[3.12] pythonGH-133410: Use commit hashes for change detection (pythongh-133416) (python#133426)

pythonGH-133410: Use commit hashes for change detection (pythongh-133416)
(cherry picked from commit d530e74)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: miss-islington

.github/workflows/reusable-context.yml

@@ -97,6 +97,8 @@ jobs:
       run: python Tools/build/compute-changes.py
       env:
         GITHUB_DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
+        CCF_TARGET_REF: ${{ github.base_ref || github.event.repository.default_branch }}
+        CCF_HEAD_REF: ${{ github.event.pull_request.head.sha || github.sha }}
 
     - name: Compute hash for config cache key
       id: config-hash

Tools/build/compute-changes.py

@@ -56,12 +56,10 @@ class Outputs:
 
 
 def compute_changes() -> None:
-    target_branch, head_branch = git_branches()
-    if target_branch and head_branch:
+    target_branch, head_ref = git_refs()
+    if target_branch and head_ref:
         # Getting changed files only makes sense on a pull request
-        files = get_changed_files(
-            f"origin/{target_branch}", f"origin/{head_branch}"
-        )
+        files = get_changed_files(target_branch, head_ref)
         outputs = process_changed_files(files)
     else:
         # Otherwise, just run the tests
@@ -89,15 +87,15 @@ def compute_changes() -> None:
     write_github_output(outputs)
 
 
-def git_branches() -> tuple[str, str]:
-    target_branch = os.environ.get("GITHUB_BASE_REF", "")
-    target_branch = target_branch.removeprefix("refs/heads/")
-    print(f"target branch: {target_branch!r}")
+def git_refs() -> tuple[str, str]:
+    target_ref = os.environ.get("CCF_TARGET_REF", "")
+    target_ref = target_ref.removeprefix("refs/heads/")
+    print(f"target ref: {target_ref!r}")
 
-    head_branch = os.environ.get("GITHUB_HEAD_REF", "")
-    head_branch = head_branch.removeprefix("refs/heads/")
-    print(f"head branch: {head_branch!r}")
-    return target_branch, head_branch
+    head_ref = os.environ.get("CCF_HEAD_REF", "")
+    head_ref = head_ref.removeprefix("refs/heads/")
+    print(f"head ref: {head_ref!r}")
+    return f"origin/{target_ref}", head_ref
 
 
 def get_changed_files(