Disallow blocking nested workflow

[BoPeng]

#1167 Implements a way to run nested workflows concurrently, with the limitations that only workflows that are the last statement can be concurrent, and multiple workflows in sos_riun(['A', 'B']) can be concurrent but blocks the step executor.

However, with the new strict -j worker implementation, sos_run('A') not as the last statement will make the worker wait for the completion of A,

sos_run('A')
other_func()

So in the case of -j1, sos_run('A') will take one worker and the workflow will hang forever.

I would suggest that we disallow the execution of blocking nested workflow so that there will be no waiting process.

[gaow]

I'm not sure if I understand it.

multiple workflows in sos_riun(['A', 'B']) can be concurrent but blocks the step executor.

block means to hang until completion of both A and B?

I would suggest that we disallow the execution of blocking nested workflow so that there will be no waiting process.

Will it impact parallel execution of sos_riun(['A', 'B'])?

[BoPeng]

OK, In the case of

whatever
sos_run('A')

or

whatever
sos_run(['A', 'B'])

the step statement can be completed (no waiting) and the step can yield (do something else) while waiting for the completion of workflows A and B. In the case of

sos_run('A')
whatever

or

sos_run(['A', 'B'])
whatever

the statement will send out workflows to be executed, but the step is blocking so nothing else can be run on the worker. Basically I am trying to make all sos statements non-blocking so that they can use the workers more efficiently and prevent deadlock.

[BoPeng]

OK, right now I just give a warning because I have some workflows like

sos_run('A')
sos_run('B')
sos_run('C')

which should be written as

sos_run(['A', 'B', 'C'])

or

sos_run('A + B + C')

if execution order matters but a warning will let the workflows run.

[BoPeng]

Final solutIon: sos will create a separate process for each blocking subworkflow, and shut it down when the workflow is completed. This means there is still a chance for sos to have more than -j + 1 processes but this is better than having sos hang forever.