Changed WithRateLimiter costFunc-arg to include the methods of al…

…l calls (#122)

* When rate limiting, include cost of batch request itself too.

This allows the cost func to provide a cost for the batch itself in
batch requests. Some providers (Infura) include this as part of their
rate limiting.

Ref: https://docs.infura.io/api/networks/ethereum/how-to/make-batch-requests

* In WithRateLimiter, change costFunc signature to receive all batch methods at once.

* simplified costFunc-example and fixed comment

* small cleanup

---------

Co-authored-by: lmittmann <lmittmann@users.noreply.github.com>
Signed-off-by: Mohammed Sohail <sohailsameja@gmail.com>

client.go

@@ -21,7 +21,7 @@ type Client struct {
 
 	// rate limiter
 	rl         *rate.Limiter
-	rlCostFunc func(method string) (cost int)
+	rlCostFunc func(methods []string) (cost int)
 }
 
 // NewClient returns a new Client given an rpc.Client client.
@@ -148,10 +148,11 @@ func (c *Client) rateLimit(ctx context.Context, batchElems []rpc.BatchElem) erro
 	}
 
 	// limit requests based on Compute Units (CUs)
-	var cost int
-	for _, batchElem := range batchElems {
-		cost += c.rlCostFunc(batchElem.Method)
+	methods := make([]string, len(batchElems))
+	for i, batchElem := range batchElems {
+		methods[i] = batchElem.Method
 	}
+	cost := c.rlCostFunc(methods)
 	return c.rl.WaitN(ctx, cost)
 }
 
@@ -192,8 +193,9 @@ type Option func(*Client)
 // WithRateLimiter sets the rate limiter for the client. Set the optional argument
 // costFunc to nil to limit the number of requests. Supply a costFunc to limit
 // the the number of requests based on individual RPC calls for advanced rate
-// limiting by Compute Units (CUs).
-func WithRateLimiter(rl *rate.Limiter, costFunc func(method string) (cost int)) Option {
+// limiting by e.g. Compute Units (CUs). Note that only if len(methods) > 1, the
+// calls are sent in a batch request.
+func WithRateLimiter(rl *rate.Limiter, costFunc func(methods []string) (cost int)) Option {
 	return func(c *Client) {
 		c.rl = rl
 		c.rlCostFunc = costFunc

client_test.go

@@ -373,3 +373,19 @@ func ExampleWithRateLimiter() {
 	)
 	defer client.Close()
 }
+
+func ExampleWithRateLimiter_costFunc() {
+	// Limit the client to 30 calls per second and allow bursts of up to
+	// 100 calls using a cost function. Batch requests have an additional charge.
+	client := w3.MustDial("https://rpc.ankr.com/eth",
+		w3.WithRateLimiter(rate.NewLimiter(rate.Every(time.Second/30), 100),
+			func(methods []string) (cost int) {
+				cost = len(methods) // charge 1 CU per call
+				if len(methods) > 1 {
+					cost += 1 // charge 1 CU extra for the batch itself
+				}
+				return cost
+			},
+		))
+	defer client.Close()
+}