DPL Analysis: use unsafe cursor when reserving

There is no point in using the slow boundary checking / buffer resizing cursor when we reserve, since it's reasonable to
expect that the memory will be there.

Author: ktf

Framework/Core/include/Framework/AnalysisHelpers.h

@@ -513,9 +513,12 @@ struct WritingCursor {
 
   /// reserve @a size rows when filling, so that we do not
   /// spend time reallocating the buffers.
+  /// Switches the internal cursor to UnsafeAppend (no capacity check),
+  /// which is safe because we just reserved enough space.
   void reserve(int64_t size)
   {
     mBuilder->reserve(typename persistent_table_t::column_types{}, size);
+    cursor = std::move(FFL(mBuilder->template unsafeCursor<persistent_table_t>()));
   }
 
   void release()

Framework/Core/include/Framework/TableBuilder.h

@@ -715,6 +715,18 @@ class TableBuilder
     };
   }
 
+  /// Same as persist, but uses UnsafeAppend (no capacity check).
+  /// Only safe after reserve() has been called with the correct size.
+  template <typename ARG0, typename... ARGS>
+    requires(sizeof...(ARGS) > 0) || ShouldNotDeconstruct<ARG0>
+  auto unsafePersist()
+  {
+    using FillTuple = std::tuple<typename BuilderMaker<ARG0>::FillType, typename BuilderMaker<ARGS>::FillType...>;
+    return [holders = mHolders](unsigned int /*slot*/, typename BuilderMaker<ARG0>::FillType arg, typename BuilderMaker<ARGS>::FillType... args) -> void {
+      TableBuilderHelpers::unsafeAppend(*(HoldersTupleIndexed<ARG0, ARGS...>*)holders, std::forward_as_tuple(arg, args...));
+    };
+  }
+
   // Same as above, but starting from a o2::soa::Table, which has all the
   // information already available.
   template <typename T>
@@ -725,6 +737,15 @@ class TableBuilder
     }(typename T::table_t::persistent_columns_t{});
   }
 
+  /// Same as cursor(), but uses UnsafeAppend. Only safe after reserve().
+  template <typename T>
+  auto unsafeCursor()
+  {
+    return [this]<typename... Cs>(pack<Cs...>) {
+      return this->template unsafePersist<typename Cs::type...>();
+    }(typename T::table_t::persistent_columns_t{});
+  }
+
   template <typename... Cs>
   auto cursor(framework::pack<Cs...>)
   {