rpc: improve getmempoolcluster output

diff --git a/src/rpc/mempool.cpp b/src/rpc/mempool.cpp
index 72cc9f8b..bc011e0b 100644
--- a/src/rpc/mempool.cpp
+++ b/src/rpc/mempool.cpp
@@ -269,12 +269,13 @@ static std::vector<RPCResult> ClusterDescription()
     return {
         RPCResult{RPCResult::Type::NUM, "weight", "total sigops-adjusted weight (as defined in BIP 141 and modified by '-bytespersigop'"},
         RPCResult{RPCResult::Type::NUM, "txcount", "number of transactions"},
-        RPCResult{RPCResult::Type::ARR, "txs", "transactions in this cluster in mining order",
-            {RPCResult{RPCResult::Type::OBJ, "txentry", "",
+        RPCResult{RPCResult::Type::ARR, "chunks", "chunks in this cluster (in mining order)",
+            {RPCResult{RPCResult::Type::OBJ, "chunk", "",
                 {
-                    RPCResult{RPCResult::Type::STR_HEX, "txid", "the transaction id"},
-                    RPCResult{RPCResult::Type::NUM, "chunkfee", "fee of the chunk containing this tx"},
-                    RPCResult{RPCResult::Type::NUM, "chunkweight", "sigops-adjusted weight of the chunk containing this transaction"}
+                    RPCResult{RPCResult::Type::NUM, "chunkfee", "fees of the transactions in this chunk"},
+                    RPCResult{RPCResult::Type::NUM, "chunkweight", "sigops-adjusted weight of all transactions in this chunk"},
+                    RPCResult{RPCResult::Type::ARR, "txs", "transactions in this chunk in mining order",
+                        {RPCResult{RPCResult::Type::STR_HEX, "txid", "transaction id"}}},
                 }
             }}
         }
@@ -311,6 +312,19 @@ static std::vector<RPCResult> MempoolEntryDescription()
     };
 }
 
+void AppendChunkInfo(UniValue& all_chunks, FeePerWeight chunk_feerate, std::vector<const CTxMemPoolEntry *> chunk_txs)
+{
+    UniValue chunk(UniValue::VOBJ);
+    chunk.pushKV("chunkfee", ValueFromAmount((int)chunk_feerate.fee));
+    chunk.pushKV("chunkweight", chunk_feerate.size);
+    UniValue chunk_txids(UniValue::VARR);
+    for (const auto& chunk_tx : chunk_txs) {
+        chunk_txids.push_back(chunk_tx->GetTx().GetHash().ToString());
+    }
+    chunk.pushKV("txs", std::move(chunk_txids));
+    all_chunks.push_back(std::move(chunk));
+}
+
 static void clusterToJSON(const CTxMemPool& pool, UniValue& info, std::vector<const CTxMemPoolEntry *> cluster) EXCLUSIVE_LOCKS_REQUIRED(pool.cs)
 {
     AssertLockHeld(pool.cs);
@@ -320,16 +334,29 @@ static void clusterToJSON(const CTxMemPool& pool, UniValue& info, std::vector<co
     }
     info.pushKV("weight", total_weight);
     info.pushKV("txcount", (int)cluster.size());
-    UniValue txs(UniValue::VARR);
+
+    // Output the cluster by chunk. This isn't handed to us by the mempool, but
+    // we can calculate it by looking at the chunk feerates of each transaction
+    // in the cluster.
+    FeePerWeight current_chunk_feerate = pool.GetMainChunkFeerate(*cluster[0]);
+    std::vector<const CTxMemPoolEntry *> current_chunk;
+    current_chunk.reserve(cluster.size());
+
+    UniValue all_chunks(UniValue::VARR);
     for (const auto& tx : cluster) {
-        UniValue txentry(UniValue::VOBJ);
-        auto feerate = pool.GetMainChunkFeerate(*tx);
-        txentry.pushKV("txid", tx->GetTx().GetHash().ToString());
-        txentry.pushKV("chunkfee", ValueFromAmount((int)feerate.fee));
-        txentry.pushKV("chunkweight", feerate.size);
-        txs.push_back(txentry);
+        if (current_chunk_feerate.size == 0) {
+            // We've iterated all the transactions in the previous chunk; so
+            // append it to the output.
+            AppendChunkInfo(all_chunks, pool.GetMainChunkFeerate(*current_chunk[0]), current_chunk);
+            current_chunk.clear();
+            current_chunk_feerate = pool.GetMainChunkFeerate(*tx);
+        }
+        current_chunk.push_back(tx);
+        current_chunk_feerate.size -= tx->GetAdjustedWeight();
     }
-    info.pushKV("txs", txs);
+    AppendChunkInfo(all_chunks, pool.GetMainChunkFeerate(*current_chunk[0]), current_chunk);
+    current_chunk.clear();
+    info.pushKV("chunks", std::move(all_chunks));
 }
 
