feat: option to modify estimated fees in /v2/fees/transaction proxy (#2172)

* feat: option to modify estimated fees in /v2/fees/transaction proxy

* chore: lazy load env STACKS_CORE_FEE_ESTIMATION_MODIFIER

* chore: use undici for rpc proxy tests

Author: zone117x

package-lock.json

@@ -147,6 +147,7 @@
     "strict-event-emitter-types": "2.0.0",
     "tiny-secp256k1": "2.2.1",
     "ts-unused-exports": "7.0.3",
+    "undici": "6.21.0",
     "uuid": "8.3.2",
     "ws": "7.5.10",
     "zone-file": "2.0.0-beta.3"

package.json

@@ -21,6 +21,26 @@ function getReqUrl(req: { url: string; hostname: string }): URL {
   return new URL(req.url, `http://${req.hostname}`);
 }
 
+// https://github.com/stacks-network/stacks-core/blob/20d5137438c7d169ea97dd2b6a4d51b8374a4751/stackslib/src/chainstate/stacks/db/blocks.rs#L338
+const MINIMUM_TX_FEE_RATE_PER_BYTE = 1;
+
+interface FeeEstimation {
+  fee: number;
+  fee_rate: number;
+}
+interface FeeEstimateResponse {
+  cost_scalar_change_by_byte: number;
+  estimated_cost: {
+    read_count: number;
+    read_length: number;
+    runtime: number;
+    write_count: number;
+    write_length: number;
+  };
+  estimated_cost_scalar: number;
+  estimations: [FeeEstimation, FeeEstimation, FeeEstimation];
+}
+
 export const CoreNodeRpcProxyRouter: FastifyPluginAsync<
   Record<never, never>,
   Server,
@@ -117,10 +137,22 @@ export const CoreNodeRpcProxyRouter: FastifyPluginAsync<
     }
   );
 
+  let feeEstimationModifier: number | null = null;
+  fastify.addHook('onReady', () => {
+    const feeEstEnvVar = process.env['STACKS_CORE_FEE_ESTIMATION_MODIFIER'];
+    if (feeEstEnvVar) {
+      const parsed = parseFloat(feeEstEnvVar);
+      if (!isNaN(parsed) && parsed > 0) {
+        feeEstimationModifier = parsed;
+      }
+    }
+  });
+
   await fastify.register(fastifyHttpProxy, {
     upstream: `http://${stacksNodeRpcEndpoint}`,
     rewritePrefix: '/v2',
     http2: false,
+    globalAgent: true,
     preValidation: async (req, reply) => {
       if (getReqUrl(req).pathname !== '/v2/transactions') {
         return;
@@ -201,6 +233,29 @@ export const CoreNodeRpcProxyRouter: FastifyPluginAsync<
           const txId = responseBuffer.toString();
           await logTxBroadcast(txId);
           await reply.send(responseBuffer);
+        } else if (
+          getReqUrl(req).pathname === '/v2/fees/transaction' &&
+          reply.statusCode === 200 &&
+          feeEstimationModifier !== null
+        ) {
+          const reqBody = req.body as {
+            estimated_len?: number;
+            transaction_payload: string;
+          };
+          // https://github.com/stacks-network/stacks-core/blob/20d5137438c7d169ea97dd2b6a4d51b8374a4751/stackslib/src/net/api/postfeerate.rs#L200-L201
+          const txSize = Math.max(
+            reqBody.estimated_len ?? 0,
+            reqBody.transaction_payload.length / 2
+          );
+          const minFee = txSize * MINIMUM_TX_FEE_RATE_PER_BYTE;
+          const modifier = feeEstimationModifier;
+          const responseBuffer = await readRequestBody(response as ServerResponse);
+          const responseJson = JSON.parse(responseBuffer.toString()) as FeeEstimateResponse;
+          responseJson.estimations.forEach(estimation => {
+            // max(min fee, estimate returned by node * configurable modifier)
+            estimation.fee = Math.max(minFee, Math.round(estimation.fee * modifier));
+          });
+          await reply.removeHeader('content-length').send(JSON.stringify(responseJson));
         } else {
           await reply.send(response);
         }

