perf: Make mempool update async from block.Commit (#3008)

.changelog/v0.38.6/features/3008-mempool-async-update.md

@@ -0,0 +1,3 @@
+- `[consensus]` Make mempool updates asynchronous from consensus Commit's,
+  reducing latency for reaching consensus timeouts.
+  ([#3008](https://github.com/cometbft/cometbft/pull/3008))

spec/abci/abci++_app_requirements.md

@@ -297,7 +297,9 @@ will be received on the mempool connection during this processing step, providin
 update all four
 connection states to the latest committed state at the same time.
 
-When `Commit` returns, CometBFT unlocks the mempool.
+CometBFT unlocks the mempool after it has finished updating for the new block,
+which occurs asynchronously from `Commit`.
+See [Mempool Update](../mempool/mempool.md) for more information on what the `update` task does.
 
 WARNING: if the ABCI app logic processing the `Commit` message sends a
 `/broadcast_tx_sync` or `/broadcast_tx` and waits for the response

state/execution.go

@@ -375,34 +375,38 @@ func (blockExec *BlockExecutor) VerifyVoteExtension(ctx context.Context, vote *t
 	return nil
 }
 
-// Commit locks the mempool, runs the ABCI Commit message, and updates the
+// Commit locks the mempool, runs the ABCI Commit message, and asynchronously starts updating the
 // mempool.
-// It returns the result of calling abci.Commit which is the height to retain (if any)).
+// Commit returns the result of calling abci.Commit which is the height to retain (if any)).
 // The application is expected to have persisted its state (if any) before returning
 // from the ABCI Commit call. This is the only place where the application should
 // persist its state.
 // The Mempool must be locked during commit and update because state is
 // typically reset on Commit and old txs must be replayed against committed
 // state before new txs are run in the mempool, lest they be invalid.
+// The mempool is unlocked when the Update routine completes, which is
+// asynchronous from Commit.
 func (blockExec *BlockExecutor) Commit(
 	state State,
 	block *types.Block,
 	abciResponse *abci.ResponseFinalizeBlock,
 ) (int64, error) {
 	blockExec.mempool.Lock()
-	defer blockExec.mempool.Unlock()
+	unlockMempool := func() { blockExec.mempool.Unlock() }
 
 	// while mempool is Locked, flush to ensure all async requests have completed
 	// in the ABCI app before Commit.
 	err := blockExec.mempool.FlushAppConn()
 	if err != nil {
-		blockExec.logger.Error("client error during mempool.FlushAppConn", "err", err)
+		unlockMempool()
+		blockExec.logger.Error("client error during mempool.FlushAppConn, flushing mempool", "err", err)
 		return 0, err
 	}
 
 	// Commit block, get hash back
 	res, err := blockExec.proxyApp.Commit(context.TODO())
 	if err != nil {
+		unlockMempool()
 		blockExec.logger.Error("client error during proxyAppConn.CommitSync", "err", err)
 		return 0, err
 	}
@@ -415,15 +419,36 @@ func (blockExec *BlockExecutor) Commit(
 	)
 
 	// Update mempool.
-	err = blockExec.mempool.Update(
+	go blockExec.asyncUpdateMempool(unlockMempool, block, state.Copy(), abciResponse)
+
+	return res.RetainHeight, nil
+}
+
+// updates the mempool with the latest state asynchronously.
+func (blockExec *BlockExecutor) asyncUpdateMempool(
+	unlockMempool func(),
+	block *types.Block,
+	state State,
+	abciResponse *abci.ResponseFinalizeBlock,
+) {
+	defer unlockMempool()
+
+	err := blockExec.mempool.Update(
 		block.Height,
 		block.Txs,
 		abciResponse.TxResults,
 		TxPreCheck(state),
 		TxPostCheck(state),
 	)
-
-	return res.RetainHeight, err
+	if err != nil {
+		// We panic in this case, out of legacy behavior. Before we made the mempool
+		// update complete asynchronously from Commit, we would panic if the mempool
+		// update failed. This is because we panic on any error within commit.
+		// We should consider changing this behavior in the future, as there is no
+		// need to panic if the mempool update failed. The most severe thing we
+		// would need to do is dump the mempool and restart it.
+		panic(fmt.Sprintf("client error during mempool.Update; error %v", err))
+	}
 }
 
 //---------------------------------------------------------