use a mutex to prevent thread conflicts

src/engine/Operation.h

@@ -65,8 +65,7 @@ class Operation {
   // Add a warning to the `Operation`. The warning will be returned by
   // `collectWarnings()` above.
   void addWarning(std::string warning) const {
-    // TODO<ullingerc>
-    // warnings_.wlock()->push_back(std::move(warning));
+    std::lock_guard l{warningsMutex_};
     warnings_.push_back(std::move(warning));
   }
 
@@ -256,9 +255,12 @@ class Operation {
   [[nodiscard]] virtual vector<ColumnIndex> resultSortedOn() const = 0;
 
   /// interface to the generated warnings of this operation
-  // TODO<ullingerc>
-  std::vector<std::string>& getWarnings() { return warnings_; }
+  std::vector<std::string>& getWarnings() {
+    std::lock_guard l{warningsMutex_};
+    return warnings_;
+  }
   [[nodiscard]] const std::vector<std::string>& getWarnings() const {
+    std::lock_guard l{warningsMutex_};
     return warnings_;
   }
 
@@ -386,11 +388,11 @@ class Operation {
   std::shared_ptr<const RuntimeInformation> _rootRuntimeInfo = _runtimeInfo;
   RuntimeInformationWholeQuery _runtimeInfoWholeQuery;
 
+  // Mutex that protects the `warnings_` below.
+  mutable ad_utility::CopyableMutex warningsMutex_;
   // Collect all the warnings that were created during the creation or
   // execution of this operation. This attribute is declared mutable in order to
   // allow const-functions in subclasses of Operation to add warnings.
-  // TODO<ullingerc> use a
-  // ad_utility::Synchronized<std::vector<std::string>, std::shared_mutex>?
   mutable std::vector<std::string> warnings_;
 
   // The limit from a SPARQL `LIMIT` clause.