wrong usage of gRPC interface

[lance6716]

I see stack is attached to raw gRPC error io.EOF when client is closed

pd/server/grpc_service.go

Lines 1045 to 1052 in f18af79

	func (b *bucketHeartbeatServer) recv() (*pdpb.ReportBucketsRequest, error) {
	if atomic.LoadInt32(&b.closed) == 1 {
	return nil, io.EOF
	}
	req, err := b.stream.Recv()
	if err != nil {
	atomic.StoreInt32(&b.closed, 1)
	return nil, errors.WithStack(err)

but at caller it directly compares with io.EOF, so the extra stack fails the comparison

pd/server/grpc_service.go

Lines 1079 to 1084 in f18af79

	request, err := server.recv()
	failpoint.Inject("grpcClientClosed", func() {
	err = errs.ErrStreamClosed
	request = nil
	})
	if err == io.EOF {