validate-pr: relax Launchpad link check; recognize UBUNTU-local commits

Two improvements addressing false-positive failures on tracked-branch PRs:

1. Launchpad bug link: drop the requirement that the URL appear on a
   line prefixed with 'BugLink:' or 'LP:'.  The check now only ensures
   a https://bugs.launchpad.net/... URL exists somewhere in the PR body.
   Avoids false negatives when authors paste the link in prose.

2. UBUNTU-local commits (e.g. 'UBUNTU: [Config] ...', 'UBUNTU: [Packaging] ...',
   'UBUNTU: SAUCE: ...') have no upstream equivalent and previously triggered
   R6 ("not SAUCE/Revert but has no upstream reference trailer").  Add an
   is_ubuntu_local() helper that matches the 'UBUNTU: ' prefix; exempt those
   commits from R6 and R9, label them [UBUNTU] in the digest table, and
   verify only the Signed-off-by trailer is present.

Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>

Author: nirmoy

.github/scripts/validate-pr

@@ -40,6 +40,14 @@ def get_backport_url(message):
 def is_sauce(subject):
     return bool(re.match(r'^NVIDIA:.*SAUCE:', subject))
 
+def is_ubuntu_local(subject):
+    """Ubuntu-local commits with no upstream equivalent.
+
+    Catches 'UBUNTU: SAUCE: ...', 'UBUNTU: [Config] ...',
+    'UBUNTU: [Packaging] ...', 'UBUNTU: Ubuntu-X.X.X-...', etc.
+    """
+    return bool(re.match(r'^UBUNTU:\s', subject))
+
 def is_revert(subject):
     return subject.startswith('Revert "')
 
@@ -336,6 +344,9 @@ def build_digest(commits, repo, upstream_remote=None):
                 # Strip outer Revert "..." wrapper and SAUCE prefix for display
                 inner = re.sub(r'^Revert\s+"', '', subj).rstrip('"')
                 display = _norm_sauce_subject(inner) or inner
+            elif is_ubuntu_local(subj):
+                kind    = 'UBUNTU'
+                display = re.sub(r'^UBUNTU:\s*', '', subj)
             else:
                 kind    = 'SAUCE'
                 display = _norm_sauce_subject(subj) or subj
@@ -460,20 +471,24 @@ def lint_commits(commits):
 
         # Classification for R6/R7/R9/R10
         sauce   = is_sauce(subject)
+        ubuntu  = is_ubuntu_local(subject)
         revert  = is_revert(subject)
 
-        # R9: subject length — exempt SAUCE and Reverts of SAUCE; the mandatory
-        # "NVIDIA: [VR: ]SAUCE:" prefix already consumes 15–20 characters.
+        # R9: subject length — exempt SAUCE, UBUNTU local and Reverts of SAUCE;
+        # the mandatory "NVIDIA: [VR: ]SAUCE:" / "UBUNTU: [Config]" prefix
+        # already consumes 12–20 characters.
         revert_of_sauce = revert and bool(re.match(r'^Revert "NVIDIA:.*SAUCE:', subject))
-        if len(subject) > 72 and not sauce and not revert_of_sauce:
+        if len(subject) > 72 and not sauce and not ubuntu and not revert_of_sauce:
             warnings.append("W: {}: subject {} chars (>72)".format(label, len(subject)))
         cp_sha  = get_cherry_pick_sha(commit.message)
         bp_url  = get_backport_url(commit.message)
 
-        # R6: non-SAUCE, non-Revert commits must have an upstream reference trailer
-        if not sauce and not revert and cp_sha is None and bp_url is None:
+        # R6: non-SAUCE, non-UBUNTU, non-Revert commits must have an upstream
+        # reference trailer.  UBUNTU local commits (e.g. UBUNTU: [Config]) have
+        # no upstream equivalent.
+        if not sauce and not ubuntu and not revert and cp_sha is None and bp_url is None:
             errors.append(
-                "E: {}: not SAUCE/Revert but has no upstream reference trailer"
+                "E: {}: not SAUCE/UBUNTU/Revert but has no upstream reference trailer"
                 " (cherry picked from commit ... or backported from ...)".format(label))
 
         # R7: detect wrong trailer for LKML in-review backports
@@ -508,19 +523,18 @@ def check_pr_metadata(pr_title, pr_body_path, base_branch):
             warnings.append(
                 "W: PR title missing [<branch>] prefix: \"{}\"".format(pr_title[:80]))
 
-    # R3: BugLink required for tracked branches
+    # R3: Launchpad bug link required for tracked branches
     if pr_body_path and base_branch and TRACKED_BRANCH_RE.match(base_branch):
         try:
             body = open(pr_body_path).read()
         except OSError as e:
             errors.append("E: cannot read --pr-body {}: {}".format(pr_body_path, e))
             body = ''
-        buglink_re = re.compile(
-            r'^(?:BugLink:|LP:)\s+https://bugs\.launchpad\.net/', re.MULTILINE)
+        buglink_re = re.compile(r'https://bugs\.launchpad\.net/')
         if not buglink_re.search(body):
             errors.append(
-                "E: PR targets {} but body has no 'BugLink:' or 'LP:'"
-                " https://bugs.launchpad.net/... line".format(base_branch))
+                "E: PR targets {} but body has no"
+                " https://bugs.launchpad.net/... link".format(base_branch))
 
     return warnings, errors