concretizer: concretize a virtual root

Before this modification the root of a DAG has to be
a real package. This commit adds rules to concretize
virtual roots.

lib/spack/spack/cmd/solve.py

@@ -119,7 +119,13 @@ def solve(parser, args):
 
         # iterate over roots from command line
         for input_spec in specs:
-            spec = answer[input_spec.name]
+            key = input_spec.name
+            if input_spec.virtual:
+                providers = [spec.name for spec in answer.values()
+                             if spec.package.provides(key)]
+                key = providers[0]
+
+            spec = answer[key]
 
             # With -y, just print YAML to output.
             if args.format == 'yaml':

lib/spack/spack/solver/asp.py

@@ -1380,7 +1380,9 @@ def setup(self, driver, specs):
         check_packages_exist(specs)
 
         # get list of all possible dependencies
-        self.possible_virtuals = set()
+        self.possible_virtuals = set(
+            x.name for x in specs if x.virtual
+        )
         possible = spack.package.possible_dependencies(
             *specs,
             virtuals=self.possible_virtuals,
@@ -1424,7 +1426,11 @@ def setup(self, driver, specs):
 
         self.gen.h1('Spec Constraints')
         for spec in sorted(specs):
+            if not spec.virtual:
                 self.gen.fact(fn.root(spec.name))
+            else:
+                self.gen.fact(fn.virtual_root(spec.name))
+
             for dep in spec.traverse():
                 self.gen.h2('Spec: %s' % str(dep))
 

lib/spack/spack/solver/concretize.lp

@@ -65,6 +65,7 @@ provider_weight(Dependency, Weight)
  :- virtual(Virtual), depends_on(Package, Dependency),
     provider(Dependency, Virtual),
     pkg_provider_preference(Package, Virtual, Dependency, Weight).
+
 provider_weight(Dependency, Weight)
  :- virtual(Virtual), depends_on(Package, Dependency),
     provider(Dependency, Virtual),
@@ -80,8 +81,23 @@ provider_weight(Dependency, 100)
     not pkg_provider_preference(Package, Virtual, Dependency, _),
     not default_provider_preference(Virtual, Dependency, _).
 
+% Do the same for virtual roots
+provider_weight(Package, Weight)
+ :- virtual_root(Virtual),
+    provider(Package, Virtual),
+    default_provider_preference(Virtual, Package, Weight).
+
+provider_weight(Package, 100)
+ :- virtual_root(Virtual),
+    provider(Package, Virtual),
+    not default_provider_preference(Virtual, Package, _).
+
 % all nodes must be reachable from some root
 node(Package) :- root(Package).
+
+1 { root(Package) : provides_virtual(Package, Virtual) } 1
+  :- virtual_root(Virtual).
+
 needed(Package) :- root(Package).
 needed(Dependency) :- needed(Package), depends_on(Package, Dependency).
 :- node(Package), not needed(Package).
@@ -94,6 +110,7 @@ node(Dependency) :- node(Package), depends_on(Package, Dependency).
 #defined declared_dependency/3.
 #defined virtual/1.
 #defined virtual_node/1.
+#defined virtual_root/1.
 #defined provides_virtual/2.
 #defined external/1.
 #defined external_spec/2.

lib/spack/spack/spec.py

@@ -2434,9 +2434,16 @@ def _new_concretize(self, tests=False):
 
         # take the best answer
         opt, i, answer = min(result.answers)
-        assert self.name in answer
+        name = self.name
+        # TODO: Consolidate this code with similar code in solve.py
+        if self.virtual:
+            providers = [spec.name for spec in answer.values()
+                         if spec.package.provides(name)]
+            name = providers[0]
 
-        concretized = answer[self.name]
+        assert name in answer
+
+        concretized = answer[name]
         self._dup(concretized)
         self._mark_concrete()