split: Improve GNU compatibility.

* Like GNU split, turn autoextend back on if given -a0.
* Add a test case that verifies that -a<non-zero> turns autoextend off.
* Add a test case that verifies that -a0 turns autoextend back on.

Sponsored by:	Klara, Inc.
Reviewed by:	christos, kevans
Differential Revision:	https://reviews.freebsd.org/D42011

usr.bin/split/split.c

@@ -115,7 +115,12 @@ main(int argc, char **argv)
 				errx(EX_USAGE, "%s: suffix length is %s",
 				    optarg, errstr);
 			}
-			autosfx = false;
+			if (sufflen == 0) {
+				sufflen = 2;
+				autosfx = true;
+			} else {
+				autosfx = false;
+			}
 			break;
 		case 'b':		/* Byte count. */
 			if (expand_number(optarg, &bytecnt) != 0) {

usr.bin/split/tests/split_test.sh

@@ -209,6 +209,23 @@ autoextend_body()
 	atf_check -o inline:"$((26*25+1))\n" cat xzaaa
 }
 
+atf_test_case noautoextend
+noautoextend_body()
+{
+	seq $((26*26)) >input
+	atf_check split -a2 -l1 input
+	atf_check -o inline:"$((26*26))\n" cat xzz
+}
+
+atf_test_case reautoextend
+reautoextend_body()
+{
+	seq $((26*25+1)) >input
+	atf_check split -a2 -a0 -l1 input
+	atf_check -o inline:"$((26*25))\n" cat xyz
+	atf_check -o inline:"$((26*25+1))\n" cat xzaaa
+}
+
 atf_test_case continue
 continue_body()
 {
@@ -249,6 +266,8 @@ atf_init_test_cases()
 	atf_add_test_case larger_suffix_length
 	atf_add_test_case pattern
 	atf_add_test_case autoextend
+	atf_add_test_case noautoextend
+	atf_add_test_case reautoextend
 	atf_add_test_case continue
 	atf_add_test_case undocumented_kludge
 	atf_add_test_case duplicate_linecount