Allow overwriting a list with an empty list using (=) operatorsam/overwrite-list-with-empty-list

I found myself writing the following split rules for an element:

    public:
      bst:
        split-rules:
          runtime:
            (=): []

          devel:
            (>):
            - |
              %{bindir}/*

The aim was to put all of this element's binaries into the 'devel'
domain, and make sure nothing went into the 'runtime' domain by
replacing the built-in rules for that domain with an empty list.

That wasn't working though because BuildStream was using [] (empty list)
as a sentinel to mean "do nothing", which is fine for the prepend and
append operators (prepending an empty list to a list has no effect) but
is not really correct for the overwrite operator.

This commit fixes that issue and adds a test.

1 files changed, 4 insertions, 2 deletions

diff --git a/buildstream/_yaml.py b/buildstream/_yaml.py
index 153e744f4..11b5fd219 100644
--- a/buildstream/_yaml.py
+++ b/buildstream/_yaml.py
@@ -552,8 +552,10 @@ def composite_list_append(target_node, target_key, source_node, source_key):
 #
 def composite_list_overwrite(target_node, target_key, source_node, source_key):
 
-    source_list = node_get(source_node, list, source_key, default_value=[])
-    if not source_list:
+    # We need to handle the legitimate case of overwriting a list with an empty
+    # list, hence the slightly odd default_value of [None] rather than [].
+    source_list = node_get(source_node, list, source_key, default_value=[None])
+    if source_list == [None]:
         return False
 
     target_provenance = node_get_provenance(target_node)