Fail if two elements provide the same thing

The current check only validates that an element that specifies
"element-provides" doesn't conflict with a "real" element.  We also
want to check this against the provides of other elements.

A real example is with a "block-device" element.  There is no actual
"block-device" element; we can have multiple elements provide it
(block-device-[gpt,mbr,efi], say) but we only want one of them at a
time.

Update the unit test for this.

Change-Id: I59d4aa5f6f09e2892b213e154befa10d85e95ca3

diskimage_builder/element_dependencies.py

@@ -151,9 +151,14 @@ def _expand_element_dependencies(user_elements, all_elements):
 
         element_deps = element_obj.depends
         element_provides = element_obj.provides
-        # save which elements provide another element for potential
+        # Check that we are not providing an element which has already
-        # error message
+        # been provided by someone else, and additionally save which
+        # elements provide another element
         for provide in element_provides:
+            if provide in provided:
+                raise AlreadyProvidedException(
+                    "%s: already provided by %s" %
+                    (provide, provided_by[provide]))
             provided_by[provide].append(element)
         provided.update(element_provides)
         check_queue.extend(element_deps - (final_elements | provided))

diskimage_builder/tests/test_elementdeps.py

@@ -67,6 +67,10 @@ class TestElementDeps(testtools.TestCase):
                           'provides_virtual',
                           [],
                           ['virtual'])
+        _populate_element(self.element_dir,
+                          'also_provides_virtual',
+                          [],
+                          ['virtual'])
         _populate_element(self.element_dir,
                           'requires_virtual',
                           ['virtual'],
@@ -168,6 +172,14 @@ class TestElementDeps(testtools.TestCase):
                 [self._e('requires_new_virtual'),
                  self._e('provides_new_virtual')], result)
 
+    def test_elements_provide_same(self):
+        msg = "virtual: already provided by \['provides_virtual'\]"
+        self.assertRaisesRegexp(element_dependencies.AlreadyProvidedException,
+                                msg,
+                                element_dependencies.get_elements,
+                                ['provides_virtual', 'also_provides_virtual'],
+                                self.element_dirs)
+
     def test_no_os_element(self):
         self.assertRaises(element_dependencies.MissingOSException,
                           element_dependencies.get_elements,