_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>

1 files changed, 6 insertions, 1 deletions

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(