concretizer: handle version preferences from packages.yaml

lib/spack/spack/solver/asp.py

@@ -1031,6 +1031,17 @@ def preferred_targets(self, pkg_name):
             str(preferred.architecture.target), pkg_name, -10
         ))
 
+    def preferred_versions(self, pkg_name):
+        packages_yaml = spack.config.get('packages')
+        versions = packages_yaml.get(pkg_name, {}).get('version', [])
+        if not versions:
+            return
+
+        for idx, version in enumerate(reversed(versions)):
+            self.gen.fact(
+                fn.preferred_version_declared(pkg_name, version, -(idx + 1))
+            )
+
     def flag_defaults(self):
         self.gen.h2("Compiler flag defaults")
 
@@ -1404,10 +1415,12 @@ def setup(self, driver, specs):
 
         self.gen.h1('Package Constraints')
         for pkg in sorted(pkgs):
-            self.gen.h2('Package: %s' % pkg)
+            self.gen.h2('Package rules: %s' % pkg)
             self.pkg_rules(pkg)
+            self.gen.h2('Package preferences: %s' % pkg)
             self.preferred_variants(pkg)
             self.preferred_targets(pkg)
+            self.preferred_versions(pkg)
 
         self.gen.h1('Spec Constraints')
         for spec in sorted(specs):

lib/spack/spack/solver/concretize.lp

@@ -15,9 +15,14 @@ version_declared(Package, Version) :- version_declared(Package, Version, _).
  :- node(Package).
 
 version_weight(Package, Weight)
- :- version(Package, Version), version_declared(Package, Version, Weight).
+ :- version(Package, Version), version_declared(Package, Version, Weight),
+    not preferred_version_declared(Package, Version, _).
+
+version_weight(Package, Weight)
+  :- version(Package, Version), preferred_version_declared(Package, Version, Weight).
 
 #defined version_conflict/2.
+#defined preferred_version_declared/3.
 
 %-----------------------------------------------------------------------------
 % Dependency semantics
@@ -243,6 +248,8 @@ target_weight(Target, Package, Weight)
      not derive_target_from_parent(_, Package),
      not package_target_weight(Target, Package, _).
 
+% TODO: Need to account for the case of more than one parent
+% TODO: each of which sets different targets
 target_weight(Target, Dependency, Weight)
   :- depends_on(Package, Dependency),
      derive_target_from_parent(Package, Dependency),