fs-util: add access_fd() which is like access() but for fds

Linux doesn't have faccess(), hence let's emulate it. Linux has access()
and faccessat() but neither allows checking the access rights of an fd
passed in directly.

src/test/test-fs-util.c

@@ -315,6 +315,32 @@ static void test_dot_or_dot_dot(void) {
         assert_se(!dot_or_dot_dot("..foo"));
 }
 
+static void test_access_fd(void) {
+        _cleanup_(rmdir_and_freep) char *p = NULL;
+        _cleanup_close_ int fd = -1;
+
+        assert_se(mkdtemp_malloc("/tmp/access-fd.XXXXXX", &p) >= 0);
+
+        fd = open(p, O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+        assert_se(fd >= 0);
+
+        assert_se(access_fd(fd, R_OK) >= 0);
+        assert_se(access_fd(fd, F_OK) >= 0);
+        assert_se(access_fd(fd, W_OK) >= 0);
+
+        assert_se(fchmod(fd, 0000) >= 0);
+
+        assert_se(access_fd(fd, F_OK) >= 0);
+
+        if (geteuid() == 0) {
+                assert_se(access_fd(fd, R_OK) >= 0);
+                assert_se(access_fd(fd, W_OK) >= 0);
+        } else {
+                assert_se(access_fd(fd, R_OK) == -EACCES);
+                assert_se(access_fd(fd, W_OK) == -EACCES);
+        }
+}
+
 int main(int argc, char *argv[]) {
         test_unlink_noerrno();
         test_get_files_in_directory();
@@ -322,6 +348,7 @@ int main(int argc, char *argv[]) {
         test_var_tmp();
         test_chase_symlinks();
         test_dot_or_dot_dot();
+        test_access_fd();
 
         return 0;
 }