rpc: avoid unnecessary RST_STREAM, PING frames sent by client (ethereum#33122)

Context from Cloudflare blog:
https://blog.cloudflare.com/go-and-enhance-your-calm/#reading-bodies-in-go-can-be-unintuitive

We were able to reproduce the same issue discussed by Cloudflare in
their recent blog post above using the `ethclient`.

Author: michaelkaplan13

rpc/http.go

@@ -168,13 +168,21 @@ func newClientTransportHTTP(endpoint string, cfg *clientConfig) reconnectFunc {
 	}
 }
 
+// cleanlyCloseBody avoids sending unnecessary RST_STREAM and PING frames by
+// ensuring the whole body is read before being closed.
+// See https://blog.cloudflare.com/go-and-enhance-your-calm/#reading-bodies-in-go-can-be-unintuitive
+func cleanlyCloseBody(body io.ReadCloser) error {
+	io.Copy(io.Discard, body)
+	return body.Close()
+}
+
 func (c *Client) sendHTTP(ctx context.Context, op *requestOp, msg interface{}) error {
 	hc := c.writeConn.(*httpConn)
 	respBody, err := hc.doRequest(ctx, msg)
 	if err != nil {
 		return err
 	}
-	defer respBody.Close()
+	defer cleanlyCloseBody(respBody)
 
 	var resp jsonrpcMessage
 	batch := [1]*jsonrpcMessage{&resp}
@@ -191,7 +199,7 @@ func (c *Client) sendBatchHTTP(ctx context.Context, op *requestOp, msgs []*jsonr
 	if err != nil {
 		return err
 	}
-	defer respBody.Close()
+	defer cleanlyCloseBody(respBody)
 
 	var respmsgs []*jsonrpcMessage
 	if err := json.NewDecoder(respBody).Decode(&respmsgs); err != nil {
@@ -236,7 +244,7 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 		if _, err := buf.ReadFrom(resp.Body); err == nil {
 			body = buf.Bytes()
 		}
-		resp.Body.Close()
+		cleanlyCloseBody(resp.Body)
 		return nil, HTTPError{
 			Status:     resp.Status,
 			StatusCode: resp.StatusCode,

rpc/http_test.go

@@ -96,7 +96,7 @@ func confirmHTTPRequestYieldsStatusCode(t *testing.T, method, contentType, body
 	if err != nil {
 		t.Fatalf("request failed: %v", err)
 	}
-	resp.Body.Close()
+	cleanlyCloseBody(resp.Body)
 	confirmStatusCode(t, resp.StatusCode, expectedStatusCode)
 }