util: rework fgetxattrat_fake() to use O_PATH

That way, we don't ever open the file, thus leave the atime untouched,
and this works even when unprivileged.

src/test/test-util.c

@@ -26,7 +26,9 @@
 #include <math.h>
 #include <signal.h>
 #include <string.h>
+#include <sys/types.h>
 #include <sys/wait.h>
+#include <sys/xattr.h>
 #include <unistd.h>
 
 #include "conf-parser.h"
@@ -2263,6 +2265,38 @@ static void test_strcmp_ptr(void) {
         assert_se(strcmp_ptr("", "") == 0);
 }
 
+static void test_fgetxattrat_fake(void) {
+        char t[] = "/var/tmp/xattrtestXXXXXX";
+        _cleanup_close_ int fd = -1;
+        const char *x;
+        char v[3] = {};
+        int r;
+
+        assert_se(mkdtemp(t));
+        x = strjoina(t, "/test");
+        assert_se(touch(x) >= 0);
+
+        r = setxattr(x, "user.foo", "bar", 3, 0);
+        if (r < 0 && errno == EOPNOTSUPP) /* no xattrs supported on /var/tmp... */
+                goto cleanup;
+        assert_se(r >= 0);
+
+        fd = open(t, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
+        assert_se(fd >= 0);
+
+        assert_se(fgetxattrat_fake(fd, "test", "user.foo", v, 3, 0) >= 0);
+        assert_se(memcmp(v, "bar", 3) == 0);
+
+        safe_close(fd);
+        fd = open("/", O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
+        assert_se(fd >= 0);
+        assert_se(fgetxattrat_fake(fd, "usr", "user.idontexist", v, 3, 0) == -ENODATA);
+
+cleanup:
+        assert_se(unlink(x) >= 0);
+        assert_se(rmdir(t) >= 0);
+}
+
 int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
@@ -2353,6 +2387,7 @@ int main(int argc, char *argv[]) {
         test_parse_mode();
         test_tempfn();
         test_strcmp_ptr();
+        test_fgetxattrat_fake();
 
         return 0;
 }