src/api/routes/core-node-rpc-proxy.ts

@@ -9,6 +9,7 @@ import * as nock from 'nock';
 import { DbBlock } from '../../src/datastore/common';
 import { PgWriteStore } from '../../src/datastore/pg-write-store';
 import { migrate } from '../utils/test-helpers';
+import { MockAgent, setGlobalDispatcher, getGlobalDispatcher } from 'undici';
 
 describe('v2-proxy tests', () => {
   let db: PgWriteStore;
@@ -27,6 +28,95 @@ describe('v2-proxy tests', () => {
     await migrate('down');
   });
 
+  test('tx fee estimation', async () => {
+    const primaryProxyEndpoint = 'proxy-stacks-node:12345';
+    const feeEstimationModifier = 0.5;
+    await useWithCleanup(
+      () => {
+        const restoreEnvVars = withEnvVars(
+          ['STACKS_CORE_FEE_ESTIMATION_MODIFIER', feeEstimationModifier.toString()],
+          ['STACKS_CORE_PROXY_HOST', primaryProxyEndpoint.split(':')[0]],
+          ['STACKS_CORE_PROXY_PORT', primaryProxyEndpoint.split(':')[1]]
+        );
+        return [, () => restoreEnvVars()] as const;
+      },
+      () => {
+        const agent = new MockAgent();
+        const originalAgent = getGlobalDispatcher();
+        setGlobalDispatcher(agent);
+        return [agent, () => setGlobalDispatcher(originalAgent)] as const;
+      },
+      async () => {
+        const apiServer = await startApiServer({
+          datastore: db,
+          chainId: ChainID.Mainnet,
+        });
+        return [apiServer, apiServer.terminate] as const;
+      },
+      async (_, mockAgent, api) => {
+        const primaryStubbedResponse = {
+          cost_scalar_change_by_byte: 0.00476837158203125,
+          estimated_cost: {
+            read_count: 19,
+            read_length: 4814,
+            runtime: 7175000,
+            write_count: 2,
+            write_length: 1020,
+          },
+          estimated_cost_scalar: 14,
+          estimations: [
+            {
+              fee: 400,
+              fee_rate: 1.2410714285714286,
+            },
+            {
+              fee: 800,
+              fee_rate: 8.958333333333332,
+            },
+            {
+              fee: 1000,
+              fee_rate: 10,
+            },
+          ],
+        };
+        const testRequest = {
+          estimated_len: 350,
+          transaction_payload:
+            '021af942874ce525e87f21bbe8c121b12fac831d02f4086765742d696e666f0b7570646174652d696e666f00000000',
+        };
+
+        mockAgent
+          .get(`http://${primaryProxyEndpoint}`)
+          .intercept({
+            path: '/v2/fees/transaction',
+            method: 'POST',
+          })
+          .reply(200, JSON.stringify(primaryStubbedResponse), {
+            headers: { 'Content-Type': 'application/json' },
+          });
+
+        const postTxReq = await supertest(api.server)
+          .post(`/v2/fees/transaction`)
+          .set('Content-Type', 'application/json')
+          .send(JSON.stringify(testRequest));
+        expect(postTxReq.status).toBe(200);
+        // Expected min fee is the byte size because MINIMUM_TX_FEE_RATE_PER_BYTE=1
+        const expectedMinFee = Math.max(
+          testRequest.estimated_len ?? 0,
+          testRequest.transaction_payload.length / 2
+        );
+        const expectedResponse = {
+          ...primaryStubbedResponse,
+        };
+        expectedResponse.estimations = expectedResponse.estimations.map(est => ({
+          ...est,
+          fee: Math.max(expectedMinFee, Math.round(est.fee * feeEstimationModifier)),
+        }));
+        expect(postTxReq.body).toEqual(expectedResponse);
+      }
+    );
+  });
+
   test('tx post multicast', async () => {
     const primaryProxyEndpoint = 'proxy-stacks-node:12345';
     const extraTxEndpoint = 'http://extra-tx-endpoint-a/test';