 static void entryToJSON(const CTxMemPool& pool, UniValue& info, const CTxMemPoolEntry& e) EXCLUSIVE_LOCKS_REQUIRED(pool.cs)
@@ -673,7 +700,13 @@ static RPCHelpMan getmempoolcluster()
     const CTxMemPool& mempool = EnsureAnyMemPool(request.context);
     LOCK(mempool.cs);
 
-    auto cluster = mempool.GetCluster(Txid::FromUint256(hash));
+    auto txid = Txid::FromUint256(hash);
+    const auto entry{mempool.GetEntry(txid)};
+    if (entry == nullptr) {
+        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not in mempool");
+    }
+
+    auto cluster = mempool.GetCluster(txid);
 
     UniValue info(UniValue::VOBJ);
     clusterToJSON(mempool, info, cluster);
diff --git a/test/functional/mempool_cluster.py b/test/functional/mempool_cluster.py
index 4b4812e6..cbe2684c 100755
--- a/test/functional/mempool_cluster.py
+++ b/test/functional/mempool_cluster.py
@@ -298,12 +298,87 @@ class MempoolClusterTest(BitcoinTestFramework):
         assert tx_replacer_sponsor["txid"] in node.getrawmempool()
         assert_equal(node.getmempoolcluster(tx_replacer["txid"])['txcount'], 2)
 
