In the SPANK plugin pyxis (https://github.com/NVIDIA/pyxis) we use the spank_option_register function of the SPANK API to register our options callbacks (of type https://github.com/SchedMD/slurm/blob/slurm-20-11-1-1/slurm/spank.h#L209-L216)

For example, we check if one argument is not passed an empty string here:
https://github.com/NVIDIA/pyxis/blob/c8f732fe77b0da39a1fe1a44d87897e265872837/args.c#L116-L119

And it aborst the job launch as expected in Slurm 20.02:

> $ srun -A admin -p admin -N1 --container-image= hostname ; echo $?
> srun: error: pyxis: --container-image: argument required
> srun: error: Invalid --container-image argument: 
> 255

After upgrading to 20.11 on a test system, we noticed that we can't filter out invalid arguments, the job proceeds and executes normally:

> $ srun -A admin -p admin -N1 --container-image= hostname ; echo $?
> srun: error: pyxis: --container-image: argument required
> srun: error: Invalid --container-image argument: 
> sc-sdgx-735
> 0

After a git bisect:

> $ git bisect good
> e9bb9ce288c6284a801b2bd4422d519da6751783 is the first bad commit
> commit e9bb9ce288c6284a801b2bd4422d519da6751783
> Author: Tim Wickberg <tim@schedmd.com>
> Date:   Fri Oct 23 02:07:43 2020 -0600
> 
>    Quietly error out when an option is unknown.
    
>    Let the rest of the argument handling figure this out.

> src/common/plugstack.c | 2 +-
> src/common/slurm_opt.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
 
It's unclear to me what is meant by "Let the rest of the argument handling figure this out" since spank_process_option is only called here in the Slurm code and the return value of slurm_process_option is ignored in most places.

Given that this is not mentioned in the release notes and that the commit message mentions "... when an option is unknown", it looks like this new behavior was not intended. Here the option is not unknown, it's a valid SPANK option rejected by our callback.