complete.py: Ensure paths get completed from 'element-path'

Element paths should always be completed from the root element folder
defined by the element-path key in project.conf. Fix complete_path() to
always search into its given base_directory argument.

See issue #448

buildstream/_frontend/complete.py

@@ -68,9 +68,10 @@ def complete_path(path_type, incomplete, base_directory='.'):
         # If there was nothing on the left of the last separator,
         # we are completing files in the filesystem root
         base_path = os.path.join(base_directory, base_path)
-
-    elif os.path.isdir(incomplete):
-        base_path = incomplete
+    else:
+        incomplete_base_path = os.path.join(base_directory, incomplete)
+        if os.path.isdir(incomplete_base_path):
+            base_path = incomplete_base_path
 
     try:
         if base_path:

tests/completions/completions.py

@@ -212,6 +212,10 @@ def test_option_directory(datafiles, cli, cmd, word_idx, expected, subdir):
     # Also try multi arguments together
     ('no-element-path', 'bst --directory ../ checkout t ', 4, ['target.bst '], 'files'),
     ('no-element-path', 'bst --directory ../ checkout target.bst ', 5, ['bin-files/', 'dev-files/'], 'files'),
+
+    # When element-path have sub-folders
+    ('sub-folders', 'bst show base', 2, ['base/wanted.bst '], None),
+    ('sub-folders', 'bst show base/', 2, ['base/wanted.bst '], None),
 ])
 def test_argument_element(datafiles, cli, project, cmd, word_idx, expected, subdir):
     cwd = os.path.join(str(datafiles), project)

tests/completions/sub-folders/base/unwanted.bst

+kind: autotools
+description: |
+
+    Not auto-completed element

tests/completions/sub-folders/elements/base.bst

+kind: stack
+description: Base stack
+
+depends:
+- base/wanted.bst

tests/completions/sub-folders/elements/base/wanted.bst

+kind: autotools
+description: |
+
+    Auto-completed element

tests/completions/sub-folders/elements/hello.bst

+kind: autotools
+description: |
+
+    Hello world

tests/completions/sub-folders/project.conf

+# Project config for frontend build test
+name: test
+
+element-path: elements