checkpatch: emit an error when there's a diff in a changelog

People often put diff snippets in changelogs.  This causes problems
when one tries to apply a file containing both the changelog and the
diff because patch(1) tries to apply the diff which it found in the
changelog.

Warn once when what seems to be a diff snippet in the changelog exists.
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -1954,6 +1954,7 @@ sub process {
 	my $in_header_lines = $file ? 0 : 1;
 	my $in_commit_log = 0;		#Scanning lines before patch
 	my $commit_log_long_line = 0;
+	my $commit_log_has_diff = 0;
 	my $reported_maintainer_file = 0;
 	my $non_utf8_charset = 0;
 
@@ -2087,7 +2088,8 @@ sub process {
 		my $rawline = $rawlines[$linenr - 1];
 
 #extract the line range in the file after the patch is applied
-		if ($line=~/^\@\@ -\d+(?:,\d+)? \+(\d+)(,(\d+))? \@\@/) {
+		if (!$in_commit_log &&
+		    $line =~ /^\@\@ -\d+(?:,\d+)? \+(\d+)(,(\d+))? \@\@/) {
 			$is_patch = 1;
 			$first_line = $linenr + 1;
 			$realline=$1-1;
@@ -2181,6 +2183,17 @@ sub process {
 
 		$cnt_lines++ if ($realcnt != 0);
 
+# Check if the commit log has what seems like a diff which can confuse patch
+		if ($in_commit_log && !$commit_log_has_diff &&
+		    (($line =~ m@^\s+diff\b.*a/[\w/]+@ &&
+		      $line =~ m@^\s+diff\b.*a/([\w/]+)\s+b/$1\b@) ||
+		     $line =~ m@^\s*(?:\-\-\-\s+a/|\+\+\+\s+b/)@ ||
+		     $line =~ m/^\s*\@\@ \-\d+,\d+ \+\d+,\d+ \@\@/)) {
+			ERROR("DIFF_IN_COMMIT_MSG",
+			      "Avoid using diff content in the commit message - patch(1) might not work\n" . $herecurr);
+			$commit_log_has_diff = 1;
+		}
+
 # Check for incorrect file permissions
 		if ($line =~ /^new (file )?mode.*[7531]\d{0,2}$/) {
 			my $permhere = $here . "FILE: $realfile\n";