presubmit: Require bug in LOW_COVERAGE_REASON

Enforce having a bug listed in the LOW_COVERAGE_REASON to track the
issue that impedes coverage for that specific CL. This encourages
developers to go back and fix those problems. Also update the
documentation.

BUG=None
TEST=util/check_low_coverage_reason.py

Change-Id: If35f918b11771d10944e0b6e7a4aeeba84e05eaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/4537010
Reviewed-by: Jeremy Bettis <jbettis@chromium.org>
Commit-Queue: Jeremy Bettis <jbettis@chromium.org>
Auto-Submit: Tristan Honscheid <honscheid@google.com>
Tested-by: Tristan Honscheid <honscheid@google.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/util/check_low_coverage_reason.py b/util/check_low_coverage_reason.py
new file mode 100755
index 0000000000..c6119f6a6a
--- /dev/null
+++ b/util/check_low_coverage_reason.py
@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+# Copyright 2023 The ChromiumOS Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Ensure commit messages using LOW_COVERAGE_REASON include a bug."""
+
+import logging
+import pathlib
+import re
+import sys
+
+from chromite.lib import commandline
+from chromite.lib import cros_build_lib
+from chromite.lib import git
+
+
+# Look for LOW_COVERAGE_REASON and then an optional b{:|/}number bug reference.
+LOW_COV_REGEX = re.compile(
+    r"\s*(LOW_COVERAGE_REASON=)(?:(?!b[/|:][\d]+).)*(b[/|:]([\d]+))?"
+)
+EC_BASE = pathlib.Path(__file__).resolve().parent.parent
+
+
+def main(argv=None):
+    """Check for bug in LOW_COVERAGE_REASON."""
+    parser = commandline.ArgumentParser()
+    parser.add_argument(
+        "commit_id",
+        help="Commit whose message will be checked.",
+    )
+    opts = parser.parse_args(argv)
+
+    if opts.commit_id == "pre-submit":
+        # Only run check if verifying an actual commit.
+        return 0
+
+    try:
+        commit_log = git.Log(EC_BASE, rev=opts.commit_id, max_count=1)
+    except cros_build_lib.RunCommandError as err:
+        logging.error("Unable to query git log: %s", str(err))
+        return 1
+
+    # Search commit message for LOW_COVERAGE_REASON and bug
+    matches = LOW_COV_REGEX.findall(commit_log)
+    if matches and not any({m[2] for m in matches}):
+        # We have LOW_COVERAGE_REASON line(s) but none include a bug
+        logging.error(
+            "LOW_COVERAGE_REASON line must include one or more bugs "
+            "tracking the reason for missing coverage"
+        )
+        return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))