Add support for composing strings

src/buildstream/node.pxd

@@ -65,6 +65,7 @@ cdef class MappingNode(Node):
     # Protected Methods
     cdef void _compose_on_composite_dict(self, MappingNode target)
     cdef void _compose_on_list(self, SequenceNode target)
+    cdef void _compose_on_string(self, ScalarNode target)
 
     # Private Methods
     cdef void __composite(self, MappingNode target, list path) except *

src/buildstream/node.pyx

@@ -977,6 +977,9 @@ cdef class MappingNode(Node):
                 if type(target_value) is SequenceNode:
                     # Composite list composes into a list
                     self._compose_on_list(target_value)
+                elif isinstance((<ScalarNode> target_value).value, str):
+                    # Composite list composes into a string
+                    self._compose_on_string(target_value)
                 elif target_value._is_composite_list():
                     # Composite list merges into composite list
                     self._compose_on_composite_dict(target_value)
@@ -1016,6 +1019,26 @@ cdef class MappingNode(Node):
         if suffix is not None:
             target.value.extend(suffix.value)
 
+    # _compose_on_string(target)
+    #
+    # Compose the current node on the given string.
+    #
+    # Args:
+    #   target (.SequenceNode): sequence on which to compose the current composite dict
+    #
+    cdef void _compose_on_string(self, ScalarNode target):
+        cdef ScalarNode clobber = self.value.get("(=)")
+        cdef ScalarNode prefix = self.value.get("(<)")
+        cdef ScalarNode suffix = self.value.get("(>)")
+
+        if clobber is not None:
+            target.value.clear()
+            target.value.extend(clobber.value)
+        if prefix is not None:
+            target.value = prefix.value + target.value
+        if suffix is not None:
+            target.value += suffix.value
+
     # _compose_on_composite_dict(target)
     #
     # Compose the current node on the given composite dict.

tests/internals/yaml.py

@@ -183,6 +183,23 @@ def test_composite_preserve_originals(datafiles):
     # But the original node is not effected by the override.
     assert orig_extra.get_str("old") == "new"
 
+@pytest.mark.datafiles(os.path.join(DATA_DIR))
+def test_composite_strings(datafiles):
+
+    filename = os.path.join(datafiles, "basics.yaml")
+    overlayfile = os.path.join(datafiles, "stringappend.yaml")
+
+    base = _yaml.load(filename, shortname=None)
+    overlay = _yaml.load(overlayfile, shortname=None)
+    base_copy = base.clone()
+    overlay._composite(base_copy)
+
+    copy_extra = base_copy.get_mapping("extra")
+    orig_extra = base.get_mapping("extra")
+
+    assert base_copy.get_str("description") == (
+        "incoming potato(The vehicle of choice for rainbow travel)potato"
+    )
 
 # Tests for list composition
 #

tests/internals/yaml/stringappend.yaml

@@ -0,0 +1,3 @@
+description:
+    (>): ")potato"
+    (<): "incoming potato("