libsubprocess: increase priority of output check watcher

[chu11]

per conversation in #6281, consider raising the priority of the output check watcher. This can ensure that a user has a chance to empty a full buffer before more data is put into the buffer.

[garlick]

This is just something to try to clean things up before we do real flow control in this direction.

My prototype priority setter is here: 1932ce5

Just call flux_watcher_set_priority (check_watcher, 1);

That should raise the priority from the default of 0 to 1. if that doesn't work try 2 :-) 2 is the max.

It has to be called before the watcher is started.

[chu11]

it appears to do what we want it to do. some debug printfs

before

remote_output_buffered:434 len data = 128
remote_out_prep_cb:179
remote_output_buffered:434 len data = 128
remote_output_buffered:456 - call output with 128 bytes  ***
remote_out_check_cb:191
remote_out_check_cb:200 - call output with 128 bytes ***
remote_output_buffered:434 len data = 0
remote_out_prep_cb:179
remote_out_check_cb:191
remote_out_check_cb:207 - call output with EOF

asterisks lines show that we call the output callback in the output buffered function and later the check output function

after

remote_output_buffered:434 len data = 128
remote_out_prep_cb:179
remote_out_check_cb:191
remote_out_check_cb:200 - call output with 128 bytes ***
remote_output_buffered:434 len data = 128 XXX
remote_out_prep_cb:179 XXX
remote_out_check_cb:191
remote_out_check_cb:200 - call output with 128 bytes ***
remote_output_buffered:434 len data = 0
remote_out_prep_cb:179
remote_out_check_cb:191
remote_out_check_cb:207 - call output with EOF

asterisks lines show that we call the output callback in the check callback both times. the XXX lines show that output is re-added to the buffer after the check and we've started a new prep iteration.