concretizer: don't require a provider for virtual deps if spec is external

This commit introduces a new rule:

real_node(Package) :- not external(Package), node(Package).

that permits to distinguish between an external node and a
real node that shouldn't trim dependency. It solves the
case of concretizing ninja with an external Python.

lib/spack/spack/solver/asp.py

@@ -938,13 +938,13 @@ def package_dependencies_rules(self, pkg, tests):
                 # add constraints on the dependency from dep spec.
                 if spack.repo.path.is_virtual(dep.spec.name):
                     self.virtual_constraints.add(str(dep.spec))
+                    conditions = ([fn.real_node(pkg.name)] +
+                                  self.spec_clauses(named_cond, body=True))
                     self.gen.rule(
                         head=fn.single_provider_for(
                             str(dep.spec.name), str(dep.spec.versions)
                         ),
-                        body=self.gen._and(
-                            *self.spec_clauses(named_cond, body=True)
-                        )
+                        body=self.gen._and(*conditions)
                     )
                 else:
                     clauses = self.spec_clauses(dep.spec)

lib/spack/spack/solver/concretize.lp

@@ -144,6 +144,9 @@ version_declared(Package, Version, Weight) :- external_version_declared(Package,
 % if a package is not buildable (external_only), only externals are allowed
 external(Package) :- external_only(Package), node(Package).
 
+% a package is a real_node if it is not external
+real_node(Package) :- node(Package), not external(Package).
+
 % if an external version is selected, the package is external and
 % we are using the corresponding spec
 external(Package) :-

lib/spack/spack/test/concretize.py

@@ -866,3 +866,9 @@ def test_external_packages_have_consistent_hash(self):
         assert s.dag_hash() == t.dag_hash()
         assert s.build_hash() == t.build_hash()
         assert s.full_hash() == t.full_hash()
+
+    def test_external_that_would_require_a_virtual_dependency(self):
+        s = Spec('requires-virtual').concretized()
+
+        assert s.external
+        assert 'stuff' not in s

lib/spack/spack/test/data/config/packages.yaml

@@ -22,3 +22,8 @@ packages:
     - spec: externalmodule@1.0%gcc@4.5.0
       modules:
       - external-module
+  'requires-virtual':
+    buildable: False
+    externals:
+    - spec:  requires-virtual@2.0
+      prefix: /usr

var/spack/repos/builtin.mock/packages/requires-virtual/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2020 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 RequiresVirtual(Package):
+    """Package that requires a virtual dependency and is registered
+    as an external.
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/a-1.0.tar.gz"
+
+    version('2.0', '2.0_a_hash')
+
+    depends_on('stuff')