link element: Adding support for new link element

This element acts as a symbolic link, it has no other configurations
other than to specify the element (or junction) to which it refers to,
either in the local project or in a subproject.

1 files changed, 90 insertions, 0 deletions

diff --git a/src/buildstream/plugins/elements/link.py b/src/buildstream/plugins/elements/link.py
new file mode 100644
index 000000000..611108241
--- /dev/null
+++ b/src/buildstream/plugins/elements/link.py
@@ -0,0 +1,90 @@
+#
+#  Copyright (C) 2020 Codethink Limited
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library. If not, see <http://www.gnu.org/licenses/>.
+#
+#  Authors:
+#        Tristan van Berkom <tristan.vanberkom@codethink.co.uk>
+
+"""
+link - Link elements
+================================
+This element is a link to another element, allowing one to create
+a symbolic element which will be resolved to another element.
+
+
+Overview
+--------
+The only configuration allowed in a ``link`` element is the specified
+target of the link.
+
+.. code:: yaml
+
+   kind: link
+
+   config:
+     target: element.bst
+
+The ``link`` element can be used to refer to elements in subprojects, and
+can be used to symbolically link junctions as well as other elements.
+"""
+
+from buildstream import Element
+
+
+# Element implementation for the 'link' kind.
+class LinkElement(Element):
+    # pylint: disable=attribute-defined-outside-init
+
+    BST_MIN_VERSION = "2.0"
+
+    # Links are not allowed any dependencies or sources
+    BST_FORBID_BDEPENDS = True
+    BST_FORBID_RDEPENDS = True
+    BST_FORBID_SOURCES = True
+
+    def configure(self, node):
+
+        node.validate_keys(["target"])
+
+        # Hold onto the provenance of the specified target,
+        # allowing the loader to raise errors with better context.
+        #
+        target_node = node.get_scalar("target")
+        self.target = target_node.as_str()
+        self.target_provenance = target_node.get_provenance()
+
+    def preflight(self):
+        pass
+
+    def get_unique_key(self):
+        # This is only used early on but later discarded
+        return 1
+
+    def configure_sandbox(self, sandbox):
+        assert False, "link elements should be discarded at load time"
+
+    def stage(self, sandbox):
+        assert False, "link elements should be discarded at load time"
+
+    def generate_script(self):
+        assert False, "link elements should be discarded at load time"
+
+    def assemble(self, sandbox):
+        assert False, "link elements should be discarded at load time"
+
+
+# Plugin entry point
+def setup():
+    return LinkElement