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.

2 files changed, 17 insertions, 0 deletions

diff --git a/tests/yaml/data/listoverwriteempty.yaml b/tests/yaml/data/listoverwriteempty.yaml
new file mode 100644
index 000000000..f97a1eafe
--- /dev/null
+++ b/tests/yaml/data/listoverwriteempty.yaml
@@ -0,0 +1,3 @@
+# Composited on top of basics.yaml, effectively deleting its children list
+children:
+  (=): []
diff --git a/tests/yaml/yaml.py b/tests/yaml/yaml.py
index 0aae28f08..a462e18df 100644
--- a/tests/yaml/yaml.py
+++ b/tests/yaml/yaml.py
@@ -213,6 +213,20 @@ def test_list_composition(datafiles, filename,
     assert_provenance(prov_file, prov_line, prov_col, child, 'mood')
 
 
+# Test that overwriting a list with an empty list works as expected.
+@pytest.mark.datafiles(os.path.join(DATA_DIR))
+def test_list_deletion(datafiles):
+    base = os.path.join(datafiles.dirname, datafiles.basename, 'basics.yaml')
+    overlay = os.path.join(datafiles.dirname, datafiles.basename, 'listoverwriteempty.yaml')
+
+    base = _yaml.load(base, shortname='basics.yaml')
+    overlay = _yaml.load(overlay, shortname='listoverwriteempty.yaml')
+    _yaml.composite_dict(base, overlay)
+
+    children = _yaml.node_get(base, list, 'children')
+    assert len(children) == 0
+
+
 # Tests for deep list composition
 #
 # Same as test_list_composition(), but adds an additional file