plugins/elements/junction.py: Add 'cache-junction-elements' option

The 'cache-junction-elements' configuration option for junction's
is a boolean which enables elements from within the junction to
interact with the parent project's remote(s).

A test has been added to enforce this behaviour and the
BST_FORMAT_VERSION has been bumped.

4 files changed, 61 insertions, 2 deletions

diff --git a/src/buildstream/_project.py b/src/buildstream/_project.py
index 8d06c7cef..3a5923f80 100644
--- a/src/buildstream/_project.py
+++ b/src/buildstream/_project.py
@@ -645,6 +645,12 @@ class Project():
         # Load artifacts pull/push configuration for this project
         self.artifact_cache_specs = ArtifactCache.specs_from_config_node(config, self.directory)
 
+        # If there is a junction Element which specifies that we want to remotely cache
+        # its elements, append the junction's remotes to the artifact cache specs list
+        if self.junction and self.junction.cache_junction_elements:
+            parent = self.junction._get_project()
+            self.artifact_cache_specs = parent.artifact_cache_specs + self.artifact_cache_specs
+
         # Load source caches with pull/push config
         self.source_cache_specs = SourceCache.specs_from_config_node(config, self.directory)
 
diff --git a/src/buildstream/_versions.py b/src/buildstream/_versions.py
index c439f59fb..cbaa52e4f 100644
--- a/src/buildstream/_versions.py
+++ b/src/buildstream/_versions.py
@@ -23,7 +23,7 @@
 # This version is bumped whenever enhancements are made
 # to the `project.conf` format or the core element format.
 #
-BST_FORMAT_VERSION = 24
+BST_FORMAT_VERSION = 25
 
 
 # The base BuildStream artifact version
diff --git a/src/buildstream/plugins/elements/junction.py b/src/buildstream/plugins/elements/junction.py
index b21ef0777..235702b07 100644
--- a/src/buildstream/plugins/elements/junction.py
+++ b/src/buildstream/plugins/elements/junction.py
@@ -55,10 +55,16 @@ Overview
      # Note that this option cannot be used in conjunction with sources.
      target: sub-project.bst:sub-sub-project.bst
 
+     # Optionally declare whether elements within the junction project
+     # should interact with project remotes (default: False).
+     cache-junction-elements: False
+
 .. note::
 
    The configuration option to allow specifying junction targets is available
-   since :ref:`format version 24 <project_format_version>`.
+   since :ref:`format version 24 <project_format_version>` and the configuration
+   option allowing junction project elements to interact with parent remotes is
+   available since :ref:`format version 25 <project_format_version>`.
 
 .. note::
 
@@ -180,6 +186,7 @@ class JunctionElement(Element):
         self.target = node.get_str('target', default=None)
         self.target_element = None
         self.target_junction = None
+        self.cache_junction_elements = node.get_bool('cache-junction-elements', default=False)
 
     def preflight(self):
         # "target" cannot be used in conjunction with:
diff --git a/tests/artifactcache/junctions.py b/tests/artifactcache/junctions.py
index c93d79c4b..bfddcedac 100644
--- a/tests/artifactcache/junctions.py
+++ b/tests/artifactcache/junctions.py
@@ -87,3 +87,49 @@ def test_push_pull(cli, tmpdir, datafiles):
         assert state == 'cached'
         state = cli.get_element_state(base_project, 'base-element.bst')
         assert state == 'cached'
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_caching_junction_elements(cli, tmpdir, datafiles):
+    project = os.path.join(str(datafiles), 'parent')
+    base_project = os.path.join(str(project), 'base')
+
+    # Load the junction element
+    junction_element = os.path.join(project, 'base.bst')
+    junction_data = _yaml.roundtrip_load(junction_element)
+
+    # Add the "cache-junction-elements" boolean to the junction Element
+    junction_data['config'] = {"cache-junction-elements": True}
+    _yaml.roundtrip_dump(junction_data, junction_element)
+
+    with create_artifact_share(os.path.join(str(tmpdir), 'artifactshare-parent')) as share,\
+        create_artifact_share(os.path.join(str(tmpdir), 'artifactshare-base')) as base_share:
+
+        # First build it without the artifact cache configured
+        result = cli.run(project=project, args=['build', 'target.bst'])
+        assert result.exit_code == 0
+
+        # Assert that we are now cached locally
+        state = cli.get_element_state(project, 'target.bst')
+        assert state == 'cached'
+        state = cli.get_element_state(base_project, 'base-element.bst')
+        assert state == 'cached'
+
+        project_set_artifacts(project, share.repo)
+        project_set_artifacts(base_project, base_share.repo)
+
+        # Now try bst artifact push
+        result = cli.run(project=project, args=['artifact', 'push', '--deps', 'all', 'target.bst'])
+        assert result.exit_code == 0
+
+        # And finally assert that the artifacts are in the right shares
+        #
+        # The parent project's cache should *also* contain elements from the junction
+        assert_shared(cli, share, project, 'target.bst', project_name='parent')
+        assert_shared(cli, share, project, 'app.bst', project_name='parent')
+        assert_shared(cli, share, base_project, 'base-element.bst', project_name='base')
+
+        # The junction project's cache should only contain elements in the junction project
+        assert_not_shared(cli, base_share, project, 'target.bst', project_name='parent')
+        assert_not_shared(cli, base_share, project, 'app.bst', project_name='parent')
+        assert_shared(cli, base_share, base_project, 'base-element.bst', project_name='base')