element.py: Abstract not producing artifacts

Currently the stack element does not produce an artifact, and sometimes
has to be treated differently because of this. It is conceivable that
someone will write a plugin that must be treated similarly and doesn't
produce an artifact. As a result this commit abstracts this "not
producing an artifact" feature of elements.

4 files changed, 16 insertions, 6 deletions

diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 2efdd3abb..044b97458 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -211,6 +211,12 @@ class Element(Plugin):
     *Since: 1.4*
     """
 
+    BST_ELEMENT_HAS_ARTIFACT = True
+    """Whether the element produces an artifact when built.
+
+    *Since: 1.90*
+    """
+
     def __init__(self, context: 'Context', project: 'Project', meta: 'MetaElement', plugin_conf: Dict[str, Any]):
 
         self.__cache_key_dict = None            # Dict for cache key calculation
diff --git a/src/buildstream/plugins/elements/filter.py b/src/buildstream/plugins/elements/filter.py
index 2f9204215..d808c9e5a 100644
--- a/src/buildstream/plugins/elements/filter.py
+++ b/src/buildstream/plugins/elements/filter.py
@@ -200,12 +200,13 @@ class FilterElement(Element):
                                .format(self, type(self).__name__),
                                detail=detail, reason="filter-bdepend-also-rdepend")
 
-        # If a stack, fail and inform user that the dependency can't be a stack
-        if build_deps[0].get_kind() == 'stack':
-            detail = "{} is a stack element, which has no artifact".format(build_deps[0].name)
-            raise ElementError("{}: {} element's build dependency must not be a stack element"
+        # If a parent does not produce an artifact, fail and inform user that the dependency
+        # must produce artifacts
+        if not build_deps[0].BST_ELEMENT_HAS_ARTIFACT:
+            detail = "{} does not produce an artifact, so there is nothing to filter".format(build_deps[0].name)
+            raise ElementError("{}: {} element's build dependency must produce an artifact"
                                .format(self, type(self).__name__),
-                               detail=detail, reason="filter-bdepend-is-stack")
+                               detail=detail, reason="filter-bdepend-no-artifact")
 
     def get_unique_key(self):
         key = {
diff --git a/src/buildstream/plugins/elements/stack.py b/src/buildstream/plugins/elements/stack.py
index 97517ca48..dbb59a43d 100644
--- a/src/buildstream/plugins/elements/stack.py
+++ b/src/buildstream/plugins/elements/stack.py
@@ -33,6 +33,9 @@ class StackElement(Element):
     # This plugin has been modified to avoid the use of Sandbox.get_directory
     BST_VIRTUAL_DIRECTORY = True
 
+    # This plugin does not produce any artifacts when built
+    BST_ELEMENT_HAS_ARTIFACT = False
+
     def configure(self, node):
         pass
 
diff --git a/tests/elements/filter.py b/tests/elements/filter.py
index b8dd4351e..fc64c0342 100644
--- a/tests/elements/filter.py
+++ b/tests/elements/filter.py
@@ -558,4 +558,4 @@ def test_filter_stack_depend_failure(datafiles, cli):
     project = str(datafiles)
 
     result = cli.run(project=project, args=['build', 'forbidden-stack-dep.bst'])
-    result.assert_main_error(ErrorDomain.ELEMENT, "filter-bdepend-is-stack")
+    result.assert_main_error(ErrorDomain.ELEMENT, "filter-bdepend-no-artifact")