not ok 26 - attach: --stdin-ranks works

[chu11]

While churning through CI runs in #6544 hit this failure several times

expecting success: 
  	id=$(flux submit -N4 -o exit-timeout=none -t60s cat) &&
  	echo hello from 0 \
  		| flux job attach -vEX --label-io -i0 $id >stdin-ranks.out &&
  	flux job eventlog -p guest.input $id &&
  	cat <<-EOF >stdin-ranks.expected &&
  	0: hello from 0
  	EOF
  	test_cmp stdin-ranks.expected stdin-ranks.out

  0.000s: job.submit {"userid":1001,"urgency":16,"flags":0,"version":1}
  0.015s: job.validate
  0.027s: job.depend
  0.027s: job.priority {"priority":16}
  0.029s: job.alloc {"annotations":{"sched":{"resource_summary":"rank[0-3]/core[0-1]"}}}
  0.051s: job.start
  0.032s: exec.init
  0.035s: exec.starting
  0.091s: exec.shell.init {"service":"1001-shell-f8AVminB","leader-rank":0,"size":4}
  0.100s: exec.shell.start {"taskmap":{"version":1,"map":[[0,4,1,1]]}}
  0.107s: exec.shell.task-exit {"localid":0,"rank":0,"state":"Exited","pid":172646,"wait_status":0,"signaled":0,"exitcode":0}
  60.052s: job.exception f8AVminB type=timeout severity=0 resource allocation expired
  60.071s: exec.complete {"status":36352}
  60.071s: exec.done
  flux-job: task(s) Alarm clock
  60.071s: job.finish {"status":36352}
  60.077s: job.release {"ranks":"all","final":true}
  60.077s: job.free
  60.077s: job.clean
  not ok 26 - attach: --stdin-ranks works

[chu11]

huh, I just noticed

  0.091s: exec.shell.init {"service":"1001-shell-f8AVminB","leader-rank":0,"size":4}
  0.100s: exec.shell.start {"taskmap":{"version":1,"map":[[0,4,1,1]]}}
  0.107s: exec.shell.task-exit {"localid":0,"rank":0,"state":"Exited","pid":172646,"wait_status":0,"signaled":0,"exitcode":0}
  60.052s: job.exception f8AVminB type=timeout severity=0 resource allocation expired
  60.071s: exec.complete {"status":36352}

It appears the task exited immediately, but somehow the shell lingered until the 60s timeout?

Unclear if the stdin "reached" the cat.

Perhaps some very subtle racy thing involving stdin and a shell that isn't ready for it??