MOVEONLY: DisconnectedBlockTransactions to its own file

This struct is only used in validation + tests and has very little to do
with txmempool.

1 files changed, 128 insertions, 0 deletions

diff --git a/src/kernel/disconnected_transactions.h b/src/kernel/disconnected_transactions.h
new file mode 100644
index 0000000000..a5d02c33ee
--- /dev/null
+++ b/src/kernel/disconnected_transactions.h
@@ -0,0 +1,128 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_KERNEL_DISCONNECTED_TRANSACTIONS_H
+#define BITCOIN_KERNEL_DISCONNECTED_TRANSACTIONS_H
+
+#include <core_memusage.h>
+#include <memusage.h>
+#include <primitives/transaction.h>
+#include <util/hasher.h>
+
+#include <list>
+#include <unordered_map>
+
+/**
+ * DisconnectedBlockTransactions
+
+ * During the reorg, it's desirable to re-add previously confirmed transactions
+ * to the mempool, so that anything not re-confirmed in the new chain is
+ * available to be mined. However, it's more efficient to wait until the reorg
+ * is complete and process all still-unconfirmed transactions at that time,
+ * since we expect most confirmed transactions to (typically) still be
+ * confirmed in the new chain, and re-accepting to the memory pool is expensive
+ * (and therefore better to not do in the middle of reorg-processing).
+ * Instead, store the disconnected transactions (in order!) as we go, remove any
+ * that are included in blocks in the new chain, and then process the remaining
+ * still-unconfirmed transactions at the end.
+ *
+ * Order of queuedTx:
+ * The front of the list should be the most recently-confirmed transactions (transactions at the
+ * end of vtx of blocks closer to the tip). If memory usage grows too large, we trim from the front
+ * of the list. After trimming, transactions can be re-added to the mempool from the back of the
+ * list to the front without running into missing inputs.
+ */
+class DisconnectedBlockTransactions {
+private:
+    /** Cached dynamic memory usage for the CTransactions (memory for the shared pointers is
+     * included in the container calculations). */
+    uint64_t cachedInnerUsage = 0;
+    std::list<CTransactionRef> queuedTx;
+    using TxList = decltype(queuedTx);
+    std::unordered_map<uint256, TxList::iterator, SaltedTxidHasher> iters_by_txid;
+
+public:
+    // It's almost certainly a logic bug if we don't clear out queuedTx before
+    // destruction, as we add to it while disconnecting blocks, and then we
+    // need to re-process remaining transactions to ensure mempool consistency.
+    // For now, assert() that we've emptied out this object on destruction.
+    // This assert() can always be removed if the reorg-processing code were
+    // to be refactored such that this assumption is no longer true (for
+    // instance if there was some other way we cleaned up the mempool after a
+    // reorg, besides draining this object).
+    ~DisconnectedBlockTransactions() {
+        assert(queuedTx.empty());
+        assert(iters_by_txid.empty());
+        assert(cachedInnerUsage == 0);
+    }
+
+    size_t DynamicMemoryUsage() const {
+        return cachedInnerUsage + memusage::DynamicUsage(iters_by_txid) + memusage::DynamicUsage(queuedTx);
+    }
+
+    /** Add transactions from the block, iterating through vtx in reverse order. Callers should call
+     * this function for blocks in descending order by block height.
+     * We assume that callers never pass multiple transactions with the same txid, otherwise things
+     * can go very wrong in removeForBlock due to queuedTx containing an item without a
+     * corresponding entry in iters_by_txid.
+     */
+    void AddTransactionsFromBlock(const std::vector<CTransactionRef>& vtx)
+    {
+        iters_by_txid.reserve(iters_by_txid.size() + vtx.size());
+        for (auto block_it = vtx.rbegin(); block_it != vtx.rend(); ++block_it) {
+            auto it = queuedTx.insert(queuedTx.end(), *block_it);
+            iters_by_txid.emplace((*block_it)->GetHash(), it);
+            cachedInnerUsage += RecursiveDynamicUsage(**block_it);
+        }
+    }
+
+    /** Remove any entries that are in this block. */
+    void removeForBlock(const std::vector<CTransactionRef>& vtx)
+    {
+        // Short-circuit in the common case of a block being added to the tip
+        if (queuedTx.empty()) {
+            return;
+        }
+        for (const auto& tx : vtx) {
+            auto iter = iters_by_txid.find(tx->GetHash());
+            if (iter != iters_by_txid.end()) {
+                auto list_iter = iter->second;
+                iters_by_txid.erase(iter);
+                cachedInnerUsage -= RecursiveDynamicUsage(**list_iter);
+                queuedTx.erase(list_iter);
+            }
+        }
+    }
+
+    /** Remove the first entry and update memory usage. */
+    CTransactionRef take_first()
+    {
+        CTransactionRef first_tx;
+        if (!queuedTx.empty()) {
+            first_tx = queuedTx.front();
+            cachedInnerUsage -= RecursiveDynamicUsage(*queuedTx.front());
+            iters_by_txid.erase(queuedTx.front()->GetHash());
+            queuedTx.pop_front();
+        }
+        return first_tx;
+    }
+
+    size_t size() const { return queuedTx.size(); }
+
+    void clear()
+    {
+        cachedInnerUsage = 0;
+        iters_by_txid.clear();
+        queuedTx.clear();
+    }
+
+    /** Clear all data structures and return the list of transactions. */
+    std::list<CTransactionRef> take()
+    {
+        std::list<CTransactionRef> ret = std::move(queuedTx);
+        clear();
+        return ret;
+    }
+};
+#endif // BITCOIN_KERNEL_DISCONNECTED_TRANSACTIONS_H