Improve stability of mempool tx position arrow

backend/src/api/mempool-blocks.ts

@@ -252,9 +252,17 @@ class MempoolBlocks {
 
   private processBlockTemplates(mempool, blocks, clusters, saveResults): MempoolBlockWithTransactions[] {
     // update this thread's mempool with the results
-    blocks.forEach(block => {
+    blocks.forEach((block, blockIndex) => {
+      let runningVsize = 0;
       block.forEach(tx => {
         if (tx.txid && tx.txid in mempool) {
+          // save position in projected blocks
+          mempool[tx.txid].position = {
+            block: blockIndex,
+            vsize: runningVsize + (mempool[tx.txid].vsize / 2),
+          };
+          runningVsize += mempool[tx.txid].vsize;
+
           if (tx.effectiveFeePerVsize != null) {
             mempool[tx.txid].effectiveFeePerVsize = tx.effectiveFeePerVsize;
           }

backend/src/api/tx-selection-worker.ts

@@ -2,7 +2,6 @@ import config from '../config';
 import logger from '../logger';
 import { ThreadTransaction, MempoolBlockWithTransactions, AuditTransaction } from '../mempool.interfaces';
 import { PairingHeap } from '../utils/pairing-heap';
-import { Common } from './common';
 import { parentPort } from 'worker_threads';
 
 let mempool: { [txid: string]: ThreadTransaction } = {};
@@ -72,7 +71,14 @@ function makeBlockTemplates(mempool: { [txid: string]: ThreadTransaction })
   }
 
   // Sort by descending ancestor score
-  mempoolArray.sort((a, b) => (b.score || 0) - (a.score || 0));
+  mempoolArray.sort((a, b) => {
+    if (b.score === a.score) {
+      // tie-break by lexicographic txid order for stability
+      return a.txid < b.txid ? -1 : 1;
+    } else {
+      return (b.score || 0) - (a.score || 0);
+    }
+  });
 
   // Build blocks by greedily choosing the highest feerate package
   // (i.e. the package rooted in the transaction with the best ancestor score)
@@ -80,7 +86,14 @@ function makeBlockTemplates(mempool: { [txid: string]: ThreadTransaction })
   let blockWeight = 4000;
   let blockSize = 0;
   let transactions: AuditTransaction[] = [];
-  const modified: PairingHeap<AuditTransaction> = new PairingHeap((a, b): boolean => (a.score || 0) > (b.score || 0));
+  const modified: PairingHeap<AuditTransaction> = new PairingHeap((a, b): boolean => {
+    if (a.score === b.score) {
+      // tie-break by lexicographic txid order for stability
+      return a.txid > b.txid;
+    } else {
+      return (a.score || 0) > (b.score || 0);
+    }
+  });
   let overflow: AuditTransaction[] = [];
   let failures = 0;
   let top = 0;

backend/src/api/websocket-handler.ts

@@ -66,9 +66,10 @@ class WebsocketHandler {
           if (parsedMessage && parsedMessage['track-tx']) {
             if (/^[a-fA-F0-9]{64}$/.test(parsedMessage['track-tx'])) {
               client['track-tx'] = parsedMessage['track-tx'];
+              const trackTxid = client['track-tx'];
               // Client is telling the transaction wasn't found
               if (parsedMessage['watch-mempool']) {
-                const rbfCacheTxid = rbfCache.getReplacedBy(client['track-tx']);
+                const rbfCacheTxid = rbfCache.getReplacedBy(trackTxid);
                 if (rbfCacheTxid) {
                   response['txReplaced'] = {
                     txid: rbfCacheTxid,
@@ -76,7 +77,7 @@ class WebsocketHandler {
                   client['track-tx'] = null;
                 } else {
                   // It might have appeared before we had the time to start watching for it
-                  const tx = memPool.getMempool()[client['track-tx']];
+                  const tx = memPool.getMempool()[trackTxid];
                   if (tx) {
                     if (config.MEMPOOL.BACKEND === 'esplora') {
                       response['tx'] = tx;
@@ -100,6 +101,13 @@ class WebsocketHandler {
                   }
                 }
               }
+              const tx = memPool.getMempool()[trackTxid];
+              if (tx && tx.position) {
+                response['txPosition'] = {
+                  txid: trackTxid,
+                  position: tx.position,
+                };
+              }
             } else {
               client['track-tx'] = null;
             }
@@ -401,9 +409,10 @@ class WebsocketHandler {
       }
 
       if (client['track-tx']) {
+        const trackTxid = client['track-tx'];
         const outspends: object = {};
         newTransactions.forEach((tx) => tx.vin.forEach((vin, i) => {
-          if (vin.txid === client['track-tx']) {
+          if (vin.txid === trackTxid) {
             outspends[vin.vout] = {
               vin: i,
               txid: tx.txid,
@@ -426,6 +435,14 @@ class WebsocketHandler {
         if (rbfChange) {
           response['rbfInfo'] = rbfChanges.trees[rbfChange];
         }
+
+        const mempoolTx = newMempool[trackTxid];
+        if (mempoolTx && mempoolTx.position) {
+          response['txPosition'] = {
+            txid: trackTxid,
+            position: mempoolTx.position,
+          };
+        }
       }
 
       if (client['track-mempool-block'] >= 0) {
@@ -556,8 +573,19 @@ class WebsocketHandler {
         response['mempool-blocks'] = mBlocks;
       }
 
-      if (client['track-tx'] && txIds.indexOf(client['track-tx']) > -1) {
-        response['txConfirmed'] = true;
+      if (client['track-tx']) {
+        const trackTxid = client['track-tx'];
+        if (txIds.indexOf(trackTxid) > -1) {
+          response['txConfirmed'] = true;
+        } else {
+          const mempoolTx = _memPool[trackTxid];
+          if (mempoolTx && mempoolTx.position) {
+            response['txPosition'] = {
+              txid: trackTxid,
+              position: mempoolTx.position,
+            };
+          }
+        }
       }
 
       if (client['track-address']) {

backend/src/mempool.interfaces.ts

@@ -80,6 +80,10 @@ export interface TransactionExtended extends IEsploraApi.Transaction {
   bestDescendant?: BestDescendant | null;
   cpfpChecked?: boolean;
   deleteAfter?: number;
+  position?: {
+    block: number,
+    vsize: number,
+  };
 }
 
 export interface AuditTransaction {