_yaml.py, source.py: Fix tracking of refs inside conditions

Previously if you had a source of the form:

```yaml
- (?):
    somecondition:
      url: blah
      ref: foo
    othercondition:
      url: blah
      ref: bar
```

And you did `bst source track` on the element then you'd get something
output of the form:
```yaml
- (?)...
  ...
  ref: wibble
```

With this patch, the *correct* ref inside the conditionals is updated
instead.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

2 files changed, 16 insertions, 2 deletions

diff --git a/buildstream/_yaml.py b/buildstream/_yaml.py
index c1297e9dd..cdab4269e 100644
--- a/buildstream/_yaml.py
+++ b/buildstream/_yaml.py
@@ -1204,18 +1204,27 @@ def assert_symbol_name(provenance, symbol_name, purpose, *, allow_dashes=True):
 # This is typically used when trying to walk a path to a given node
 # for the purpose of then modifying a similar tree of objects elsewhere
 #
+# If the key is provided, then we actually hunt for the node represented by
+# target[key] and return its container, rather than hunting for target directly
+#
 # Args:
 #    node (Node): The node at the root of the tree to search
 #    target (Node): The node you are looking for in that tree
+#    key (str): Optional string key within target node
 #
 # Returns:
 #    (list): A path from `node` to `target` or None if `target` is not in the subtree
-def node_find_target(node, target):
+def node_find_target(node, target, *, key=None):
     assert type(node) is Node
     assert type(target) is Node
+    if key is not None:
+        target = target[0][key]
 
     path = []
     if _walk_find_target(node, path, target):
+        if key:
+            # Remove key from end of path
+            path = path[:-1]
         return path
     return None
 
diff --git a/buildstream/source.py b/buildstream/source.py
index daf2cb27c..0c07dd41e 100644
--- a/buildstream/source.py
+++ b/buildstream/source.py
@@ -951,10 +951,15 @@ class Source(Plugin):
                 provenance = _yaml.node_get_provenance(node)
             else:
                 provenance = _yaml.node_get_provenance(node, key=key)
+
             toplevel_node = provenance.toplevel
 
             # Get the path to whatever changed
-            path = _yaml.node_find_target(toplevel_node, node)
+            if action == 'add':
+                path = _yaml.node_find_target(toplevel_node, node)
+            else:
+                path = _yaml.node_find_target(toplevel_node, node, key=key)
+
             roundtrip_file = roundtrip_cache.get(provenance.filename)
             if not roundtrip_file:
                 roundtrip_file = roundtrip_cache[provenance.filename] = _yaml.roundtrip_load(