txgraph: Allow Refs to outlive the TxGraph (feature)

Author: sipa

src/test/fuzz/txgraph.cpp

@@ -587,4 +587,10 @@ FUZZ_TARGET(txgraph)
 
     // Sanity check again (because invoking inspectors may modify internal unobservable state).
     real->SanityCheck();
+
+    // Kill the TxGraph object.
+    real.reset();
+    // Kill the simulated graphs, with all remaining Refs in it. If any, this verifies that Refs
+    // can outlive the TxGraph that created them.
+    sims.clear();
 }

src/txgraph.cpp

@@ -320,6 +320,9 @@ class TxGraphImpl final : public TxGraph
         Assume(max_cluster_count <= MAX_CLUSTER_COUNT_LIMIT);
     }
 
+    /** Destructor. */
+    ~TxGraphImpl() noexcept;
+
     // Cannot move or copy (would invalidate TxGraphImpl* in Ref, MiningOrder, EvictionOrder).
     TxGraphImpl(const TxGraphImpl&) = delete;
     TxGraphImpl& operator=(const TxGraphImpl&) = delete;
@@ -809,6 +812,17 @@ void Cluster::ApplyDependencies(TxGraphImpl& graph, std::span<std::pair<GraphInd
     Updated(graph);
 }
 
+TxGraphImpl::~TxGraphImpl() noexcept
+{
+    // If Refs outlive the TxGraphImpl they refer to, unlink them, so that their destructor does not
+    // try to reach into a non-existing TxGraphImpl anymore.
+    for (auto& entry : m_entries) {
+        if (entry.m_ref != nullptr) {
+            GetRefGraph(*entry.m_ref) = nullptr;
+        }
+    }
+}
+
 std::unique_ptr<Cluster> TxGraphImpl::ExtractCluster(int level, QualityLevel quality, ClusterSetIndex setindex) noexcept
 {
     Assume(quality != QualityLevel::NONE);

src/txgraph.h

@@ -63,7 +63,8 @@ class TxGraph
     /** Construct a new transaction with the specified feerate, and return a Ref to it.
      *  If a staging graph exists, the new transaction is only created there. In all
      *  further calls, only Refs created by AddTransaction() are allowed to be passed to this
-     *  TxGraph object (or empty Ref objects). */
+     *  TxGraph object (or empty Ref objects). Ref objects may outlive the TxGraph they were
+     *  created for. */
     [[nodiscard]] virtual Ref AddTransaction(const FeePerWeight& feerate) noexcept = 0;
     /** Remove the specified transaction. If a staging graph exists, the removal only happens
      *  there. This is a no-op if the transaction was already removed.