fix race on select scan (#4216)

Author: ivanmorozov333

ydb/core/testlib/common_helper.cpp

@@ -60,7 +60,9 @@ void THelper::StartScanRequest(const TString& request, const bool expectSuccess,
     });
     const TInstant start = TInstant::Now();
     while (!resultReady && start + TDuration::Seconds(60) > TInstant::Now()) {
+        Cerr << "START_SLEEP" << Endl;
         Server.GetRuntime()->SimulateSleep(TDuration::Seconds(1));
+        Cerr << "FINISHED_SLEEP" << Endl;
         if (scanIterator && !resultReady) {
             scanIterator->ReadNext().Subscribe([&](NThreading::TFuture<NYdb::NTable::TScanQueryPart> streamPartFuture) {
                 NYdb::NTable::TScanQueryPart streamPart = streamPartFuture.GetValueSync();

ydb/core/tx/columnshard/engines/reader/plain_reader/iterator/interval.cpp

@@ -24,10 +24,10 @@ void TFetchingInterval::OnInitResourcesGuard(const std::shared_ptr<NResourceBrok
     AFL_VERIFY(guard);
     AFL_VERIFY(!ResourcesGuard);
     ResourcesGuard = guard;
-    AFL_VERIFY(ReadyGuards.Inc() <= 1);
     for (auto&& i : Sources) {
         i.second->OnInitResourcesGuard(i.second);
     }
+    AFL_VERIFY(ReadyGuards.Inc() <= 1);
     ConstructResult();
 }