Introduce fftw-api virtual package for Intel-MKL and FFTW (#9618)

* Introduce FFTW2 and FFT3 providers for Intel-MKL and FFTW Spack packages.

* make fftw default package for fftw-api virtual package

* virtual package test assertion now provides location of default virtual packages.

* Change name of virtual package to fftw-api and used versioned interface.

etc/spack/defaults/packages.yaml

@@ -22,6 +22,7 @@ packages:
       blas: [openblas]
       daal: [intel-daal]
       elf: [elfutils]
+      fftw-api: [fftw]
       gl: [mesa, opengl]
       glu: [mesa-glu, openglu]
       golang: [gcc]

lib/spack/spack/test/package_sanity.py

@@ -47,9 +47,12 @@ def test_all_virtual_packages_have_default_providers():
     defaults = spack.config.get('packages', scope='defaults')
     default_providers = defaults['all']['providers']
     providers = spack.repo.path.provider_index.providers
-
+    default_providers_filename = \
+        spack.config.config.scopes['defaults'].get_section_filename('packages')
     for provider in providers:
-        assert provider in default_providers
+        assert provider in default_providers, \
+            "all providers must have a default in %s" \
+            % default_providers_filename
 
 
 def test_package_version_consistency():

var/spack/repos/builtin/packages/fftw/package.py

@@ -72,6 +72,9 @@ class Fftw(AutotoolsPackage):
     # https://github.com/FFTW/fftw3/commit/902d0982522cdf6f0acd60f01f59203824e8e6f3
     conflicts('%gcc@8:8.9999', when="@3.3.7")
 
+    provides('fftw-api@2', when='@2.1.5')
+    provides('fftw-api@3', when='@3:')
+
     @property
     def libs(self):
 

var/spack/repos/builtin/packages/intel-mkl/package.py

@@ -53,6 +53,7 @@ class IntelMkl(IntelPackage):
     provides('lapack')
     provides('scalapack')
     provides('mkl')
+    provides('fftw-api@3', when='@2017:')
 
     if sys.platform == 'darwin':
         # there is no libmkl_gnu_thread on macOS