_loader.py: Use Source._get_local_path() for subprojects

This eliminates unneeded copies of the project directory for junctions
with a single local source.

1 files changed, 11 insertions, 5 deletions

diff --git a/buildstream/_loader/loader.py b/buildstream/_loader/loader.py
index 22600b9e9..a3c4ea8f0 100644
--- a/buildstream/_loader/loader.py
+++ b/buildstream/_loader/loader.py
@@ -563,17 +563,23 @@ class Loader():
                                 "Subproject has no ref for junction: {}".format(filename),
                                 detail=detail)
 
-        # Stage sources
-        os.makedirs(self._context.builddir, exist_ok=True)
-        basedir = tempfile.mkdtemp(prefix="{}-".format(element.normal_name), dir=self._context.builddir)
-        element._stage_sources_at(basedir, mount_workspaces=False)
+        if len(sources) == 1 and sources[0]._get_local_path():
+            # Optimization for junctions with a single local source
+            basedir = sources[0]._get_local_path()
+            tempdir = None
+        else:
+            # Stage sources
+            os.makedirs(self._context.builddir, exist_ok=True)
+            basedir = tempfile.mkdtemp(prefix="{}-".format(element.normal_name), dir=self._context.builddir)
+            element._stage_sources_at(basedir, mount_workspaces=False)
+            tempdir = basedir
 
         # Load the project
         project_dir = os.path.join(basedir, element.path)
         try:
             from .._project import Project
             project = Project(project_dir, self._context, junction=element,
-                              parent_loader=self, tempdir=basedir)
+                              parent_loader=self, tempdir=tempdir)
         except LoadError as e:
             if e.reason == LoadErrorReason.MISSING_PROJECT_CONF:
                 raise LoadError(reason=LoadErrorReason.INVALID_JUNCTION,