t.Errorf("data mismatch, got %s, want %s", dstData, data)
	}
}

// newSendFileTest initializes a new test for sendfile.
//
// It creates source file and destination sockets, and populates the source file
// with random data of the specified size. It also hooks package os' call
// to poll.Sendfile and returns the hook so it can be inspected.