+    @cleanup
+    def test_getmempoolcluster(self):
+        node = self.nodes[0]
+
+        self.log.info("Testing getmempoolcluster")
+
+        assert_equal(node.getrawmempool(), [])
+
+        # Not in-mempool
+        not_mempool_tx = self.wallet.create_self_transfer()
+        assert_raises_rpc_error(-5, "Transaction not in mempool", node.getmempoolcluster, not_mempool_tx["txid"])
+
+        # Test that chunks are being recomputed properly
+
+        # One chunk with one tx
+        first_chunk_tx = self.wallet.send_self_transfer(from_node=node)
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        assert_equal(first_chunk_info, {'weight': first_chunk_tx["tx"].get_weight(), 'txcount': 1, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunk_tx["tx"].get_weight(), 'txs': [first_chunk_tx["txid"]]}]})
+
+        # Another unconnected tx, nothing should change
+        self.wallet.send_self_transfer(from_node=node)
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        assert_equal(first_chunk_info, {'weight': first_chunk_tx["tx"].get_weight(), 'txcount': 1, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunk_tx["tx"].get_weight(), 'txs': [first_chunk_tx["txid"]]}]})
+
+        # Second connected tx, makes one chunk still with high enough fee
+        second_chunk_tx = self.wallet.send_self_transfer(from_node=node, utxo_to_spend=first_chunk_tx["new_utxo"], fee_rate=Decimal("0.01"))
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        # output is same across same cluster transactions
+        assert_equal(first_chunk_info, node.getmempoolcluster(second_chunk_tx["txid"]))
+        chunkweight = first_chunk_tx["tx"].get_weight() + second_chunk_tx["tx"].get_weight()
+        chunkfee = first_chunk_tx["fee"] + second_chunk_tx["fee"]
+        assert_equal(first_chunk_info, {'weight': chunkweight, 'txcount': 2, 'chunks': [{'chunkfee': chunkfee, 'chunkweight': chunkweight, 'txs': [first_chunk_tx["txid"], second_chunk_tx["txid"]]}]})
+
+        # Third connected tx, makes one chunk still with high enough fee
+        third_chunk_tx = self.wallet.send_self_transfer(from_node=node, utxo_to_spend=second_chunk_tx["new_utxo"], fee_rate=Decimal("0.1"))
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        # output is same across same cluster transactions
+        assert_equal(first_chunk_info, node.getmempoolcluster(third_chunk_tx["txid"]))
+        chunkweight = first_chunk_tx["tx"].get_weight() + second_chunk_tx["tx"].get_weight() + third_chunk_tx["tx"].get_weight()
+        chunkfee = first_chunk_tx["fee"] + second_chunk_tx["fee"] + third_chunk_tx["fee"]
+        assert_equal(first_chunk_info, {'weight': chunkweight, 'txcount': 3, 'chunks': [{'chunkfee': chunkfee, 'chunkweight': chunkweight, 'txs': [first_chunk_tx["txid"], second_chunk_tx["txid"], third_chunk_tx["txid"]]}]})
+
+        # Now test single cluster with each tx being its own chunk
+
+        # One chunk with one tx
+        first_chunk_tx = self.wallet.send_self_transfer(from_node=node)
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        assert_equal(first_chunk_info, {'weight': first_chunk_tx["tx"].get_weight(), 'txcount': 1, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunk_tx["tx"].get_weight(), 'txs': [first_chunk_tx["txid"]]}]})
+
+        # Second connected tx, lower fee
+        second_chunk_tx = self.wallet.send_self_transfer(from_node=node, utxo_to_spend=first_chunk_tx["new_utxo"], fee_rate=Decimal("0.000002"))
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        # output is same across same cluster transactions
+        assert_equal(first_chunk_info, node.getmempoolcluster(second_chunk_tx["txid"]))
+        first_chunkweight = first_chunk_tx["tx"].get_weight()
+        second_chunkweight = second_chunk_tx["tx"].get_weight()
+        assert_equal(first_chunk_info, {'weight': first_chunkweight + second_chunkweight, 'txcount': 2, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunkweight, 'txs': [first_chunk_tx["txid"]]}, {'chunkfee': second_chunk_tx["fee"], 'chunkweight': second_chunkweight, 'txs': [second_chunk_tx["txid"]]}]})
+
+        # Third connected tx, even lower fee
+        third_chunk_tx = self.wallet.send_self_transfer(from_node=node, utxo_to_spend=second_chunk_tx["new_utxo"], fee_rate=Decimal("0.000001"))
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        # output is same across same cluster transactions
+        assert_equal(first_chunk_info, node.getmempoolcluster(third_chunk_tx["txid"]))
+        first_chunkweight = first_chunk_tx["tx"].get_weight()
+        second_chunkweight = second_chunk_tx["tx"].get_weight()
+        third_chunkweight = third_chunk_tx["tx"].get_weight()
+        chunkfee = first_chunk_tx["fee"] + second_chunk_tx["fee"] + third_chunk_tx["fee"]
+        assert_equal(first_chunk_info, {'weight': first_chunkweight + second_chunkweight + third_chunkweight, 'txcount': 3, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunkweight, 'txs': [first_chunk_tx["txid"]]}, {'chunkfee': second_chunk_tx["fee"], 'chunkweight': second_chunkweight, 'txs': [second_chunk_tx["txid"]]}, {'chunkfee': third_chunk_tx["fee"], 'chunkweight': third_chunkweight, 'txs': [third_chunk_tx["txid"]]}]})
+
+        # If we prioritise the last transaction it can join the second transaction's chunk.
+        node.prioritisetransaction(third_chunk_tx["txid"], 0, int(third_chunk_tx["fee"]*COIN) + 1)
+        first_chunk_info = node.getmempoolcluster(first_chunk_tx["txid"])
+        assert_equal(first_chunk_info, {'weight': first_chunkweight + second_chunkweight + third_chunkweight, 'txcount': 3, 'chunks': [{'chunkfee': first_chunk_tx["fee"], 'chunkweight': first_chunkweight, 'txs': [first_chunk_tx["txid"]]}, {'chunkfee': second_chunk_tx["fee"] + 2*third_chunk_tx["fee"] + Decimal("0.00000001"), 'chunkweight': second_chunkweight + third_chunkweight, 'txs': [second_chunk_tx["txid"], third_chunk_tx["txid"]]}]})
 
     def run_test(self):
         node = self.nodes[0]
         self.wallet = MiniWallet(node)
         self.generate(self.wallet, 400)
 
+        self.test_getmempoolcluster()
+
         self.test_cluster_limit_rbf(DEFAULT_CLUSTER_LIMIT)
 
         for cluster_size_limit_kvb in [10, 20, 33, 100, DEFAULT_CLUSTER_SIZE_LIMIT_KVB]: