Merge #12479: RPC: Add child transactions to getrawmempool verbose output

1dfb4e7d7 [Tests] Check output of parent/child tx list from getrawmempool, getmempooldescendants, getmempoolancestors, and REST interface (Conor Scott)
fc44cb108 [RPC] Add list of child transactions to verbose output of getrawmempool (Conor Scott)

Pull request description:

  `bitcoin-cli getrawmempool true` only lists a transaction's parents in the `depends` field. This change adds a `spentby` field to the json response, which lists the transaction's children in the mempool.

  Currently the only way to find child transactions is to use `getrawmempool` or make another call to `getmempooldescendants` and search the response for transactions that list the parent_txid in the `depends` list, which is inefficient.

  This change allows direct lookup of children.

  Example Output
  ```
    "9a9b5733c0d89f207908cfa3fe17809bee71f629aa095c9f8754524e29e98ba4": {
      ...other geterawmempool data...
      "wtxid": "9a9b5733c0d89f207908cfa3fe17809bee71f629aa095c9f8754524e29e98ba4",
      "depends": [
        "bdd92851d5766a42aeb62af667bb422a116cab4e032bba5e3dd6efe5b4b40aa0"
      ],
      "spentby": [
        "dc5d3ec388a9121421208738a041ac30a22163bc2e17758f2275b6c51a15ba7b"
      ]
    },
  ```

Tree-SHA512: 83da7d421c9799a40ef65af3b7fdb586d6d87385f3f2ede3afd2c311725444b858f9d91cc110422a0fa31905779934fee07211ca6fe6b746792b83692c94b3ce

src/rpc/blockchain.cpp

@@ -372,6 +372,9 @@ std::string EntryDescriptionString()
            "    \"wtxid\" : hash,         (string) hash of serialized transaction, including witness data\n"
            "    \"depends\" : [           (array) unconfirmed transactions used as inputs for this transaction\n"
            "        \"transactionid\",    (string) parent transaction id\n"
+           "       ... ]\n"
+           "    \"spentby\" : [           (array) unconfirmed transactions spending outputs from this transaction\n"
+           "        \"transactionid\",    (string) child transaction id\n"
            "       ... ]\n";
 }
 
@@ -406,6 +409,15 @@ void entryToJSON(UniValue &info, const CTxMemPoolEntry &e)
     }
 
     info.pushKV("depends", depends);
+
+    UniValue spent(UniValue::VARR);
+    const CTxMemPool::txiter &it = mempool.mapTx.find(tx.GetHash());
+    const CTxMemPool::setEntries &setChildren = mempool.GetMemPoolChildren(it);
+    for (const CTxMemPool::txiter &childiter : setChildren) {
+        spent.push_back(childiter->GetTx().GetHash().ToString());
+    }
+
+    info.pushKV("spentby", spent);
 }
 
 UniValue mempoolToJSON(bool fVerbose)