test: cover cluster limits during reorgs #36214

This PR adds functional test coverage for cluster-limit handling during reorgs.

Transactions resurrected from disconnected blocks are added before their existing mempool children are linked in the transaction graph. Once those dependencies are restored, oversized clusters must be trimmed back within their configured count and size limits.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36214
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36214.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36214#author-ralph

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Reviews

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cc @instagibbs @sipa

I think the only thing this test adds over test_chainlimits_exceeded and transaction_graph_test is testing the pre-existing mempool children are added back in to the clusters. Thoughts?

I think the only thing this test adds over test_chainlimits_exceeded and transaction_graph_test is testing the pre-existing mempool children are added back in to the clusters.

Agree, transaction_graph_test covers reconnecting existing children. These tests cover clusters that become oversized only after reconnecting existing children, exercising Trim() for count, size, and merged clusters.

for count, size, and merged clusters.

Not sure it's useful to run through all of them. Can you trim this down to just the case that actually kills a mutant?

Trimmed to a single case. The mutant this kills is skipping the cluster trim in UpdateTransactionsFromBlock after a reorg.

<details> <summary> diff </summary>

--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -112,7 +112,7 @@
         }
     }

-    auto txs_to_remove = m_txgraph->Trim(); // Enforce cluster size limits.
+    std::vector<TxGraph::Ref*> txs_to_remove; // Enforce cluster size limits.
     for (auto txptr : txs_to_remove) {
         const CTxMemPoolEntry& entry = *(static_cast<const CTxMemPoolEntry*>(txptr));
         removeUnchecked(mapTx.iterator_to(entry), MemPoolRemovalReason::SIZELIMIT);

</details>

If there is a killed mutant, could include the diff in a comment?

If there is a killed mutant, could include the diff in a comment?

I've updated the above comment.