ci/cxx: Add out-of-tree build CI test.

Introduce a new CI test to validate out-of-tree builds
and prevent regressions in the `make export` workflow.
Specifically, this change adds `test-oot-build.sh` that:
* Builds NuttX using a selected configuration.
* Runs `make export` to generate the export package.
* Copies the `nuttx-export` to the OOT build project.
* Verifies that compilation and linking succeed.

Signed-off-by: trns1997 <trns1997@gmail.com>

.github/workflows/build.yml

@@ -201,6 +201,51 @@ jobs:
           path: buildartifacts/
         continue-on-error: true
 
+  # Test the out-of-tree build
+  OOT-Build:
+    needs: Linux
+    runs-on: ubuntu-latest
+    env:
+      DOCKER_BUILDKIT: 1
+    steps:
+      - name: Download Source Artifact
+        uses: actions/download-artifact@v5
+        with:
+          name: source-bundle
+          path: .
+
+      - name: Extract sources
+        run: tar zxf sources.tar.gz
+
+      - name: Docker Login
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Export NuttX Repo SHA
+        run: echo "nuttx_sha=`git -C sources/nuttx rev-parse HEAD`" >> $GITHUB_ENV
+
+      - name: Run Out-of-Tree Build Test
+        uses: ./sources/nuttx/.github/actions/ci-container
+        env:
+          BLOBDIR: /tools/blobs
+        with:
+          run: |
+            echo "::add-matcher::sources/nuttx/.github/gcc.json"
+            git config --global --add safe.directory /github/workspace/sources/nuttx
+            git config --global --add safe.directory /github/workspace/sources/apps
+            cd sources/nuttx
+            ./tools/ci/cibuild-oot.sh
+
+      - uses: actions/upload-artifact@v4
+        if: ${{ always() }}
+        with:
+          name: oot-build-artifacts
+          path: sources/apps/testing/cxx-oot-build
+        continue-on-error: true
+
   # Select the macOS Builds based on PR Arch Label
   macOS-Arch:
     uses: apache/nuttx/.github/workflows/arch.yml@master

Documentation/applications/testing/cxx-oot-build/index.rst

@@ -0,0 +1,75 @@
+========================================
+``cxx-oot-build`` Out-of-Tree Build Test
+========================================
+
+The ``cxx-oot-build`` test automates building an **Out-of-Tree (OOT)** NuttX
+project using a NuttX export tarball. Its primary purpose is to validate that
+NuttX can be built outside of the main source tree and to prevent regressions
+in the build process for C++ projects.
+
+**Important:** This test uses a specialized defconfig that is **not functional**
+for running actual applications. It is intended purely for CI/build
+verification. Functional OOT projects should be configured according to
+the instructions in :ref:`cpp_cmake`.
+
+The test script is located at:
+
+- ``tools/ci/cibuild-oot.sh``
+
+### Out-of-Tree App Content
+
+The source content for this OOT test can be found in:
+
+- ``apps/testing/cxx-oot-build``
+
+Its structure provides a basic skeleton for building a C++ NuttX application:
+
+.. code-block:: text
+
+    testing/cxx-oot-build
+    ├── CMakeLists.txt
+    ├── include
+    │   └── HelloWorld.hpp
+    └── src
+        ├── HelloWorld.cpp
+        └── main.cpp
+
+This minimal structure includes:
+
+- `CMakeLists.txt` - Build instructions for the OOT C++ project  
+- `include/HelloWorld.hpp` - Example header file  
+- `src/HelloWorld.cpp` - Example class implementation  
+- `src/main.cpp` - Entry point for the test application  
+
+### How to Run the Test
+
+Execute the test script from the NuttX CI tools directory:
+
+    cd ${NUTTX_PATH}/tools/ci
+    ./cibuild-oot.sh
+
+The script performs the following steps:
+
+1. Configures NuttX for the ``cxx-oot-build`` board profile
+2. Builds an export tarball of NuttX
+3. Prepares the Out-of-Tree project by extracting the tarball
+4. Builds the OOT project using CMake
+5. Verifies that the output binaries ``oot`` and ``oot.bin`` exist
+
+### Expected Output
+
+On success, you should see:
+
+    ✅ SUCCESS: OOT build completed. Output:
+    -rwxrwxr-x 1 <user> <group> 94K <date> /path/to/oot
+    -rwxrwxr-x 1 <user> <group> 46K <date> /path/to/oot.bin
+
+If any step fails, the script will exit immediately with an error message.
+
+### Notes
+
+- No additional configuration options are required for this test. The
+  ``cxx-oot-build`` defconfig is preconfigured to build correctly but is
+  **not suitable for running applications**.
+- For functional OOT builds, please follow the procedure documented in
+  :doc:`here </guides/cpp_cmake>`.

