Adds "element-provides" support to element dependencies

An element can define a list of elements provided by itself. This allows
using an alternative element and keeping element dependencies at the
same time. This will be needed for example for mariadb element which will be
an alternative to mysql element - mysql dependency can be kept everywhere
but user can choose mariadb explicitly as a replacement by putting "mysql"
into "element-provides" file.

Change-Id: I7f193612f279dd71d9f1f1edfc60009838a00c5a

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):