Improve --strip-trailing-cr handling:

- Advance ctold for f1 and ctnew for f2
 - ungetc() if the character is unexpected
 - Don't break early when we hit the combination on one side

PR:		230049
Reported by:	maskray <emacsray gmail com>
Reviewed by:	bapt, maskray
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D16451

usr.bin/diff/diffreg.c

@@ -720,19 +720,22 @@ check(FILE *f1, FILE *f2, int flags)
 				}
 				ctold++;
 				ctnew++;
-				if (flags & D_STRIPCR) {
+				if (flags & D_STRIPCR && (c == '\r' || d == '\r')) {
 					if (c == '\r') {
 						if ((c = getc(f1)) == '\n') {
-							ctnew++;
-							break;
+							ctold++;
+						} else {
+							ungetc(c, f1);
 						}
 					}
 					if (d == '\r') {
 						if ((d = getc(f2)) == '\n') {
-							ctold++;
-							break;
+							ctnew++;
+						} else {
+							ungetc(d, f2);
 						}
 					}
+					break;
 				}
 				if ((flags & D_FOLDBLANKS) && isspace(c) &&
 				    isspace(d)) {

usr.bin/diff/tests/diff_test.sh

@@ -8,6 +8,7 @@ atf_test_case ifdef
 atf_test_case group_format
 atf_test_case side_by_side
 atf_test_case brief_format
+atf_test_case b230049
 
 simple_body()
 {
@@ -52,6 +53,15 @@ unified_body()
 		diff -u9999 -L input_c1.in -L input_c2.in "$(atf_get_srcdir)/input_c1.in" "$(atf_get_srcdir)/input_c2.in"
 }
 
+b230049_body()
+{
+	printf 'a\nb\r\nc\n' > b230049_a.in
+	printf 'a\r\nb\r\nc\r\n' > b230049_b.in
+	atf_check -o empty -s eq:0 \
+		diff -up --strip-trailing-cr -L b230049_a.in -L b230049_b.in \
+		    b230049_a.in b230049_b.in
+}
+
 header_body()
 {
 	export TZ=UTC
@@ -150,4 +160,5 @@ atf_init_test_cases()
 	atf_add_test_case group_format
 	atf_add_test_case side_by_side
 	atf_add_test_case brief_format
+	atf_add_test_case b230049
 }