feat(ci): add parent_branch.txt, make sure it's on the merge path (#3564

)

scripts/merge_branches.py

@@ -17,7 +17,7 @@
 
 FINAL_BRANCH = "main"
 MERGE_PATHS_FILE = "scripts/merge_paths.json"
-FILES_TO_PRESERVE = {"rust-toolchain.toml"}
+FILES_TO_PRESERVE = {"rust-toolchain.toml", "scripts/parent_branch.txt"}
 
 
 def load_merge_paths() -> Dict[str, str]:

scripts/merge_paths_test.py

@@ -1,19 +1,28 @@
+import pytest
+from typing import Dict
+
 from merge_branches import FINAL_BRANCH, MERGE_PATHS_FILE, load_merge_paths
 
 
-def test_linear_path():
-    merge_paths = load_merge_paths()
+@pytest.fixture
+def parent_branch() -> str:
+    return open("scripts/parent_branch.txt").read().strip()
+
+
+@pytest.fixture
+def merge_paths() -> Dict[str, str]:
+    return load_merge_paths()
 
+
+def test_linear_path(merge_paths: Dict[str, str]):
     src_dst_iter = iter(merge_paths.items())
     (oldest_branch, prev_dst_branch) = next(src_dst_iter)
     assert (
         oldest_branch not in merge_paths.values()
     ), f"Oldest branch '{oldest_branch}' cannot be a destination branch."
 
     for src_branch, dst_branch in src_dst_iter:
-        assert (
-            prev_dst_branch == src_branch
-        ), (
+        assert prev_dst_branch == src_branch, (
             f"Since the merge graph is linear, the source branch '{src_branch}' must be the same "
             f"as the previous destination branch, which is '{prev_dst_branch}'. Check out "
             f"{MERGE_PATHS_FILE}."
@@ -23,3 +32,11 @@ def test_linear_path():
     assert (
         prev_dst_branch == FINAL_BRANCH
     ), f"The last destination is '{prev_dst_branch}' but must be '{FINAL_BRANCH}'."
+
+
+def test_parent_branch_is_on_path(parent_branch: str, merge_paths: Dict[str, str]):
+    known_branches = set(merge_paths.keys()) | set(merge_paths.values())
+    assert parent_branch in known_branches, (
+        f"Parent branch '{parent_branch}' is not on the merge path (branches in merge path: "
+        f"{known_branches})."
+    )

scripts/parent_branch.txt

@@ -0,0 +1 @@
+main