Catch attempts to compose a listsam/compose-list-exception

Attempting to compose a list property would result in an unhandled
exception:

    AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that
the frontend will report neatly:

    Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the
sources list:

    sources:
      (?):
        arch == "x86_64":
          - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent
dict, e.g.:

    (?):
      arch == "x86_64":
        sources:
          - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do
what they want, but it doesn't seem possible in this case.

1 files changed, 7 insertions, 3 deletions

diff --git a/buildstream/_yaml.py b/buildstream/_yaml.py
index bba59c73c..8aa2ec714 100644
--- a/buildstream/_yaml.py
+++ b/buildstream/_yaml.py
@@ -743,13 +743,17 @@ def composite_dict(target, source, path=None):
 # Like composite_dict(), but raises an all purpose LoadError for convenience
 #
 def composite(target, source):
-    provenance = node_get_provenance(source)
+    if not hasattr(source, 'get'):
+        raise LoadError(LoadErrorReason.ILLEGAL_COMPOSITE,
+                        "Only values of type 'dict' can be composed.")
+
+    source_provenance = node_get_provenance(source)
     try:
         composite_dict(target, source)
     except CompositeTypeError as e:
         error_prefix = ""
-        if provenance:
-            error_prefix = "[%s]: " % str(provenance)
+        if source_provenance:
+            error_prefix = "[%s]: " % str(source_provenance)
         raise LoadError(LoadErrorReason.ILLEGAL_COMPOSITE,
                         "%sExpected '%s' type for configuration '%s', instead received '%s'" %
                         (error_prefix,