This causes an INTERRUPT to happen, which is useful to see in
practice.

When adding a sleep to the READ operation, this yields the following
trace:

 rx 28: OPEN i2 {O_RDONLY,0x8000} 
 tx 28:     OK, {Fh 1 }
 rx 30: READ i2 {Fh 1 [0 +4096)  L 0 RDONLY,0x8000} 
 killing subprocess
 rx 32: FLUSH i2 {Fh 1} 
 tx 32:     OK
 rx 33: INTERRUPT i0 {ix 32} 
 tx 33:     OK
 tx 30:     OK,  1024b data "********"...
 rx 34: RELEASE i2 {Fh 1 0x8000  L0} 
 tx 34:     OK


ie. rather than canceling the READ, a FLUSH is issued which is then
INTERRUPTed.