element: validate configuration variables

Ensure that protected variables are not being redefined by the user.

2 files changed, 12 insertions, 1 deletions

diff --git a/buildstream/_exceptions.py b/buildstream/_exceptions.py
index a4e3ce69b..fa595852b 100644
--- a/buildstream/_exceptions.py
+++ b/buildstream/_exceptions.py
@@ -219,6 +219,9 @@ class LoadErrorReason(Enum):
     # A recursive variable has been encountered
     RECURSIVE_VARIABLE = 22
 
+    # An attempt so set the value of a protected variable
+    PROTECTED_VARIABLE_REDEFINED = 23
+
 
 # LoadError
 #
diff --git a/buildstream/element.py b/buildstream/element.py
index 4f7fc0564..6b0a728e6 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -2166,7 +2166,8 @@ class Element(Plugin):
     # substituting command strings to be run in the sandbox
     #
     def __extract_variables(self, meta):
-        default_vars = _yaml.node_get(self.__defaults, Mapping, 'variables', default_value={})
+        default_vars = _yaml.node_get(self.__defaults, Mapping, 'variables',
+                                      default_value={})
 
         project = self._get_project()
         if self.__is_junction:
@@ -2179,6 +2180,13 @@ class Element(Plugin):
         _yaml.composite(variables, meta.variables)
         _yaml.node_final_assertions(variables)
 
+        for var in ('project-name', 'element-name', 'max-jobs'):
+            provenance = _yaml.node_get_provenance(variables, var)
+            if provenance and provenance.filename != '':
+                raise LoadError(LoadErrorReason.PROTECTED_VARIABLE_REDEFINED,
+                                "{}: invalid redefinition of protected variable '{}'"
+                                .format(provenance, var))
+
         return variables
 
     # This will resolve the final configuration to be handed