[PATCH v3 1/1] checkpatch: don't suggest else is not useful with chained if blocks

[Ilpo Järvinen]

When if blocks are chained with "else if" like this:

	if (a)
		c = 0;
	else if (b)
		break;
	else
		c = 1;

checkpatch recommends removing else:

WARNING: else is not generally useful after a break or return

Removing else will easily introduce logic errors in this situation
so it's better to check if the preceding line has "else if" before
issuing that warning.

Fixes: 032a4c0f9a77 ("checkpatch: warn on unnecessary else after return or break")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
---

v2:
- Don't match spuriously to removed lines
- Extend check backwards to the first line with the same or lower indent 
  as "else" line

v3:
- Add back the check to ensure line is defined

 scripts/checkpatch.pl | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index b01c36a15d9d..3ea73155c42b 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -4037,14 +4037,24 @@ sub process {
 # check indentation of any line with a bare else
 # (but not if it is a multiple line "if (foo) return bar; else return baz;")
 # if the previous line is a break or return and is indented 1 tab more...
+# but don't warn when there is "else if" before that line to avoid logic errors
 		if ($sline =~ /^\+([\t]+)(?:}[ \t]*)?else(?:[ \t]*{)?\s*$/) {
 			my $tabs = length($1) + 1;
 			if ($prevline =~ /^\+\t{$tabs,$tabs}break\b/ ||
 			    ($prevline =~ /^\+\t{$tabs,$tabs}return\b/ &&
 			     defined $lines[$linenr] &&
 			     $lines[$linenr] !~ /^[ \+]\t{$tabs,$tabs}return/)) {
-				WARN("UNNECESSARY_ELSE",
-				     "else is not generally useful after a break or return\n" . $hereprev);
+				my $ifline = $linenr - 3;
+				$tabs--;
+				while ($ifline >= 1 &&
+				       $lines[$ifline] !~ /^[^-]\t{0,$tabs}[^\t]/) {
+					$ifline--;
+				}
+				if (!defined $lines[$ifline] ||
+				    $lines[$ifline] !~ /^[\+ ]\t{$tabs,$tabs}(\} *|)else if\s*\(/) {
+					WARN("UNNECESSARY_ELSE",
+					     "else is not generally useful after a break or return\n" . $hereprev);
+				}
 			}
 		}
 
-- 
2.30.2