element.py: Add BST_ALLOW_SHELL to improve errors

src/buildstream/element.py

diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index fa4f63b..16f1fdf 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -185,6 +185,8 @@
     *Since: 1.4*
     """
 
+    BST_ALLOW_SHELL = True
+
     def __init__(self, context, project, meta, plugin_conf):
 
         self.__cache_key_dict = None            # Dict for cache key calculation
@@ -1957,6 +1959,9 @@
     def _shell(self, scope=None, directory=None, *, mounts=None, isolate=False, prompt=None, command=None,
                usebuildtree=False):
 
+        if scope == Scope.BUILD and self.BST_ALLOW_SHELL == False:    
+            raise ElementError("{} does not allow build shell access".format(self))
+        
         with self._prepare_sandbox(scope, directory, shell=True, usebuildtree=usebuildtree) as sandbox:
             environment = self.get_environment()
             environment = copy.copy(environment)

src/buildstream/plugins/elements/filter.py

diff --git a/src/buildstream/plugins/elements/filter.py b/src/buildstream/plugins/elements/filter.py
index 45847e6..7f786a4 100644
--- a/src/buildstream/plugins/elements/filter.py
+++ b/src/buildstream/plugins/elements/filter.py
@@ -166,6 +166,8 @@
     # Filter elements do not run any commands
     BST_RUN_COMMANDS = False
 
+    BST_ALLOW_SHELL = False
+
     def configure(self, node):
         self.node_validate(node, [
             'include', 'exclude', 'include-orphans'

src/buildstream/plugins/elements/import.py

diff --git a/src/buildstream/plugins/elements/import.py b/src/buildstream/plugins/elements/import.py
index 61e353d..3885fa3 100644
--- a/src/buildstream/plugins/elements/import.py
+++ b/src/buildstream/plugins/elements/import.py
@@ -44,6 +44,8 @@
     # Import elements do not run any commands
     BST_RUN_COMMANDS = False
 
+    BST_ALLOW_SHELL = False
+
     def configure(self, node):
         self.node_validate(node, [
             'source', 'target'

src/buildstream/plugins/elements/junction.py

diff --git a/src/buildstream/plugins/elements/junction.py b/src/buildstream/plugins/elements/junction.py
index 4222de3..d3f8cf5 100644
--- a/src/buildstream/plugins/elements/junction.py
+++ b/src/buildstream/plugins/elements/junction.py
@@ -173,6 +173,7 @@
     # Junctions are not allowed any dependencies
     BST_FORBID_BDEPENDS = True
     BST_FORBID_RDEPENDS = True
+    BST_ALLOW_SHELL = False
 
     def configure(self, node):
         self.path = self.node_get_member(node, str, 'path', default='')

src/buildstream/plugins/elements/stack.py

diff --git a/src/buildstream/plugins/elements/stack.py b/src/buildstream/plugins/elements/stack.py
index 97517ca..e4eba2c 100644
--- a/src/buildstream/plugins/elements/stack.py
+++ b/src/buildstream/plugins/elements/stack.py
@@ -32,6 +32,7 @@
 
     # This plugin has been modified to avoid the use of Sandbox.get_directory
     BST_VIRTUAL_DIRECTORY = True
+    BST_ALLOW_SHELL = False
 
     def configure(self, node):
         pass