prevent lock from being moved from one thread to another

arangod/Utils/CollectionExport.cpp

@@ -57,6 +57,7 @@ CollectionExport::~CollectionExport() {
     _ditch->ditches()->freeDocumentDitch(_ditch, false);
   }
 
+  // if not already done
   delete _guard;
 }
 
@@ -136,4 +137,14 @@ void CollectionExport::run(uint64_t maxWaitTime, size_t limit) {
 
     trx.finish(res);
   }
+
+  // delete guard right now as we're about to return
+  // if we would continue holding the guard's collection lock and return,
+  // and the export object gets later freed in a different thread, then all
+  // would be lost. so we'll release the lock here and rely on the cleanup
+  // thread not unloading the collection (as we've acquired a document ditch
+  // for the collection already - this will prevent unloading of the collection's
+  // datafiles etc.)
+  delete _guard;
+  _guard = nullptr;
 }