let CMakeMake easyblock also set Python_EXECUTABLE option, as well as Python3_EXECUTABLE and Python2_EXECUTABLE derivatives (when appropriate)

easybuild/easyblocks/generic/cmakemake.py

@@ -87,6 +87,52 @@ def setup_cmake_env(tc):
     setvar("CMAKE_LIBRARY_PATH", library_paths)
 
 
+def setup_cmake_env_python_hints(cmake_version=None):
+    """Set environment variables as hints for CMake to prefer the Python module, if loaded.
+    Useful when there is no way to specify arguments for CMake directly,
+    e.g. when CMake is called from within another build system.
+    Otherwise get_cmake_python_config_[str/dict] should be used instead.
+    """
+    if cmake_version is None:
+        cmake_version = det_cmake_version()
+    if LooseVersion(cmake_version) < '3.12':
+        raise EasyBuildError("Setting Python hints for CMake requires CMake version 3.12 or newer")
+    python_root = get_software_root('Python')
+    if python_root:
+        python_version = LooseVersion(get_software_version('Python'))
+        setvar('Python_ROOT_DIR', python_root)
+        if python_version >= "3":
+            setvar('Python3_ROOT_DIR', python_root)
+        else:
+            setvar('Python2_ROOT_DIR', python_root)
+
+
+def get_cmake_python_config_dict():
+    """Get a dictionary with CMake configuration options for finding Python if loaded as a module."""
+    options = {}
+    python_root = get_software_root('Python')
+    if python_root:
+        python_version = LooseVersion(get_software_version('Python'))
+        python_exe = os.path.join(python_root, 'bin', 'python')
+        # This is required for (deprecated) `find_package(PythonInterp ...)`
+        options['PYTHON_EXECUTABLE'] = python_exe
+        # Ensure that both `find_package(Python) and find_package(Python2/3)` work as intended
+        options['Python_EXECUTABLE'] = python_exe
+        if python_version >= "3":
+            options['Python3_EXECUTABLE'] = python_exe
+        else:
+            options['Python2_EXECUTABLE'] = python_exe
+    return options
+
+
+def get_cmake_python_config_str():
+    """Get CMake configuration arguments for finding Python if loaded as a module.
+    This string is intended to be passed to the invocation of `cmake`.
+    """
+    options = get_cmake_python_config_dict()
+    return ' '.join('-D%s=%s' % (key, value) for key, value in options.items())
+
+
 class CMakeMake(ConfigureMake):
     """Support for configuring build with CMake instead of traditional configure script"""
 
@@ -273,17 +319,15 @@ def configure_step(self, srcdir=None, builddir=None):
             # see https://github.com/Kitware/CMake/commit/3ec9226779776811240bde88a3f173c29aa935b5
             options['CMAKE_SKIP_RPATH'] = 'ON'
 
-        # make sure that newer CMAKE picks python based on location, not just the newest python
-        # Avoids issues like e.g. https://github.com/EESSI/software-layer/pull/370#issuecomment-1785594932
-        if LooseVersion(self.cmake_version) >= '3.15':
-            options['CMAKE_POLICY_DEFAULT_CMP0094'] = 'NEW'
-
         # show what CMake is doing by default
         options['CMAKE_VERBOSE_MAKEFILE'] = 'ON'
 
         # disable CMake user package repository
         options['CMAKE_FIND_USE_PACKAGE_REGISTRY'] = 'OFF'
 
+        # ensure CMake uses EB python, not system or virtualenv python
+        options.update(get_cmake_python_config_dict())
+
         if not self.cfg.get('allow_system_boost', False):
             boost_root = get_software_root('Boost')
             if boost_root:
@@ -302,6 +346,8 @@ def configure_step(self, srcdir=None, builddir=None):
                 options['BOOST_ROOT'] = boost_root
                 options['Boost_NO_SYSTEM_PATHS'] = 'ON'
 
+        self.cmake_options = options
+
         if self.cfg.get('configure_cmd') == DEFAULT_CONFIGURE_CMD:
             self.prepend_config_opts(options)
             command = ' '.join([