From 83fde600f12dcfd213a9ddd9a38c03e412ccf5bf Mon Sep 17 00:00:00 2001
From: Mononaut <github@monospace.live>
Date: Wed, 25 Oct 2023 16:53:05 +0000
Subject: [PATCH] Enforce purging rate minimum on recommended fees

---
 backend/src/api/fee-api.ts | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/backend/src/api/fee-api.ts b/backend/src/api/fee-api.ts
index 82778825e..97dfc29d2 100644
--- a/backend/src/api/fee-api.ts
+++ b/backend/src/api/fee-api.ts
@@ -3,21 +3,30 @@ import { Common } from './common';
 import mempool from './mempool';
 import projectedBlocks from './mempool-blocks';
 
+interface RecommendedFees {
+  fastestFee: number,
+  halfHourFee: number,
+  hourFee: number,
+  economyFee: number,
+  minimumFee: number,
+}
+
 class FeeApi {
   constructor() { }
 
   defaultFee = Common.isLiquid() ? 0.1 : 1;
 
-  public getRecommendedFee() {
+  public getRecommendedFee(): RecommendedFees {
     const pBlocks = projectedBlocks.getMempoolBlocks();
     const mPool = mempool.getMempoolInfo();
     const minimumFee = Math.ceil(mPool.mempoolminfee * 100000);
+    const defaultMinFee = Math.max(minimumFee, this.defaultFee);
 
     if (!pBlocks.length) {
       return {
-        'fastestFee': this.defaultFee,
-        'halfHourFee': this.defaultFee,
-        'hourFee': this.defaultFee,
+        'fastestFee': defaultMinFee,
+        'halfHourFee': defaultMinFee,
+        'hourFee': defaultMinFee,
         'economyFee': minimumFee,
         'minimumFee': minimumFee,
       };
@@ -27,11 +36,15 @@ class FeeApi {
     const secondMedianFee = pBlocks[1] ? this.optimizeMedianFee(pBlocks[1], pBlocks[2], firstMedianFee) : this.defaultFee;
     const thirdMedianFee = pBlocks[2] ? this.optimizeMedianFee(pBlocks[2], pBlocks[3], secondMedianFee) : this.defaultFee;
 
+    // explicitly enforce a minimum of ceil(mempoolminfee) on all recommendations.
+    // simply rounding up recommended rates is insufficient, as the purging rate
+    // can exceed the median rate of projected blocks in some extreme scenarios
+    // (see https://bitcoin.stackexchange.com/a/120024)
     return {
-      'fastestFee': firstMedianFee,
-      'halfHourFee': secondMedianFee,
-      'hourFee': thirdMedianFee,
-      'economyFee': Math.min(2 * minimumFee, thirdMedianFee),
+      'fastestFee': Math.max(minimumFee, firstMedianFee),
+      'halfHourFee': Math.max(minimumFee, secondMedianFee),
+      'hourFee': Math.max(minimumFee, thirdMedianFee),
+      'economyFee': Math.max(minimumFee, Math.min(2 * minimumFee, thirdMedianFee)),
       'minimumFee': minimumFee,
     };
   }