import.py: Raise an error for elements that have no sources

Previously the build would start but then fail during assemble() once it
became clear there were no files to import. Now an error is raised right
away:

        Error loading pipeline: import element at gnu-toolchain/base-platform.bst [line 1 column 0]: An import element must have at least one source

The usual cause of this error in my experience is when sources
are chosen using the 'arches' conditional. If the user specifies
an arch that the import element doesn't know about then they
will get see this error which may not be particularly helpful.

1 files changed, 6 insertions, 2 deletions

diff --git a/buildstream/plugins/elements/import.py b/buildstream/plugins/elements/import.py
index 23ba270f7..a6178a179 100644
--- a/buildstream/plugins/elements/import.py
+++ b/buildstream/plugins/elements/import.py
@@ -43,7 +43,11 @@ class ImportElement(BuildElement):
         self.target = self.node_subst_member(node, 'target')
 
     def preflight(self):
-        pass
+        # Assert that we have at least one source to fetch.
+
+        sources = list(self.sources())
+        if not sources:
+            raise ElementError("{}: An import element must have at least one source.".format(self))
 
     def get_unique_key(self):
         return {
@@ -78,7 +82,7 @@ class ImportElement(BuildElement):
         os.makedirs(os.path.dirname(outputdir), exist_ok=True)
 
         if not os.path.exists(inputdir):
-            raise ElementError("{}: No files were found at path '{}'"
+            raise ElementError("{}: No files were found inside directory '{}'"
                                .format(self, self.source))
 
         # Move it over