[PATCH 1/2] checkpatch: Add support for Checkpatch-ignore patch footer

[Brendan Jackman <jackmanb@google.com>]

Checkpatch sometimes has false positives. This makes it less useful for
automatic usage: tools like b4 [0] can run checkpatch on all of your
patches and give you a quick overview. When iterating on a branch, it's
tiresome to manually re-check that any errors are known false positives.

This patch adds a feature to record in the commit message that a patch
might produce a certain checkpatch error, and that this is an expected
false positive. Recording this information in the patch itself can also
highlight it to reviewers, so they can make a judgment as to whether
it's appropriate to ignore.

To avoid significant reworks to the Perl code, this is implemented by
mutating a global variable while processing each patch. (The variable
name refers to a patch as a "file" for consistency with other code).

This feature is immediately adopted for this commit itself, which
falls afoul of EMAIL_SUBJECT due to the word "checkpatch" appearing in
the "Checkpatch-ignore" reference in the title - a good example of a
false positive.

[0] b4 - see "--check" arg
    https://b4.docs.kernel.org/en/latest/contributor/prep.html

Checkpatch-ignore: EMAIL_SUBJECT
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 scripts/checkpatch.pl | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 9eed3683ad76caffbbb2418e5dbea7551d374406..1a2ae442068d870903d46bd2dc63b757609e142d 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -53,7 +53,10 @@ my %debug;
 my %camelcase = ();
 my %use_type = ();
 my @use = ();
+# Error types to ignore during the whole invocation.
 my %ignore_type = ();
+# Error types to be ignored in the present "file" (i.e. patch).
+my %file_ignore_type = ();
 my @ignore = ();
 my $help = 0;
 my $configuration_file = ".checkpatch.conf";
@@ -2329,7 +2332,7 @@ sub show_type {
 
 	return defined $use_type{$type} if (scalar keys %use_type > 0);
 
-	return !defined $ignore_type{$type};
+	return !defined $ignore_type{$type} && !defined $file_ignore_type{$type};
 }
 
 sub report {
@@ -2701,6 +2704,8 @@ sub process {
 
 	my $checklicenseline = 1;
 
+	%file_ignore_type = ();
+
 	sanitise_line_reset();
 	my $line;
 	foreach my $rawline (@rawlines) {
@@ -2778,6 +2783,10 @@ sub process {
 		if ($setup_docs && $line =~ /^\+/) {
 			push(@setup_docs, $line);
 		}
+
+		if ($line =~ /^Checkpatch-ignore:\s+(.*)/) {
+			hash_save_array_words(\%file_ignore_type, [$1]);
+		}
 	}
 
 	$prefix = '';

-- 
2.47.1.613.gc27f4b7a9f-goog