Generalize package relations like depends_on, provides, conflicts.

All of these do the same thing.  So they are all now generalized
to a single closure function; just the name of the updated variable
in the package is different.

lib/spack/spack/package.py

@@ -255,6 +255,11 @@ class SomePackage(Package):
     """List of specs of virtual packages provided by this package."""
     provided = {}
 
+    """List of specs of conflicting packages.
+       TODO: implement conflicts.
+    """
+    conflicted = {}
+
     #
     # These are default values for instance variables.
     #

lib/spack/spack/relations.py

@@ -62,23 +62,28 @@ def _caller_locals():
         del stack
 
 
-def depends_on(*specs):
-    """Adds a dependencies local variable in the locals of
-       the calling class, based on args.
-    """
-    # Get the enclosing package's scope and add deps to it.
-    dependencies = _caller_locals().setdefault("dependencies", {})
-    for string in specs:
-        for spec in spack.spec.parse(string):
-            dependencies[spec.name] = spec
+def _make_relation(map_name):
+    def relation_fun(*specs):
+        package_map = _caller_locals().setdefault(map_name, {})
+        for string in specs:
+            for spec in spack.spec.parse(string):
+                package_map[spec.name] = spec
+    return relation_fun
 
 
-def provides(*args):
-    """Allows packages to provide a virtual dependency.  If a package provides
-       "mpi", other packages can declare that they depend on "mpi", and spack
-       can use the providing package to satisfy the dependency.
-    """
-    # Get the enclosing package's scope and add deps to it.
-    provided = _caller_locals().setdefault("provided", [])
-    for name in args:
-        provided.append(name)
+"""Adds a dependencies local variable in the locals of
+   the calling class, based on args. """
+depends_on = _make_relation("dependencies")
+
+
+"""Allows packages to provide a virtual dependency.  If a package provides
+   'mpi', other packages can declare that they depend on "mpi", and spack
+   can use the providing package to satisfy the dependency.
+"""
+provides   = _make_relation("provided")
+
+
+"""Packages can declare conflicts with other packages.
+   This can be as specific as you like: use regular spec syntax.
+"""
+conflicts  = _make_relation("conflicted")

lib/spack/spack/spec.py

@@ -394,7 +394,12 @@ def preorder_traversal(self, visited=None, d=0, **kwargs):
 
 
     def _concretize_helper(self, presets):
-        """Recursive helper function for concretize()."""
+        """Recursive helper function for concretize().
+           This concretizes everything bottom-up.  As things are
+           concretized, they're added to the presets, and ancestors
+           will prefer the settings of their children.
+        """
+        # Concretize deps first -- this is a bottom-up process.
         for name in sorted(self.dependencies.keys()):
             self.dependencies[name]._concretize_helper(presets)