hwloc: Add netloc variant (#16001)

hwloc depends on MPI when netloc is enabled. Note that OpenMPI depends on
netloc, so hwloc cannot use OpenMPI as the MPI provider when netloc is
enabled (this would result in a cyclic dependency).

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

@@ -57,6 +57,14 @@ class Hwloc(AutotoolsPackage):
         default=False,
         description='Enable the Cairo back-end of hwloc\'s lstopo command'
     )
+    variant(
+        'netloc',
+        default=False,
+        description="Enable netloc [requires MPI]"
+    )
+
+    # netloc isn't available until version 2.0.0
+    conflicts('+netloc', when="@:1.99.99")
 
     depends_on('pkgconfig', type='build')
     depends_on('m4', type='build', when='@master')
@@ -71,6 +79,10 @@ class Hwloc(AutotoolsPackage):
     depends_on('cairo', when='+cairo')
     depends_on('numactl', when='@:1.11.11 platform=linux')
 
+    # When mpi=openmpi, this introduces an unresolvable dependency.
+    # See https://github.com/spack/spack/issues/15836 for details
+    depends_on('mpi', when='+netloc')
+
     def url_for_version(self, version):
         return "http://www.open-mpi.org/software/hwloc/v%s/downloads/hwloc-%s.tar.gz" % (version.up_to(2), version)
 
@@ -81,7 +93,7 @@ def configure_args(self):
             # (Alternatively, we could require OpenCL as dependency.)
             "--disable-opencl",
         ]
-        if '@2.0.0:' in self.spec:
+        if '+netloc' in self.spec:
             args.append('--enable-netloc')
 
         args.extend(self.enable_or_disable('cairo'))