[PATCH] checkpatch: warn on unhandled placeholders in cover letters

["Onur Özkan" <work@onurozkan.dev>]

Adds a new check to detect unhandled placeholders in cover letters.
This prevents sending patch series with incomplete cover letters
containing auto generated subject or blurb lines such as:

    *** SUBJECT HERE ***
    *** BLURB HERE ***

These placeholders can be seen on mailing lists (e.g., searching
for "BLURB HERE" on lore.kernel.org). With this patch, checkpatch
will emit a warning when such text is found.

Example output on an invalid cover letter:

    WARNING: Incomplete cover letter: placeholder text detected
    #4: FILE: ./0000-cover-letter.patch:4:
    +Subject: [PATCH 0/4] *** SUBJECT HERE ***

    WARNING: Incomplete cover letter: placeholder text detected
    #9: FILE: ./0000-cover-letter.patch:9:
    +*** BLURB HERE ***

    total: 0 errors, 2 warnings, 24 lines checked

    NOTE: For some of the reported defects, checkpatch may be able to
	  mechanically convert to the typical style using --fix or --fix-inplace.

    ./0000-cover-letter.patch has style problems, please review.

    NOTE: If any of the errors are false positives, please report
	  them to the maintainer, see CHECKPATCH in MAINTAINERS.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
---
 scripts/checkpatch.pl | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index e722dd6fa8ef..9d5ded376112 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3339,6 +3339,15 @@ sub process {
 				$fixed[$fixlinenr] =~ s/^/ /;
 			}
 		}
+# Check for unhandled placeholder text in cover letters
+		if ($filename =~ /cover-letter\.patch$/) {
+			if ($rawline =~ /^\+Subject:.*\*\*\* SUBJECT HERE \*\*\*/ ||
+				$rawline =~ /^\+\*\*\* BLURB HERE \*\*\*/) {
+				my $placeholder = $1 || $2;
+				WARN("COVER_LETTER_PLACEHOLDER",
+					 "Incomplete cover letter: placeholder text detected\n" . $herecurr);
+			}
+		}
 
 # Check for git id commit length and improperly formed commit descriptions
 # A correctly formed commit description is:
-- 
2.51.0