refactor: move GetTransaction(...) to node/transaction.cpp

can be reviewed with --color-moved

src/node/transaction.h

@@ -10,7 +10,12 @@
 #include <primitives/transaction.h>
 #include <util/error.h>
 
+class CBlockIndex;
+class CTxMemPool;
 struct NodeContext;
+namespace Consensus {
+struct Params;
+}
 
 /** Maximum fee rate for sendrawtransaction and testmempoolaccept RPC calls.
  * Also used by the GUI when broadcasting a completed PSBT.
@@ -38,4 +43,19 @@ static const CFeeRate DEFAULT_MAX_RAW_TX_FEE_RATE{COIN / 10};
  */
 [[nodiscard]] TransactionError BroadcastTransaction(NodeContext& node, CTransactionRef tx, std::string& err_string, const CAmount& max_tx_fee, bool relay, bool wait_callback);
 
+/**
+ * Return transaction with a given hash.
+ * If mempool is provided and block_index is not provided, check it first for the tx.
+ * If -txindex is available, check it next for the tx.
+ * Finally, if block_index is provided, check for tx by reading entire block from disk.
+ *
+ * @param[in]  block_index     The block to read from disk, or nullptr
+ * @param[in]  mempool         If provided, check mempool for tx
+ * @param[in]  hash            The txid
+ * @param[in]  consensusParams The params
+ * @param[out] hashBlock       The block hash, if the tx was found via -txindex or block_index
+ * @returns                    The tx if found, otherwise nullptr
+ */
+CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock);
+
 #endif // BITCOIN_NODE_TRANSACTION_H