Fix a capi bug: we were passing the wrong value through as passthrough

Most of the time this just gets ignored (its usually the parent module,
and functions don't usually care about that) but here it doesn't.

src/runtime/capi.cpp

@@ -1523,12 +1523,13 @@ Box* BoxedCApiFunction::callInternal(BoxedFunctionBase* func, CallRewriteArgs* r
 
     if (rewrite_args) {
         rewrite_args->arg1->addGuard((intptr_t)arg1);
+        RewriterVar* r_passthrough = rewrite_args->arg1->getAttr(offsetof(BoxedCApiFunction, passthrough));
         rewrite_args->out_rtn
-            = rewrite_args->rewriter->call(true, (void*)capifunc->func, rewrite_args->arg1, rewrite_args->arg2);
+            = rewrite_args->rewriter->call(true, (void*)capifunc->func, r_passthrough, rewrite_args->arg2);
         rewrite_args->rewriter->call(true, (void*)checkAndThrowCAPIException);
         rewrite_args->out_success = true;
     }
-    Box* r = capifunc->func(arg1, arg2);
+    Box* r = capifunc->func(capifunc->passthrough, arg2);
     checkAndThrowCAPIException();
     assert(r);
     return r;

test/tests/select_test.py

@@ -16,8 +16,11 @@ for k in sorted(dir(select)):
 
 p = select.poll()
 f = open('/dev/null')
+p.register(f)
+gc.collect()
+p.unregister(f)
+p.register(f)
 try:
-    p.register(f)
     print p.poll(10)
 finally:
     f.close()