fix: drain the req.body into io.Discard correctly (#17881)

internal/event/target/webhook.go

@@ -34,6 +34,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/dustin/go-humanize"
 	"github.com/minio/minio/internal/event"
 	"github.com/minio/minio/internal/logger"
 	"github.com/minio/minio/internal/once"
@@ -194,8 +195,7 @@ func (target *WebhookTarget) send(eventData event.Event) error {
 	if err != nil {
 		return err
 	}
-	defer resp.Body.Close()
-	io.Copy(io.Discard, resp.Body)
+	defer DrainBody(resp.Body)
 
 	if resp.StatusCode < 200 || resp.StatusCode > 299 {
 		return fmt.Errorf("sending event failed with %v", resp.Status)
@@ -297,3 +297,23 @@ func NewWebhookTarget(ctx context.Context, id string, args WebhookArgs, loggerOn
 
 	return target, nil
 }
+
+// DrainBody close non nil response with any response Body.
+// convenient wrapper to drain any remaining data on response body.
+//
+// Subsequently this allows golang http RoundTripper
+// to re-use the same connection for future requests.
+func DrainBody(respBody io.ReadCloser) {
+	// Callers should close resp.Body when done reading from it.
+	// If resp.Body is not closed, the Client's underlying RoundTripper
+	// (typically Transport) may not be able to re-use a persistent TCP
+	// connection to the server for a subsequent "keep-alive" request.
+	if respBody != nil {
+		// Drain any remaining Body and then close the connection.
+		// Without this closing connection would disallow re-using
+		// the same connection for future uses.
+		//  - http://stackoverflow.com/a/17961593/4465767
+		defer respBody.Close()
+		io.CopyN(io.Discard, respBody, 1*humanize.MiByte)
+	}
+}