Allow plugins to forbid runtime dependencies and sources

The flags BST_FORBID_RDEPENDS and BST_FORBID_SOURCES can be set in the
plugin's class declaration, and exceptions will be raised during
pre-flight checks

4 files changed, 36 insertions, 2 deletions

diff --git a/buildstream/_loader.py b/buildstream/_loader.py
index 2ced3abfa..de9efdd25 100644
--- a/buildstream/_loader.py
+++ b/buildstream/_loader.py
@@ -350,7 +350,7 @@ class Loader():
             source = meta_element.project._create_source(meta_source.kind,
                                                          meta_source)
 
-            source.preflight()
+            source._preflight()
 
             if source.get_consistency() != Consistency.CACHED:
                 if self.context._fetch_subprojects:
diff --git a/buildstream/_pipeline.py b/buildstream/_pipeline.py
index 7328d7d38..d1ab0f27c 100644
--- a/buildstream/_pipeline.py
+++ b/buildstream/_pipeline.py
@@ -164,7 +164,7 @@ class Pipeline():
     def preflight(self):
         for plugin in self.dependencies(Scope.ALL, include_sources=True):
             try:
-                plugin.preflight()
+                plugin._preflight()
             except BstError as e:
                 # Prepend the plugin identifier string to the error raised by
                 # the plugin so that users can more quickly identify the issue
diff --git a/buildstream/element.py b/buildstream/element.py
index 2bd7abfca..8362f81ca 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -128,6 +128,14 @@ class Element(Plugin):
     any of the dependencies have changed.
     """
 
+    BST_FORBID_RDEPENDS = False
+    """Whether to raise exceptions if an element has runtime-dependencies.
+    """
+
+    BST_FORBID_SOURCES = False
+    """Whether to raise exceptions if an element has sources.
+    """
+
     def __init__(self, context, project, artifacts, meta, plugin_conf):
 
         super().__init__(meta.name, context, project, meta.provenance, "element")
@@ -1662,6 +1670,21 @@ class Element(Plugin):
                 # Strong cache key could not be calculated yet
                 return
 
+    def _preflight(self):
+        if self.BST_FORBID_RDEPENDS:
+            runtime_deps = list(self.dependencies(Scope.RUN, recurse=False))
+            if runtime_deps:
+                raise ElementError("{}: Runtime dependencies are forbidden for elements of type {}"
+                                   .format(self, type(self).__name__), reason="element-forbidden-rdepends")
+
+        if self.BST_FORBID_SOURCES:
+            sources = list(self.sources())
+            if sources:
+                raise ElementError("{}: Sources are forbidden for elements of type {}"
+                                   .format(self, type(self).__name__), reason="element-forbidden-sources")
+
+        self.preflight()
+
     #############################################################
     #                   Private Local Methods                   #
     #############################################################
diff --git a/buildstream/plugin.py b/buildstream/plugin.py
index bfa37ef5b..353f13cc1 100644
--- a/buildstream/plugin.py
+++ b/buildstream/plugin.py
@@ -577,6 +577,17 @@ class Plugin():
         else:
             yield self.__log
 
+    # _preflight():
+    # Calls preflight() for the plugin, and allows generic preflight
+    # checks to be added
+    #
+    # Raises:
+    #    SourceError: If it's a Source implementation
+    #    ElementError: If it's an Element implementation
+    #    ProgramNotFoundError: If a required host tool is not found
+    def _preflight(self):
+        self.preflight()
+
     #############################################################
     #                     Local Private Methods                 #
     #############################################################