creds: minor cleanups for --newline= handling (#41329)

Prompted by #41325

Fixes #41348

src/basic/cleanup-util.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "assert-util.h"
+#include "assert-fundamental.h"
 #include "cleanup-fundamental.h" /* IWYU pragma: export */
 
 typedef void (*free_func_t)(void *p);

src/creds/creds.c

@@ -398,8 +398,6 @@ static int transcode(
 }
 
 static int print_newline(FILE *f, const char *data, size_t l) {
-        int fd;
-
         assert(f);
         assert(data || l == 0);
 
@@ -411,12 +409,14 @@ static int print_newline(FILE *f, const char *data, size_t l) {
         if (l > 0 && data[l-1] == '\n')
                 return 0;
 
-        /* Don't bother unless this is a tty */
-        fd = fileno(f);
-        if (fd >= 0 && !isatty_safe(fd))
-                return 0;
+        /* If not explicitly requested, don't bother if the output is not a tty */
+        if (arg_newline < 0) {
+                int fd = fileno(f);
+                if (fd >= 0 && !isatty_safe(fd))
+                        return 0;
+        }
 
-        if (fputc('\n', f) != '\n')
+        if (fputc('\n', f) == EOF)
                 return log_error_errno(errno, "Failed to write trailing newline: %m");
 
         return 1;
@@ -940,15 +940,9 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_NEWLINE:
-                        if (isempty(optarg) || streq(optarg, "auto"))
-                                arg_newline = -1;
-                        else {
-                                r = parse_boolean_argument("--newline=", optarg, NULL);
-                                if (r < 0)
-                                        return r;
-
-                                arg_newline = r;
-                        }
+                        r = parse_tristate_argument_with_auto("--newline=", optarg, &arg_newline);
+                        if (r < 0)
+                                return r;
                         break;
 
                 case 'p':