migrate exec hangs for several minutes if the pipe is closed before all its data is written

Binary package hint: kvm

Using

  migrate "exec:true"

in the monitor hangs the VM for several minutes. What I expect is that the VM stops attempting to migrate after the pipe has been closed.

Indicating a background migrate with -d doesn't help. Presumably the migration is not backgrounded until a certain amount of data is written to the pipe, or the migration times out What I expect is that the migration is backgrounded immediately.