danger: check backport links to the original MR

When doing archeology, it is much easier to find stuff if it's properly
linked. This check ensures that backport MR are linked to their original
MR via a "Backport of !XXXX" message.

The regular expression is fairly broad and has been tested to accept the
following variants of the message:
Backport of MR !XXXX
Backport of: !XXXX
backport of mr !XXXX
Backport of   !XXXX
Backport of https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/XXXX

(cherry picked from commit 12e0b05738cbd456c8c691fd96364b1ac8b6b259)

diff --git a/dangerfile.py b/dangerfile.py
index 46a8baee5c4e10aee56f7fa5b2866a66b4cf1a18..a5fb883c8f377bb2809f88d45b0e703267b971e0 100644 (file)
--- a/dangerfile.py
+++ b/dangerfile.py
@@ -129,7 +129,13 @@ if not danger.gitlab.mr.milestone:
 #   backporting preferences.)
 #
 # * The Backport MR doesn't have target branch in the merge request title.
+#
+# * The Backport MR doesn't link to the original MR is its description.
 
+BACKPORT_OF_RE = re.compile(
+    r"Backport\s+of.*(merge_requests/|!)([0-9]+)", flags=re.IGNORECASE
+)
+backport_desc = BACKPORT_OF_RE.search(danger.gitlab.mr.description)
 version_labels = [l for l in mr_labels if l.startswith("v9.")]
 if is_backport:
     if len(version_labels) != 1:
@@ -144,6 +150,11 @@ if is_backport:
                 "Backport MRs must have their target branch in the "
                 f"title. Please put `{mr_title_version}` in the MR title."
             )
+    if backport_desc is None:
+        fail(
+            "Backport MRs must link to the original MR. Please put "
+            "`Backport of MR !XXXX` in the MR description."
+        )
 if not is_backport and not version_labels:
     fail(
         "If this merge request is a backport, set the *Backport* label and "