12 files changed, 76 insertions, 1 deletions
diff --git a/src/buildstream/_loader/loader.py b/src/buildstream/_loader/loader.py
index 3032f9036..06e3fbfd5 100644
--- a/src/buildstream/_loader/loader.py
+++ b/src/buildstream/_loader/loader.py
@@ -311,6 +311,10 @@ class Loader:
         if filename in self._elements:
             return self._elements[filename]
 
+        if self.project.junction and filename in self.project.junction.replacements:
+            replacement = self.project.junction.replacements.get_str(filename)
+            return self._parent._load_file(replacement, rewritable, ticker, provenance)
+
         # Call the ticker
         if ticker:
             ticker(filename)
@@ -339,6 +343,9 @@ class Loader:
                         dep.junction, rewritable=rewritable, ticker=ticker, provenance=dep.provenance
                     )
                     dep_element = loader._load_file(dep.name, rewritable, ticker, dep.provenance)
+                elif self.project.junction and dep.name in self.project.junction.replacements:
+                    replacement = self.project.junction.replacements.get_str(dep.name)
+                    dep_element = self._parent._load_file(replacement, rewritable, ticker, dep.provenance)
                 else:
                     dep_element = self._elements.get(dep.name)
 
diff --git a/src/buildstream/plugins/elements/junction.py b/src/buildstream/plugins/elements/junction.py
index 86d1de8f8..f450d67f1 100644
--- a/src/buildstream/plugins/elements/junction.py
+++ b/src/buildstream/plugins/elements/junction.py
@@ -173,7 +173,9 @@ class JunctionElement(Element):
 
     def configure(self, node):
 
-        node.validate_keys(["path", "options", "target", "cache-junction-elements", "ignore-junction-remotes"])
+        node.validate_keys(
+            ["path", "options", "target", "cache-junction-elements", "ignore-junction-remotes", "replacements"]
+        )
 
         self.path = node.get_str("path", default="")
         self.options = node.get_mapping("options", default={})
@@ -182,6 +184,7 @@ class JunctionElement(Element):
         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)
+        self.replacements = node.get_mapping("replacements", default={})
 
     def preflight(self):
         # "target" cannot be used in conjunction with:
diff --git a/tests/format/junctions.py b/tests/format/junctions.py
index 70572ee3e..500d3f628 100644
--- a/tests/format/junctions.py
+++ b/tests/format/junctions.py
@@ -471,3 +471,35 @@ def test_junction_show(cli, tmpdir, datafiles):
 
     # Show, assert that it says junction
     assert cli.get_element_state(project, "base.bst") == "junction"
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_replacements(cli, tmpdir, datafiles):
+    project = os.path.join(str(datafiles), "replacements")
+    copy_subprojects(project, datafiles, ["replacements-base"])
+    checkoutdir = os.path.join(str(tmpdir), "checkout")
+
+    # Build, checkout
+    result = cli.run(project=project, args=["build", "stack.bst"])
+    result.assert_success()
+    result = cli.run(project=project, args=["artifact", "checkout", "stack.bst", "--directory", checkoutdir])
+    result.assert_success()
+
+    assert os.path.exists(os.path.join(checkoutdir, "destination/injected.txt"))
+    assert not os.path.exists(os.path.join(checkoutdir, "original.txt"))
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_replacements_top_element(cli, tmpdir, datafiles):
+    project = os.path.join(str(datafiles), "replacements")
+    copy_subprojects(project, datafiles, ["replacements-base"])
+    checkoutdir = os.path.join(str(tmpdir), "checkout")
+
+    # Build, checkout
+    result = cli.run(project=project, args=["build", "base.bst:a.bst"])
+    result.assert_success()
+    result = cli.run(project=project, args=["artifact", "checkout", "base.bst:a.bst", "--directory", checkoutdir])
+    result.assert_success()
+
+    assert os.path.exists(os.path.join(checkoutdir, "destination/injected.txt"))
+    assert not os.path.exists(os.path.join(checkoutdir, "original.txt"))
diff --git a/tests/format/junctions/replacements-base/a.bst b/tests/format/junctions/replacements-base/a.bst
new file mode 100644
index 000000000..61edc6467
--- /dev/null
+++ b/tests/format/junctions/replacements-base/a.bst
@@ -0,0 +1,4 @@
+kind: import
+sources:
+- kind: local
+  path: original.txt
diff --git a/tests/format/junctions/replacements-base/compose.bst b/tests/format/junctions/replacements-base/compose.bst
new file mode 100644
index 000000000..1b4024228
--- /dev/null
+++ b/tests/format/junctions/replacements-base/compose.bst
@@ -0,0 +1,3 @@
+kind: compose
+build-depends:
+- a.bst
diff --git a/tests/format/junctions/replacements-base/original.txt b/tests/format/junctions/replacements-base/original.txt
new file mode 100644
index 000000000..5762c898d
--- /dev/null
+++ b/tests/format/junctions/replacements-base/original.txt
@@ -0,0 +1 @@
+Original
diff --git a/tests/format/junctions/replacements-base/project.conf b/tests/format/junctions/replacements-base/project.conf
new file mode 100644
index 000000000..9f5c000c9
--- /dev/null
+++ b/tests/format/junctions/replacements-base/project.conf
@@ -0,0 +1,2 @@
+name: replacements-base
+min-version: 2.0
diff --git a/tests/format/junctions/replacements/base.bst b/tests/format/junctions/replacements/base.bst
new file mode 100644
index 000000000..d128ae54d
--- /dev/null
+++ b/tests/format/junctions/replacements/base.bst
@@ -0,0 +1,7 @@
+kind: junction
+config:
+  replacements:
+    a.bst: injected.bst
+sources:
+- kind: local
+  path: replacements-base
diff --git a/tests/format/junctions/replacements/injected.bst b/tests/format/junctions/replacements/injected.bst
new file mode 100644
index 000000000..3f822c711
--- /dev/null
+++ b/tests/format/junctions/replacements/injected.bst
@@ -0,0 +1,6 @@
+kind: import
+config:
+  target: "%{dest}"
+sources:
+- kind: local
+  path: injected.txt
diff --git a/tests/format/junctions/replacements/injected.txt b/tests/format/junctions/replacements/injected.txt
new file mode 100644
index 000000000..43a3eba08
--- /dev/null
+++ b/tests/format/junctions/replacements/injected.txt
@@ -0,0 +1 @@
+Injected
diff --git a/tests/format/junctions/replacements/project.conf b/tests/format/junctions/replacements/project.conf
new file mode 100644
index 000000000..625eae6f2
--- /dev/null
+++ b/tests/format/junctions/replacements/project.conf
@@ -0,0 +1,6 @@
+name: replacements
+min-version: 2.0
+
+variables:
+  # This variable is not defined in the junctioned project
+  dest: "/destination"
diff --git a/tests/format/junctions/replacements/stack.bst b/tests/format/junctions/replacements/stack.bst
new file mode 100644
index 000000000..0ab52e878
--- /dev/null
+++ b/tests/format/junctions/replacements/stack.bst
@@ -0,0 +1,3 @@
+kind: stack
+runtime-depends:
+- base.bst:compose.bst