system instance "dry run" option

[chu11]

before loading (or re-loading) a new set of configs for a system instance, it would be nice to be able to 'dry run' the configs to ensure they parse correctly, seem to make sense, etc.

My initial thought was to support some "dry run" config option that could effectively run sleep 0 as the initial process, thus exiting immediately if everything worked. However, this technique probably wouldn't work for testing flux config reload.

not sure if a dry-run option could be parsed during flux config reload, and the broker would know "do not save this config, just report errors" or something.

just initial thoughts, haven't thought much more than this

[garlick]

Hmm, good thought. The bummer is that config is parsed separately by different components. If any part of a flux config reload fails, the caller will get an error but the "good parts" of the config would have been accepted. IOW a config update does not atomically roll back to the previous config on failure. Maybe it should.

I guess for a dry run there would need to be a flag passed with the config update to tell each component not to apply the change, just to check it. Not a bad idea!