[StorageV2] Make delta list double linked

Summary: this is necessary for garbage collection

Reviewers: teon.banek, mferencevic

Reviewed By: mferencevic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2156

src/storage/v2/delta.hpp

@@ -17,6 +17,7 @@ struct Delta {
         value(value),
         timestamp(timestamp),
         command_id(command_id),
+        prev(nullptr),
         next(nullptr) {}
 
   Delta(Delta &&other) noexcept
@@ -24,6 +25,7 @@ struct Delta {
         value(other.value),
         timestamp(other.timestamp),
         command_id(other.command_id),
+        prev(nullptr),
         next(other.next.load()) {}
 
   Delta(const Delta &) = delete;
@@ -38,6 +40,7 @@ struct Delta {
   // TODO: optimize with in-place copy
   std::atomic<uint64_t> *timestamp;
   uint64_t command_id;
+  Delta *prev;
   std::atomic<Delta *> next;
 };
 

src/storage/v2/vertex_accessor.hpp

@@ -54,6 +54,9 @@ class VertexAccessor final {
       return Result<bool>{false};
 
     auto delta = transaction_->CreateDelta(Delta::Action::REMOVE_LABEL, label);
+    if (vertex_->delta) {
+      vertex_->delta->prev = delta;
+    }
     delta->next = vertex_->delta;
     vertex_->delta = delta;
 
@@ -71,6 +74,9 @@ class VertexAccessor final {
     if (it == vertex_->labels.end()) return Result<bool>{false};
 
     auto delta = transaction_->CreateDelta(Delta::Action::ADD_LABEL, label);
+    if (vertex_->delta) {
+      vertex_->delta->prev = delta;
+    }
     delta->next = vertex_->delta;
     vertex_->delta = delta;