Teach "diff --check" about new blank lines at end

When a patch adds new blank lines at the end, "git apply --whitespace"
warns.  This teaches "diff --check" to do the same.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -823,6 +823,7 @@ extern unsigned ws_check(const char *line, int len, unsigned ws_rule);
 extern void ws_check_emit(const char *line, int len, unsigned ws_rule, FILE *stream, const char *set, const char *reset, const char *ws);
 extern char *whitespace_error_string(unsigned ws);
 extern int ws_fix_copy(char *, const char *, int, unsigned, int *);
+extern int ws_blank_line(const char *line, int len, unsigned ws_rule);
 
 /* ls-files */
 int pathspec_match(const char **spec, char *matched, const char *filename, int skiplen);

diff.c

@@ -1140,6 +1140,7 @@ struct checkdiff_t {
 	struct diff_options *o;
 	unsigned ws_rule;
 	unsigned status;
+	int trailing_blanks_start;
 };
 
 static void checkdiff_consume(void *priv, char *line, unsigned long len)
@@ -1154,6 +1155,10 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 	if (line[0] == '+') {
 		unsigned bad;
 		data->lineno++;
+		if (!ws_blank_line(line + 1, len - 1, data->ws_rule))
+			data->trailing_blanks_start = 0;
+		else if (!data->trailing_blanks_start)
+			data->trailing_blanks_start = data->lineno;
 		bad = ws_check(line + 1, len - 1, data->ws_rule);
 		if (!bad)
 			return;
@@ -1165,14 +1170,16 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 		emit_line(data->o->file, set, reset, line, 1);
 		ws_check_emit(line + 1, len - 1, data->ws_rule,
 			      data->o->file, set, reset, ws);
-	} else if (line[0] == ' ')
+	} else if (line[0] == ' ') {
 		data->lineno++;
-	else if (line[0] == '@') {
+		data->trailing_blanks_start = 0;
+	} else if (line[0] == '@') {
 		char *plus = strchr(line, '+');
 		if (plus)
 			data->lineno = strtol(plus, NULL, 10) - 1;
 		else
 			die("invalid diff");
+		data->trailing_blanks_start = 0;
 	}
 }
 
@@ -1584,6 +1591,12 @@ static void builtin_checkdiff(const char *name_a, const char *name_b,
 		ecb.outf = xdiff_outf;
 		ecb.priv = &data;
 		xdi_diff(&mf1, &mf2, &xpp, &xecfg, &ecb);
+
+		if (data.trailing_blanks_start) {
+			fprintf(o->file, "%s:%d: ends with blank lines.\n",
+				data.filename, data.trailing_blanks_start);
+			data.status = 1; /* report errors */
+		}
 	}
  free_and_return:
 	diff_free_filespec_data(one);

t/t4015-diff-whitespace.sh

@@ -335,4 +335,10 @@ test_expect_success 'line numbers in --check output are correct' '
 
 '
 
+test_expect_success 'checkdiff detects trailing blank lines' '
+	echo "foo();" >x &&
+	echo "" >>x &&
+	git diff --check | grep "ends with blank"
+'
+
 test_done

ws.c

@@ -225,6 +225,21 @@ unsigned ws_check(const char *line, int len, unsigned ws_rule)
 	return ws_check_emit_1(line, len, ws_rule, NULL, NULL, NULL, NULL);
 }
 
+int ws_blank_line(const char *line, int len, unsigned ws_rule)
+{
+	/*
+	 * We _might_ want to treat CR differently from other
+	 * whitespace characters when ws_rule has WS_CR_AT_EOL, but
+	 * for now we just use this stupid definition.
+	 */
+	while (len-- > 0) {
+		if (!isspace(*line))
+			return 0;
+		line++;
+	}
+	return 1;
+}
+
 /* Copy the line to the buffer while fixing whitespaces */
 int ws_fix_copy(char *dst, const char *src, int len, unsigned ws_rule, int *error_count)
 {