Assign background transaction futures to the tracking array so they can be cancelled

CHANGELOG.md

@@ -23,6 +23,7 @@
 - Update to tuweni 2.7.2 [#9338](https://github.com/hyperledger/besu/pull/9338)
 
 ### Bug fixes
+- Performance: Fix CompletableFuture tracking to correctly cancel parallel txs threads [#9360](https://github.com/hyperledger/besu/pull/9360)
 
 ## 25.10.0
 This is a recommended update for Hoodi users for the Fusaka hardfork.

ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/parallelization/ParallelizedConcurrentTransactionProcessor.java

@@ -116,18 +116,19 @@ public void runAsyncBlock(
       /*
        * All transactions are executed in the background by copying the world state of the block on which the transactions need to be executed, ensuring that each one has its own accumulator.
        */
-      CompletableFuture.runAsync(
-          () ->
-              runTransaction(
-                  protocolContext,
-                  blockHeader,
-                  transactionLocation,
-                  transaction,
-                  miningBeneficiary,
-                  blockHashLookup,
-                  blobGasPrice,
-                  blockAccessListBuilder),
-          executor);
+      completableFuturesForBackgroundTransactions[i] =
+          CompletableFuture.runAsync(
+              () ->
+                  runTransaction(
+                      protocolContext,
+                      blockHeader,
+                      transactionLocation,
+                      transaction,
+                      miningBeneficiary,
+                      blockHashLookup,
+                      blobGasPrice,
+                      blockAccessListBuilder),
+              executor);
     }
   }