util: split out errno related stuff

src/basic/errno-util.h

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include "macro.h"
+
+static inline void _reset_errno_(int *saved_errno) {
+        if (*saved_errno < 0) /* Invalidated by UNPROTECT_ERRNO? */
+                return;
+
+        errno = *saved_errno;
+}
+
+#define PROTECT_ERRNO                                                   \
+        _cleanup_(_reset_errno_) _unused_ int _saved_errno_ = errno
+
+#define UNPROTECT_ERRNO                         \
+        do {                                    \
+                errno = _saved_errno_;          \
+                _saved_errno_ = -1;             \
+        } while (false)
+
+static inline int negative_errno(void) {
+        /* This helper should be used to shut up gcc if you know 'errno' is
+         * negative. Instead of "return -errno;", use "return negative_errno();"
+         * It will suppress bogus gcc warnings in case it assumes 'errno' might
+         * be 0 and thus the caller's error-handling might not be triggered. */
+        assert_return(errno > 0, -EINVAL);
+        return -errno;
+}