txgraph: expose memory usage estimate function (feature)

diff --git a/src/test/fuzz/txgraph.cpp b/src/test/fuzz/txgraph.cpp
index 00001034..fbf22bfc 100644
--- a/src/test/fuzz/txgraph.cpp
+++ b/src/test/fuzz/txgraph.cpp
@@ -1012,6 +1012,21 @@ FUZZ_TARGET(txgraph)
                 }
                 assert(!top_sim.IsOversized());
                 break;
+            } else if (command-- == 0) {
+                // GetMainMemoryUsage().
+                auto usage = real->GetMainMemoryUsage();
+                // Test stability.
+                if (alt) {
+                    auto usage2 = real->GetMainMemoryUsage();
+                    assert(usage == usage2);
+                }
+                // Only empty graphs have 0 memory usage.
+                if (main_sim.GetTransactionCount() == 0) {
+                    assert(usage == 0);
+                } else {
+                    assert(usage > 0);
+                }
+                break;
             }
         }
     }
diff --git a/src/txgraph.cpp b/src/txgraph.cpp
index a2392bc8..41a59b5b 100644
--- a/src/txgraph.cpp
+++ b/src/txgraph.cpp
@@ -629,6 +629,8 @@ public:
     std::unique_ptr<BlockBuilder> GetBlockBuilder() noexcept final;
     std::pair<std::vector<Ref*>, FeePerWeight> GetWorstMainChunk() noexcept final;
 
+    size_t GetMainMemoryUsage() noexcept final;
+
     void SanityCheck() const final;
 };
 
@@ -2980,6 +2982,21 @@ std::vector<TxGraph::Ref*> TxGraphImpl::Trim() noexcept
     return ret;
 }
 
+size_t TxGraphImpl::GetMainMemoryUsage() noexcept
+{
+    // Make sure splits/merges are applied, as memory usage may not be representative otherwise.
+    SplitAll(/*up_to_level=*/0);
+    ApplyDependencies(/*level=*/0);
+    // Compute memory usage
+    size_t usage = /* From clusters */
+                   m_main_clusterset.m_cluster_usage +
+                   /* From Entry objects. */
+                   sizeof(Entry) * m_main_clusterset.m_txcount +
+                   /* From the chunk index. */
+                   memusage::DynamicUsage(m_main_chunkindex);
+    return usage;
+}
+
 } // namespace
 
 TxGraph::Ref::~Ref()
diff --git a/src/txgraph.h b/src/txgraph.h
index 20ea97ac..1d8d31a5 100644
--- a/src/txgraph.h
+++ b/src/txgraph.h
@@ -202,6 +202,13 @@ public:
      *  graph must not be oversized. If the graph is empty, {{}, FeePerWeight{}} is returned. */
     virtual std::pair<std::vector<Ref*>, FeePerWeight> GetWorstMainChunk() noexcept = 0;
 
+    /** Get the approximate memory usage for this object, just counting the main graph. If a
+     *  staging graph is present, return a number corresponding to memory usage after
+     *  AbortStaging() would be called. BlockBuilders' memory usage, memory usage of internally
+     *  queued operations, and memory due to temporary caches, is not included here. Can always be
+     *  called. */
+    virtual size_t GetMainMemoryUsage() noexcept = 0;
+
     /** Perform an internal consistency check on this object. */
     virtual void SanityCheck() const = 0;