perl: fix computeFSClosure

we must await the promise. also add a Werror argument to not break this again.

Change-Id: Ia02fee1720c0280853bc299d3d20301370d93e56

perl/lib/Nix/Store.xs

@@ -159,7 +159,9 @@ SV * computeFSClosure(int flipDirection, int includeOutputs, ...)
         try {
             StorePathSet paths;
             for (int n = 2; n < items; ++n)
-                store()->computeFSClosure(store()->parseStorePath(SvPV_nolen(ST(n))), paths, flipDirection, includeOutputs);
+                aio().blockOn(store()->computeFSClosure(
+                    store()->parseStorePath(SvPV_nolen(ST(n))), paths, flipDirection, includeOutputs
+                ));
             for (auto & i : paths)
                 XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(i).c_str(), 0)));
         } catch (Error & e) {

perl/meson.build

@@ -9,6 +9,11 @@ project('lix-perl', 'cpp',
   ],
 )
 
+add_project_arguments(
+  '-Werror=unused-result',
+  language : 'cpp',
+)
+
 fs = import('fs')
 
 prefix = get_option('prefix')