boards/arm/stm32/stm32f4discovery/configs/cxx-oot-build/defconfig

@@ -0,0 +1,48 @@
+#
+# This file is autogenerated: PLEASE DO NOT EDIT IT.
+#
+# You can use "make menuconfig" to make any modifications to the installed .config file.
+# You can then do "make savedefconfig" to generate a new defconfig file that includes your
+# modifications.
+#
+# CONFIG_ARCH_FPU is not set
+# CONFIG_ARCH_LEDS is not set
+# CONFIG_DISABLE_OS_API is not set
+# CONFIG_SYSTEM_DD is not set
+CONFIG_ARCH="arm"
+CONFIG_ARCH_BOARD="stm32f4discovery"
+CONFIG_ARCH_BOARD_COMMON=y
+CONFIG_ARCH_BOARD_STM32F4_DISCOVERY=y
+CONFIG_ARCH_CHIP="stm32"
+CONFIG_ARCH_CHIP_STM32=y
+CONFIG_ARCH_CHIP_STM32F407VG=y
+CONFIG_ARCH_STACKDUMP=y
+CONFIG_BOARD_LATE_INITIALIZE=y
+CONFIG_BOARD_LOOPSPERMSEC=16717
+CONFIG_BUILTIN=y
+CONFIG_CXX_EXCEPTION=y
+CONFIG_CXX_STANDARD="c++17"
+CONFIG_DRVR_MKRD=y
+CONFIG_FS_PROCFS=y
+CONFIG_HAVE_CXX=y
+CONFIG_INIT_NONE=y
+CONFIG_INTELHEX_BINARY=y
+CONFIG_LIBCXXTOOLCHAIN=y
+CONFIG_LINE_MAX=64
+CONFIG_MM_REGIONS=2
+CONFIG_PREALLOC_TIMERS=4
+CONFIG_RAM_SIZE=114688
+CONFIG_RAM_START=0x20000000
+CONFIG_RAW_BINARY=y
+CONFIG_RR_INTERVAL=200
+CONFIG_SCHED_WAITPID=y
+CONFIG_START_DAY=6
+CONFIG_START_MONTH=12
+CONFIG_START_YEAR=2011
+CONFIG_STM32_JTAG_SW_ENABLE=y
+CONFIG_STM32_PWR=y
+CONFIG_STM32_SPI1=y
+CONFIG_STM32_USART2=y
+CONFIG_USART2_RXBUFSIZE=128
+CONFIG_USART2_SERIAL_CONSOLE=y
+CONFIG_USART2_TXBUFSIZE=128

tools/ci/cibuild-oot.sh

@@ -0,0 +1,76 @@
+#!/usr/bin/env bash
+############################################################################
+# tools/ci/cibuild-oot.sh
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.  The
+# ASF licenses this file to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance with the
+# License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+# License for the specific language governing permissions and limitations
+# under the License.
+#
+############################################################################
+
+set -euo pipefail
+
+CID=$(cd "$(dirname "$0")" && pwd)
+CIWORKSPACE=$(cd "${CID}"/../../../ && pwd -P)
+NUTTX_PATH=${CIWORKSPACE}/nuttx
+APP_PATH=${CIWORKSPACE}/apps
+
+# === CONFIGURATION ===
+# Allow overriding from environment for CI
+EXPORT_CONFIG=${EXPORT_CONFIG:-"stm32f4discovery:cxx-oot-build"}
+OOT_SRC=${OOT_SRC:-"$APP_PATH/testing/cxx-oot-build"}
+BUILD_DIR=${BUILD_DIR:-"$OOT_SRC/build"}
+
+# Place ourselves in the nuttx dir
+cd $NUTTX_PATH
+
+echo "=== [1/5] Configuring NuttX Export ($EXPORT_CONFIG) ==="
+./tools/configure.sh -E -l "$EXPORT_CONFIG"
+
+echo "=== [2/5] Building Export Tarball ==="
+make -j"$(nproc)" export
+
+EXPORT_TARBALL=$(find . -maxdepth 1 -type f -name "nuttx-export-*.tar.gz" | sort | tail -n 1)
+if [[ -z "$EXPORT_TARBALL" ]]; then
+    echo "❌ ERROR: No export tarball found"
+    exit 1
+fi
+
+echo "=== [3/5] Preparing Out of Tree (OOT) Project ==="
+rm -rf "$OOT_SRC"/nuttx-export-*
+tar -xzf "$EXPORT_TARBALL" -C "$OOT_SRC"
+
+TOOLCHAIN_FILE=$(find "$OOT_SRC" -type f -path "*/scripts/toolchain.cmake" | head -n 1)
+if [[ ! -f "$TOOLCHAIN_FILE" ]]; then
+    echo "❌ ERROR: toolchain.cmake not found after export"
+    exit 1
+fi
+
+echo "=== [4/5] Building OOT ==="
+rm -rf "$BUILD_DIR"
+mkdir -p "$BUILD_DIR"
+cd "$BUILD_DIR"
+cmake .. -DCMAKE_TOOLCHAIN_FILE="$TOOLCHAIN_FILE" -DBUILD_OOTCPP=ON
+make -j"$(nproc)"
+
+echo "=== [5/5] Verifying Output ==="
+if [[ ! -f "$BUILD_DIR/oot" || ! -f "$BUILD_DIR/oot.bin" ]]; then
+    echo "❌ ERROR: oot or oot.bin not found in $BUILD_DIR"
+    exit 1
+fi
+
+echo "✅ SUCCESS: OOT build completed. Output:"
+ls -lh "$BUILD_DIR"/oot*