Merge "Adds "element-provides" support to element dependencies"

README.md

@@ -325,9 +325,17 @@ scripts in each phase.
 
 ### Dependencies ###
 
-Each element has a file named element-deps: a plain text, newline separated
-list of elements which will be added to the list of elements built into the
-image at image creation time.
+Each element can use the following files to define or affect dependencies:
+
+* element-deps: a plain text, newline separated list of elements which will
+  be added to the list of elements built into the image at image creation time.
+
+* element-provides: A plain text, newline separated list of elements which
+  are provided by this element. These elements will be excluded from elements
+  built into the image at image creation time. For example if element A depends
+  on element B and element C includes element B in its "element-provides"
+  file and A and C are included when building an image, then B is not used.
+
 
 ### First-boot files ###
 

diskimage_builder/element_dependencies.py

@@ -24,21 +24,12 @@ def get_elements_dir():
     return os.environ['ELEMENTS_PATH']
 
 
-def dependencies(element, elements_dir=None):
-    """Return the non-transitive list of dependencies for a single element.
-
-    :param user_elements: iterable enumerating elements a user has requested
-    :param elements_dir: the elements dir to read from. If not supplied,
-                         inferred by calling get_elements_dir().
-
-    :return: a set just containing all elements that the specified element
-             depends on.
-    """
+def _get_set(element, fname, elements_dir=None):
     if elements_dir is None:
         elements_dir = get_elements_dir()
 
     for path in elements_dir.split(':'):
-        element_deps_path = (os.path.join(path, element, 'element-deps'))
+        element_deps_path = (os.path.join(path, element, fname))
         try:
             with open(element_deps_path) as element_deps:
                 return set([line.strip() for line in element_deps])
@@ -55,6 +46,32 @@ def dependencies(element, elements_dir=None):
     exit(-1)
 
 
+def provides(element, elements_dir=None):
+    """Return the set of elements provided by the specified element.
+
+    :param element: name of a single element
+    :param elements_dir: the elements dir to read from. If not supplied,
+                         inferred by calling get_elements_dir().
+
+    :return: a set just containing all elements that the specified element
+             provides.
+    """
+    return _get_set(element, 'element-provides', elements_dir)
+
+
+def dependencies(element, elements_dir=None):
+    """Return the non-transitive set of dependencies for a single element.
+
+    :param element: name of a single element
+    :param elements_dir: the elements dir to read from. If not supplied,
+                         inferred by calling get_elements_dir().
+
+    :return: a set just containing all elements that the specified element
+             depends on.
+    """
+    return _get_set(element, 'element-deps', elements_dir)
+
+
 def expand_dependencies(user_elements, elements_dir=None):
     """Expand user requested elements using element-deps files.
 
@@ -68,14 +85,24 @@ def expand_dependencies(user_elements, elements_dir=None):
     """
     final_elements = set(user_elements)
     check_queue = list(user_elements)
+    provided = set()
 
     while check_queue:
         element = check_queue.pop()
+        if element in provided:
+            continue
         deps = dependencies(element, elements_dir)
-        check_queue.extend(deps - final_elements)
+        provided.update(provides(element, elements_dir))
+        check_queue.extend(deps - (final_elements | provided))
         final_elements.update(deps)
 
-    return final_elements
+    conflicts = set(user_elements) & provided
+    if conflicts:
+        sys.stderr.write("ERROR: Following elements were explicitly required "
+                         "but are provided by other included elements: %s\n" %
+                         ", ".join(conflicts))
+        exit(-1)
+    return final_elements - provided
 
 
 def main(argv):

diskimage_builder/tests/test_elementdeps.py

@@ -24,12 +24,15 @@ data_dir = os.path.abspath(
     os.path.join(os.path.dirname(__file__), 'test-elements'))
 
 
-def _populate_element(element_dir, element_name, element_deps=[]):
+def _populate_element(element_dir, element_name, element_deps=[], provides=[]):
         element_home = os.path.join(element_dir, element_name)
         os.mkdir(element_home)
         deps_path = os.path.join(element_home, 'element-deps')
         with open(deps_path, 'w') as deps_file:
             deps_file.write("\n".join(element_deps))
+        provides_path = os.path.join(element_home, 'element-provides')
+        with open(provides_path, 'w') as provides_file:
+            provides_file.write("\n".join(provides))
 
 
 class TestElementDeps(testtools.TestCase):
@@ -43,6 +46,12 @@ class TestElementDeps(testtools.TestCase):
                           'requires-requires-foo',
                           ['requires-foo'])
         _populate_element(self.element_dir, 'self', ['self'])
+        _populate_element(self.element_dir,
+                          'provides_virtual',
+                          [],
+                          ['virtual'])
+        _populate_element(self.element_dir, 'requires_virtual', ['virtual'])
+        _populate_element(self.element_dir, 'virtual', [])
         _populate_element(self.element_dir, 'circular1', ['circular2'])
         _populate_element(self.element_dir, 'circular2', ['circular1'])
 
@@ -79,6 +88,18 @@ class TestElementDeps(testtools.TestCase):
             ['circular1'], elements_dir=self.element_dir)
         self.assertEqual(set(['circular1', 'circular2']), result)
 
+    def test_provide(self):
+        result = element_dependencies.expand_dependencies(
+            ['requires_virtual', 'provides_virtual'],
+            elements_dir=self.element_dir)
+        self.assertEqual(set(['requires_virtual', 'provides_virtual']), result)
+
+    def test_provide_conflict(self):
+        self.assertRaises(SystemExit,
+                          element_dependencies.expand_dependencies,
+                          ['virtual', 'provides_virtual'],
+                          self.element_dir)
+
 
 class TestElements(testtools.TestCase):
     def test_depends_on_env(self):