Fail if we try to crosscompile while PYTHON_SOABI is not set.

CMakeLists.txt

@@ -13,5 +13,8 @@ ament_package(
  CONFIG_EXTRAS "python_cmake_module-extras.cmake"
 )
 
+include(cmake/Modules/CheckCrossCompilingSoabi.cmake)
+CheckCrossCompilingSoabi()
+
 install(DIRECTORY cmake
  DESTINATION share/${PROJECT_NAME})

cmake/Modules/CheckCrossCompilingSoabi.cmake

@@ -0,0 +1,30 @@
+# Copyright 2022 Open Source Robotics Foundation, Inc.
+#
+# Licensed 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.
+
+function(CheckCrossCompilingSoabi)
+ if(NOT DEFINED PYTHON_SOABI AND DEFINED CMAKE_SYSROOT)
+ message(FATAL_ERROR "We detected you defined a sysroot thus crosscompiling.\n"
+ "You have to define the PYTHON_SOABI variable in the toolchain file, which looks like cpython-39-x86_64-linux-gnu or cpython-39-aarch64-linux-gnu.\n"
+ ""
+ "To find the correct PYTHON_SOABI value, you can execute the following from the target:\n"
+ "python3 -c \"${_python_code}\"\n"
+ ""
+ "Run it with qemu with: (expecting aarch64 as your \n"
+ "qemu-aarch64-static -L ${CMAKE_SYSROOT} ${CMAKE_SYSROOT}/usr/bin/python3 -c \"${_python_code}\"\n"
+ ""
+ "The toolchain should look like:\n"
+ "set(PYTHON_SOABI cpython-39-aarch64-linux-gnu)\n"
+ )
+ endif()
+endfunction()

cmake/Modules/FindPythonExtra.cmake

@@ -154,6 +154,8 @@ if(PYTHONINTERP_FOUND)
  "from sysconfig import get_config_var"
  "print(get_config_var('SOABI'))"
  )
+ include(CheckCrossCompilingSoabi)
+ CheckCrossCompilingSoabi()
  execute_process(
  COMMAND
  "${PYTHON_EXECUTABLE}"