logging: keep only log-max-files backup files when rotating

src/ctr_logging.c

@@ -1038,9 +1038,7 @@ static gboolean shift_backup_files(void)
 	close(validation_fd);
 
 	/* Shift existing backups from highest to lowest: .N-1 -> .N, .N-2 -> .N-1, etc. */
-	int loop_start = (opt_log_max_files > 1) ? opt_log_max_files : 2;
-
-	for (int i = loop_start; i >= 2; i--) {
+	for (int i = opt_log_max_files; i >= 2; i--) {
 		_cleanup_free_ char *from = g_strdup_printf("%s.%d", k8s_log_path, i - 1);
 		_cleanup_free_ char *to = g_strdup_printf("%s.%d", k8s_log_path, i);
 

test/02-ctr-logs.bats

@@ -172,3 +172,15 @@ run_conmon_with_log_opts() {
     run cat "$LOG_PATH"
     assert "${output}" =~ "stdout P"
 }
+
+@test "ctr logs: should keep only log-max-files backup files" {
+    # The container's output rotates the 1024-byte log several times over, so
+    # with --log-max-files 1 a backup must exist, and it must be the only one
+    # left: rotation must atomically replace .1, not shift it to .2.
+    run_conmon_with_default_args \
+        --log-path "k8s-file:$LOG_PATH" \
+        --log-rotate --log-max-files 1 --log-size-max 1024
+
+    backups=("$LOG_PATH".*)
+    assert "${backups[*]}" == "$LOG_PATH.1" "only one backup file is left"
+}