rpc: linear time batch response matching (#23856)

This avoids quadratic time complexity in the lookup of the batch element
corresponding to an RPC response. Unfortunately, the new approach
requires additional memory for the mapping from ID to index.

Fixes #22805

Author: fjl

rpc/client.go

@@ -17,7 +17,6 @@
 package rpc
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -360,7 +359,10 @@ func (c *Client) BatchCall(b []BatchElem) error {
 //
 // Note that batch calls may not be executed atomically on the server side.
 func (c *Client) BatchCallContext(ctx context.Context, b []BatchElem) error {
-	msgs := make([]*jsonrpcMessage, len(b))
+	var (
+		msgs = make([]*jsonrpcMessage, len(b))
+		byID = make(map[string]int, len(b))
+	)
 	op := &requestOp{
 		ids:  make([]json.RawMessage, len(b)),
 		resp: make(chan *jsonrpcMessage, len(b)),
@@ -372,6 +374,7 @@ func (c *Client) BatchCallContext(ctx context.Context, b []BatchElem) error {
 		}
 		msgs[i] = msg
 		op.ids[i] = msg.ID
+		byID[string(msg.ID)] = i
 	}
 
 	var err error
@@ -391,13 +394,7 @@ func (c *Client) BatchCallContext(ctx context.Context, b []BatchElem) error {
 		// Find the element corresponding to this response.
 		// The element is guaranteed to be present because dispatch
 		// only sends valid IDs to our channel.
-		var elem *BatchElem
-		for i := range msgs {
-			if bytes.Equal(msgs[i].ID, resp.ID) {
-				elem = &b[i]
-				break
-			}
-		}
+		elem := &b[byID[string(resp.ID)]]
 		if resp.Error != nil {
 			elem.Error = resp.Error
 			continue