Make specifying elements optional in bst commands

Known issues:
* `bst shell` works, but `bst shell COMMANDS...` doesn't, because click
  has no way of separating optional args from variable-length args.
* `bst checkout` and `bst source-checkout`'s usage strings mark LOCATION
  as an optional argument. Because click gets confused if there's an
  optional argument before a mandatory argument, I had to mark LOCATION
  as optional internally.
* `bst workspace open` makes no sense with element being optional, so
  I skipped it.
* `bst workspace close` will probably need to be revisited when multiple
  projects can own one workspace.
* `bst workspace reset` will happily delete the directory you're
  currently in, requiring you to `cd $PWD` to see the contents of your
  directory.
  I could exclude the top-level directory of the workspace being
  deleted, but it is entirely valid to run workspace commands from deeper
  in the workspace.

This is a part of #222

1 files changed, 15 insertions, 1 deletions

diff --git a/buildstream/_context.py b/buildstream/_context.py
index 9a12b2b27..a3487a794 100644
--- a/buildstream/_context.py
+++ b/buildstream/_context.py
@@ -32,7 +32,7 @@ from ._message import Message, MessageType
 from ._profile import Topics, profile_start, profile_end
 from ._artifactcache import ArtifactCache
 from ._artifactcache.cascache import CASCache
-from ._workspaces import Workspaces, WorkspaceProjectCache
+from ._workspaces import Workspaces, WorkspaceProjectCache, WORKSPACE_PROJECT_FILE
 from .plugin import _plugin_lookup
 
 
@@ -649,6 +649,20 @@ class Context():
             self._cascache = CASCache(self.artifactdir)
         return self._cascache
 
+    # guess_element()
+    #
+    # Attempts to interpret which element the user intended to run commands on
+    #
+    # Returns:
+    #    (str) The name of the element, or None if no element can be guessed
+    def guess_element(self):
+        workspace_project_dir, _ = utils._search_upward_for_files(self._directory, [WORKSPACE_PROJECT_FILE])
+        if workspace_project_dir:
+            workspace_project = self._workspace_project_cache.get(workspace_project_dir)
+            return workspace_project.get_default_element()
+        else:
+            return None
+
 
 # _node_get_option_str()
 #