_loader.py: Some fixes in how we load sources for junctions

  o Ensure that we call Source._load_ref(), and consider project.refs this way

  o Ensure that we report a warning in the case that project.refs is in use
    and the loaded junction source has a redundant ref which is going to
    be ignored

  o Report more distinct machine readable errors for failures to load
    junction element sources

  o Handle Consistency.INCONSISTENT and Consistency.RESOLVED separately:

    - The user should be told something different depending on whether
      they need to fetch or whether they need to track.

    - It is never possible to automatically fetch the source in the
      case that the source has no ref to begin with.

This also adjusts the test/loader/junctions.py test to expect the new error

2 files changed, 30 insertions, 3 deletions

diff --git a/buildstream/_loader.py b/buildstream/_loader.py
index 047a09f2e..6d384ae2c 100644
--- a/buildstream/_loader.py
+++ b/buildstream/_loader.py
@@ -25,6 +25,7 @@ import tempfile
 import shutil
 
 from ._exceptions import LoadError, LoadErrorReason
+from ._message import Message, MessageType
 from . import Consistency
 from ._project import Project
 from . import _yaml
@@ -350,16 +351,33 @@ class Loader():
         for meta_source in meta_element.sources:
             source = meta_element.project.create_source(meta_source.kind,
                                                         meta_source)
+            redundant_ref = source._load_ref()
+            if redundant_ref:
+                self._message(MessageType.WARN,
+                              "Ignoring redundant ref in junction element {}".format(element.name))
 
             source._preflight()
 
-            if source.get_consistency() != Consistency.CACHED:
+            # Handle the case where a subproject needs to be fetched
+            #
+            if source.get_consistency() == Consistency.RESOLVED:
                 if self.context._fetch_subprojects:
                     if ticker:
                         ticker(filename, 'Fetching subproject from {} source'.format(meta_source.kind))
                     source.fetch()
                 else:
-                    raise LoadError(LoadErrorReason.MISSING_FILE, "Subproject fetch needed for {}".format(filename))
+                    detail = "Try fetching the project with `bst fetch {}`".format(filename)
+                    raise LoadError(LoadErrorReason.SUBPROJECT_FETCH_NEEDED,
+                                    "Subproject fetch needed for junction: {}".format(filename),
+                                    detail=detail)
+
+            # Handle the case where a subproject has no ref
+            #
+            elif source.get_consistency() == Consistency.INCONSISTENT:
+                detail = "Try tracking the junction element with `bst track {}`".format(filename)
+                raise LoadError(LoadErrorReason.SUBPROJECT_INCONSISTENT,
+                                "Subproject has no ref for junction: {}".format(filename),
+                                detail=detail)
 
             source._stage(basedir)
 
@@ -636,3 +654,12 @@ class Loader():
         if self.tempdir.startswith(self.context.builddir + os.sep):
             if os.path.exists(self.tempdir):
                 shutil.rmtree(self.tempdir)
+
+    # _message()
+    #
+    # Local message propagator
+    #
+    def _message(self, message_type, message, **kwargs):
+        args = dict(kwargs)
+        self.context.message(
+            Message(None, message_type, message, **args))
diff --git a/tests/loader/junctions.py b/tests/loader/junctions.py
index 0634b8aa2..df77e0064 100644
--- a/tests/loader/junctions.py
+++ b/tests/loader/junctions.py
@@ -221,7 +221,7 @@ def test_git_show(cli, tmpdir, datafiles):
     assert result.exit_code != 0
     assert result.exception
     assert isinstance(result.exception, LoadError)
-    assert result.exception.reason == LoadErrorReason.MISSING_FILE
+    assert result.exception.reason == LoadErrorReason.SUBPROJECT_FETCH_NEEDED
 
     # Explicitly fetch subproject
     result = cli.run(project=project, args=['fetch', 'base.bst'])