Create and store data inside projects when opening workspaces

Changes to _context.py:
* Context has been extended to contain a WorkspaceProjectCache, as there
  are times when we want to use it before a Workspaces can be initialised
  (looking up a WorkspaceProject to find the directory that the project is
  in)

Changes to _stream.py:
* Removed staging the elements from workspace_open() and workspace_reset()

Changes in _workspaces.py:
* A new WorkspaceProject contains all the information needed to refer back
  to a project from its workspace (currently this is the project path and
  the element used to create this workspace)
* This is stored within a new WorkspaceProjectCache object, which keeps
  WorkspaceProjects around so they don't need to be loaded from disk
  repeatedly.
* Workspaces has been extended to contain the WorkspaceProjectCache, and
  will use it when opening and closing workspaces.
* Workspaces.create_workspace has been extended to handle the staging of
  the element into the workspace, in addition to creating the equivalent
  WorkspaceProject file.

This is a part of #222

1 files changed, 13 insertions, 2 deletions

diff --git a/buildstream/_context.py b/buildstream/_context.py
index 7ca60e7aa..55d0fd489 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
+from ._workspaces import Workspaces, WorkspaceProjectCache
 from .plugin import _plugin_lookup
 
 
@@ -140,6 +140,7 @@ class Context():
         self._projects = []
         self._project_overrides = {}
         self._workspaces = None
+        self._workspace_project_cache = WorkspaceProjectCache()
         self._log_handle = None
         self._log_filename = None
         self._cascache = None
@@ -285,7 +286,7 @@ class Context():
     #
     def add_project(self, project):
         if not self._projects:
-            self._workspaces = Workspaces(project)
+            self._workspaces = Workspaces(project, self._workspace_project_cache)
         self._projects.append(project)
 
     # get_projects():
@@ -312,6 +313,16 @@ class Context():
     def get_workspaces(self):
         return self._workspaces
 
+    # get_workspace_project_cache():
+    #
+    # Return the WorkspaceProjectCache object used for this BuildStream invocation
+    #
+    # Returns:
+    #    (WorkspaceProjectCache): The WorkspaceProjectCache object
+    #
+    def get_workspace_project_cache(self):
+        return self._workspace_project_cache
+
     # get_overrides():
     #
     # Fetch the override dictionary for the active project. This returns