Avoid null reference in arch concretization (#2596)

Fixes #2587

The concretizer falls back on using the root architecture (followed
by the default system architecture) to fill in unspecified arch
properties for a spec. It failed to check cases where the root could
be None.

lib/spack/spack/concretize.py

@@ -222,9 +222,10 @@ def concretize_architecture(self, spec):
             spec.architecture = spack.spec.ArchSpec(sys_arch)
             spec_changed = True
 
-        default_archs = [root_arch, sys_arch]
-        while not spec.architecture.concrete and default_archs:
-            arch = default_archs.pop(0)
+        default_archs = list(x for x in [root_arch, sys_arch] if x)
+        for arch in default_archs:
+            if spec.architecture.concrete:
+                break
 
             replacement_fields = [k for k, v in iteritems(arch.to_cmp_dict())
                                   if v and not getattr(spec.architecture, k)]
@@ -232,6 +233,9 @@ def concretize_architecture(self, spec):
                 setattr(spec.architecture, field, getattr(arch, field))
                 spec_changed = True
 
+        if not spec.architecture.concrete:
+            raise InsufficientArchitectureInfoError(spec, default_archs)
+
         return spec_changed
 
     def concretize_variants(self, spec):
@@ -466,6 +470,17 @@ def __init__(self, spec):
             % (spec.name, spec.versions))
 
 
+class InsufficientArchitectureInfoError(spack.error.SpackError):
+
+    """Raised when details on architecture cannot be collected from the
+       system"""
+
+    def __init__(self, spec, archs):
+        super(InsufficientArchitectureInfoError, self).__init__(
+            "Cannot determine necessary architecture information for '%s': %s"
+            % (spec.name, str(archs)))
+
+
 class NoBuildError(spack.error.SpackError):
     """Raised when a package is configured with the buildable option False, but
        no satisfactory external versions can be found"""