errno-util: avoid double evaluation in STRERROR_OR_EOF()

Follow-up for f69ae8585f.

src/basic/errno-util.c

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "errno-util.h"
+
+const char* strerror_or_eof(int errnum, char *buf, size_t buflen) {
+        if (errnum != 0)
+                return strerror_r(ABS(errnum), buf, buflen);
+
+        return "Unexpected EOF";
+}

src/basic/errno-util.h

@@ -15,10 +15,9 @@
  * Note that we use the GNU variant of strerror_r() here. */
 #define STRERROR(errnum) strerror_r(ABS(errnum), (char[ERRNO_BUF_LEN]){}, ERRNO_BUF_LEN)
 
-/* A helper to print an error message or message for functions that return 0 on EOF.
- * Note that we can't use ({ … }) to define a temporary variable, so errnum is
- * evaluated twice. */
-#define STRERROR_OR_EOF(errnum) ((errnum) != 0 ? STRERROR(errnum) : "Unexpected EOF")
+/* A helper to print an error message or message for functions that return 0 on EOF. */
+const char* strerror_or_eof(int errnum, char *buf, size_t buflen);
+#define STRERROR_OR_EOF(errnum) strerror_or_eof(errnum, (char[ERRNO_BUF_LEN]){}, ERRNO_BUF_LEN)
 
 static inline void _reset_errno_(int *saved_errno) {
         if (*saved_errno < 0) /* Invalidated by UNPROTECT_ERRNO? */

src/basic/meson.build

@@ -31,6 +31,7 @@ basic_sources = files(
         'env-file.c',
         'env-util.c',
         'errno-list.c',
+        'errno-util.c',
         'escape.c',
         'ether-addr-util.c',
         'extract-word.c',