checkpatch: check signoff when reading stdin

Signoff was not checked if the filename is '-', indicating reading the
patch from stdin.  Commands such as the below would not warn about a
missing signoff, because the patch filename is '-'.  This change allows
checkpatch to warn about a missing signoff, even if the input filename
is '-', but only if the patch has a commit message.

  git show --pretty=email | scripts/checkpatch.pl -

A more common use of checkpatch with stdin is for piping git diff
through checkpatch.  The diff output would not contain a commit message,
and therefore it would not contain a signoff line.  For this common use
case, a warning should not be printed about the missing signoff.  With
this change we will only warn about a missing signoff if the input
contains a commit message.

  git diff | scripts/checkpatch.pl -

Before this patch, a workaround for the first command was to refer to
stdin by a name other than '-'.  The workaround is not an elegant
solution, because elsewhere checkpatch uses the fact that filename
equals '-', such as in setting '$vname' to 'Your patch' for stdin.  The
command below would report "/dev/stdin has style problems" instead of
"Your patch has style problems."

  git show --pretty=email | scripts/checkpatch.pl /dev/stdin

Link: http://lkml.kernel.org/r/48be31e414bddc65bccfa6b1322359be9ba032eb.1469670589.git.allenbh@gmail.com
Signed-off-by: Allen Hubbe <allenbh@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2069,6 +2069,7 @@ sub process {
 	my $is_patch = 0;
 	my $in_header_lines = $file ? 0 : 1;
 	my $in_commit_log = 0;		#Scanning lines before patch
+	my $has_commit_log = 0;		#Encountered lines before patch
        my $commit_log_possible_stack_dump = 0;
 	my $commit_log_long_line = 0;
 	my $commit_log_has_diff = 0;
@@ -2566,6 +2567,7 @@ sub process {
 		      $rawline =~ /^(commit\b|from\b|[\w-]+:).*$/i)) {
 			$in_header_lines = 0;
 			$in_commit_log = 1;
+			$has_commit_log = 1;
 		}
 
 # Check if there is UTF-8 in a commit log when a mail header has explicitly
@@ -6055,7 +6057,7 @@ sub process {
 		ERROR("NOT_UNIFIED_DIFF",
 		      "Does not appear to be a unified-diff format patch\n");
 	}
-	if ($is_patch && $filename ne '-' && $chk_signoff && $signoff == 0) {
+	if ($is_patch && $has_commit_log && $chk_signoff && $signoff == 0) {
 		ERROR("MISSING_SIGN_OFF",
 		      "Missing Signed-off-by: line(s)\n");
 	}