Attach API supports streaming output (see JDK-8319055).
In some test scenarios a test runs jcmd against main test process and analyzes jcmd output. In the scenarios streaming output can cause hung (attach operation is executed in safepoint and produces lengthy output, jcmd writes the operation reply to its output, but main process cannot drain redirected stream as it's in the safepoint).

Jcmd should allow to specify streaming/buffered output mode.