I've rebased #2014 on this. One questions I still have:

Previously we updated dependents' references to the reloaded module. None of my tests are failing without it, should we leave this part out? We need a reference to the load wrapper in order to do this.

I found another scenario we need to account for.

If module a has an execution error we don't return an update function. This might be ok if the error is on the initial load, but if the module goes from success -> error -> success we lose reference to the original importSetters.

Similarly when a module reloads that has an update function and it failed to execute we don't update the importSetters either, so we lose that information for the next reload.

Previously we updated dependents' references to the reloaded module. None of my tests are failing without it, should we leave this part out? We need a reference to the load wrapper in order to do this.

You mean the reference in the load dependencies? That isn't used after execution, so should be fine.

If module a has an execution error we don't return an update function. This might be ok if the error is on the initial load, but if the module goes from success -> error -> success we lose reference to the original importSetters.

Similarly when a module reloads that has an update function and it failed to execute we don't update the importSetters either, so we lose that information for the next reload.

Can we not keep the previously working update function around in these case when using the API? That is only apply the update function when the module is known to have executed successfully, and otherwise keep it around until it does.

That might work indeed, I will try it out.

I've added a commit here to handle the System.set issues discussed in #1976.

@guybedford can you explain the rational for changing System.has to return true for in-progress changes?

Prior to that change System.get and System.has returned the same values, after the change, only has returns true for in-progress modules.

@khangsfdc hmm that sounds weird to me... maybe we should reconsider it?