TxGraphImpl::PullIn: only allowed when staging exists

Author: instagibbs

src/txgraph.cpp

@@ -411,8 +411,8 @@ class TxGraphImpl final : public TxGraph
      *  values for remaining Entry objects, so this only does something when no to-be-applied
      *  operations or staged removals referring to GraphIndexes remain). */
     void Compact() noexcept;
-    /** If cluster is not in staging, copy it there, and return a pointer to it. This has no
-    *   effect if only a main graph exists, but if staging exists this modifies the locators of its
+    /** If cluster is not in staging, copy it there, and return a pointer to it.
+    *   Staging must exist, and this modifies the locators of its
     *   transactions from inherited (P,M) to explicit (P,P). */
     Cluster* PullIn(Cluster* cluster) noexcept;
     /** Apply all removals queued up in m_to_remove to the relevant Clusters (which get a
@@ -928,7 +928,7 @@ Cluster* TxGraphImpl::FindCluster(GraphIndex idx, int level) const noexcept
 Cluster* TxGraphImpl::PullIn(Cluster* cluster) noexcept
 {
     int to_level = GetTopLevel();
-    if (to_level == 0) return cluster;
+    Assume(to_level == 1);
     int level = cluster->m_level;
     Assume(level <= to_level);
     // Copy the Cluster from main to staging, if it's not already there.