define_from_variant: return an empty string for non-existing variants (#27503)

This permits to use conditional variants without a lot of boilerplate.

lib/spack/docs/build_systems/cmakepackage.rst

@@ -145,6 +145,20 @@ and without the :meth:`~spack.build_systems.cmake.CMakePackage.define` and
 
        return args
 
+Spack supports CMake defines from conditional variants too. Whenever the condition on
+the variant is not met, ``define_from_variant()`` will simply return an empty string,
+and CMake simply ignores the empty command line argument. For example the following
+
+.. code-block:: python
+
+   variant('example', default=True, when='@2.0:')
+
+   def cmake_args(self):
+      return [self.define_from_variant('EXAMPLE', 'example')]
+
+will generate ``'cmake' '-DEXAMPLE=ON' ...`` when `@2.0: +example` is met, but will
+result in ``'cmake' '' ...`` when the spec version is below ``2.0``.
+
 
 ^^^^^^^^^^
 Generators

lib/spack/spack/build_systems/cmake.py

@@ -267,6 +267,10 @@ def define_from_variant(self, cmake_var, variant=None):
                  "-DSWR:STRING=avx;avx2]
 
             for ``<spec-name> cxxstd=14 +shared swr=avx,avx2``
+
+        Note: if the provided variant is conditional, and the condition is not met,
+                this function returns an empty string. CMake discards empty strings
+                provided on the command line.
         """
 
         if variant is None:
@@ -276,6 +280,9 @@ def define_from_variant(self, cmake_var, variant=None):
             raise KeyError(
                 '"{0}" is not a variant of "{1}"'.format(variant, self.name))
 
+        if variant not in self.spec.variants:
+            return ''
+
         value = self.spec.variants[variant].value
         if isinstance(value, (tuple, list)):
             # Sort multi-valued variants for reproducibility

lib/spack/spack/test/build_systems.py

@@ -429,3 +429,11 @@ def test_define(self):
 
         assert pkg.urls[0] == 'https://www.x.org/archive/individual/' \
                               'util/util-macros-1.19.1.tar.bz2'
+
+
+def test_cmake_define_from_variant_conditional(config, mock_packages):
+    """Test that define_from_variant returns empty string when a condition on a variant
+    is not met. When this is the case, the variant is not set in the spec."""
+    s = Spec('cmake-conditional-variants-test').concretized()
+    assert 'example' not in s.variants
+    assert s.package.define_from_variant('EXAMPLE', 'example') == ''

var/spack/repos/builtin.mock/packages/cmake-conditional-variants-test/package.py

@@ -0,0 +1,9 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+class CmakeConditionalVariantsTest(CMakePackage):
+    homepage = "https://dev.null"
+    version('1.0')
+    variant('example', default=True, description='nope', when='@2.0:')