plugins/elements/junction.py: Add 'ignore-junction-remotes' option

This option allows us to completely ignore remotes defined in the
subproject. As a user, we may not wish to interact with subproject
remotes as we may not be able to trust them, for example.

An appropriate test and some documentation has also been added.

2 files changed, 15 insertions, 4 deletions

diff --git a/src/buildstream/_project.py b/src/buildstream/_project.py
index 3a5923f80..dff101582 100644
--- a/src/buildstream/_project.py
+++ b/src/buildstream/_project.py
@@ -647,9 +647,13 @@ class Project():
 
         # 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:
+        if self.junction:
             parent = self.junction._get_project()
-            self.artifact_cache_specs = parent.artifact_cache_specs + self.artifact_cache_specs
+            if self.junction.cache_junction_elements:
+                self.artifact_cache_specs = parent.artifact_cache_specs + self.artifact_cache_specs
+
+            if self.junction.ignore_junction_remotes:
+                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/plugins/elements/junction.py b/src/buildstream/plugins/elements/junction.py
index 235702b07..aec32516b 100644
--- a/src/buildstream/plugins/elements/junction.py
+++ b/src/buildstream/plugins/elements/junction.py
@@ -59,12 +59,18 @@ Overview
      # should interact with project remotes (default: False).
      cache-junction-elements: False
 
+     # Optionally ignore junction remotes, this means that BuildStream
+     # will not attempt to pull artifacts from the junction project's
+     # remote(s) (default: False).
+     ignore-junction-remotes: False
+
 .. note::
 
    The configuration option to allow specifying junction targets is available
    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>`.
+   options allowing for junction project elements to interact with parent remotes
+   or to completely ignore junction project remotes are available since
+   :ref:`format version 25 <project_format_version>`.
 
 .. note::
 
@@ -187,6 +193,7 @@ class JunctionElement(Element):
         self.target_element = None
         self.target_junction = None
         self.cache_junction_elements = node.get_bool('cache-junction-elements', default=False)
+        self.ignore_junction_remotes = node.get_bool('ignore-junction-remotes', default=False)
 
     def preflight(self):
         # "target" cannot be used in conjunction with: