroid:hermes-engine:buildHermesLibWithDebugger",
+      ":packages:react-native:ReactAndroid:hermes-engine:prepareHeadersForPrefabWithDebugger",
+  )
+}
+
+val configureCxxTests by
+    tasks.registering(CustomExecTask::class) {
+      dependsOn(
+          createReportsDir,
+          prepareRNCodegen,
+          prepareHermesDependencies,
+          ":private:react-native-fantom:prepareNative3pDependencies",
+      )
+      workingDir(cxxDir)
+      inputs.dir(cxxDir)
+      outputs.files(cxxBuildOutputFileTree)
+      commandLine(
+          cmakeBinaryPath,
+          "--log-level=ERROR",
+          "-S",
+          ".",
+          "-B",
+          cxxBuildDir.toString(),
+          "-DCMAKE_BUILD_TYPE=Debug",
+          "-DREACT_ANDROID_DIR=$reactAndroidDir",
+          "-DREACT_COMMON_DIR=$reactNativeDir/ReactCommon",
+          "-DREACT_THIRD_PARTY_NDK_DIR=$reactAndroidBuildDir/third-party-ndk",
+          "-DRN_STAGED_THIRD_PARTY_DIR=$stagedThirdPartyDir",
+          "-DRN_TESTER_THIRD_PARTY_SRC_DIR=$testerThirdPartySrcDir",
+          "-DRN_CODEGEN_DIR=$codegenOutDir",
+          "-DRN_ENABLE_DEBUG_STRING_CONVERTIBLE=ON",
+      )
+      standardOutputFile.set(project.file("$buildDir/reports/configure-cxx-tests.log"))
+      errorOutputFile.set(project.file("$buildDir/reports/configure-cxx-tests.error.log"))
+    }
+
+val buildCxxTests by
+    tasks.registering(CustomExecTask::class) {
+      dependsOn(configureCxxTests)
+      workingDir(cxxDir)
+      inputs.files(cxxBuildOutputFileTree)
+      commandLine(cmakeBinaryPath, "--build", cxxBuildDir.toString(), "-j", buildJobs)
+      standardOutputFile.set(project.file("$buildDir/reports/build-cxx-tests.log"))
+      errorOutputFile.set(project.file("$buildDir/reports/build-cxx-tests.error.log"))
+    }
+
+val runCxxTests by
+    tasks.registering(CustomExecTask::class) {
+      dependsOn(buildCxxTests)
+      workingDir(cxxBuildDir)
+      commandLine(
+          ctestBinaryPath,
+          "--output-on-failure",
+          "--output-junit",
+          "$reportsDir/cxx-tests-results.xml",
+      )
+      standardOutputFile.set(project.file("$buildDir/reports/run-cxx-tests.log"))
+      errorOutputFile.set(project.file("$buildDir/reports/run-cxx-tests.error.log"))
+    }

private/react-native-tests/build.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+set -e
+
+pushd ../..
+./gradlew :private:react-native-tests:buildCxxTests
+popd

private/react-native-tests/codegen/CMakeLists.txt

@@ -0,0 +1,43 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# Builds the generated codegen sources (FBReactNativeSpec) that Fabric component
+# tests depend on. Populated by the Gradle `prepareRNCodegen` task, which runs
+# `generateCodegenArtifactsFromSchema` and stages the generated headers/sources
+# next to this file. Mirrors private/react-native-fantom/tester/codegen.
+
+cmake_minimum_required(VERSION 3.13)
+set(CMAKE_VERBOSE_MAKEFILE on)
+
+file(GLOB react_codegen_SRCS CONFIGURE_DEPENDS react/renderer/components/FBReactNativeSpec/*.cpp)
+
+add_library(
+  react_codegen_rncore
+  OBJECT
+  ${react_codegen_SRCS}
+)
+
+target_include_directories(react_codegen_rncore
+  PUBLIC
+    ${CMAKE_CURRENT_SOURCE_DIR}/.
+    ${CMAKE_CURRENT_SOURCE_DIR}/react/renderer/components/FBReactNativeSpec
+    ${CMAKE_CURRENT_SOURCE_DIR}/react/renderer/components)
+
+target_link_libraries(
+  react_codegen_rncore
+  jsi
+  react_nativemodule_core
+  rrc_view
+)
+
+target_compile_options(
+  react_codegen_rncore
+  PRIVATE
+  -DLOG_TAG=\"ReactNative\"
+  -fexceptions
+  -frtti
+  -std=c++20
+  -Wall
+)