add option to retain fetchEvent handler

[karthik2804]

This PR adds the ability to use the fetchEvent Handler provided by StarlingMonkey.

fixes #114

[guybedford]

Can we also add this to the readme?

[guybedford]

I wonder if it would make sense to integrate this into the enableFeatures model and call it fetch-event?

[karthik2804]

Oooh! I like the idea of adding it via the enableFeatures. I will also add it to the readme.

[karthik2804]

I think this should wait until we get the fetch PR, so leaving it as a draft.

[guybedford]

Just like we automatically merge WASI imports into the target world, perhaps we can automatically merge the wasi:http/[email protected] export into the target world when this feature is used?

[guybedford]

Actually isn't this the default behaviour anyway? Or do we still strip the export if it's not explicitly in the target world?

[karthik2804]

With this PR we want to only optionally strip the export from the StarlingMonkey engine here based on the value of the feature. My intention with that statement is to make it such that, if the target world does not export the interface but the feature is enabled, we should probably still strip the export.

[guybedford]

Shouldn't it read "If the target world does export" though? Rather than "does not". Thinking about this further we probably want to make this an explicit error. That is, if you enable this feature and the target world already exports the incoming handler, then we should throw an error that you should either target the incoming handler export, or you should use the fetch event version of it, but not both.

[karthik2804]

I think there is a disconnect in what we each mean by the target world. I am talking about the wit definition, not the guest content. As in, consider the following world

package local:hello;

world hello {
  export hello: func(name: string) -> string;
}

and the fetch-event is set, if we do not strip the export from the engine, it will end up in the output component but maybe that is fine?

I also think that there is currently no way to check if a handler is attached to the fetch event but we can check if there is something that targets the incoming handler export so erroring on that would be good, I agree.

[karthik2804]

Maybe I am misunderstanding here. To verify my understanding - the proposal above is to not retain fetchEvent incase the target world contains the wasi:http/incoming-handler export and only have the feature take effect if it is not explicitly part of the target world.

[tschneidereit]

Oh, I see—I had missed that part of it. On that, I agree with you: I think this setting should be usable whether the targeted world includes incoming-handler or not.

@guybedford doing what you proposed would make it substantially harder to use FetchEvent, because now anyone who wants to target an environment that supports wasi:http/proxy has to edit the target world not to include incoming-handler if they want to use FetchEvent. I think that's a requirement we really shouldn't put on people—and it'd be quite hard for us to satisfy using Spin as a development tool.

[karthik2804]

I am happy to update this PR once there is consensus on this discussion.

[tschneidereit]

Guy and I just discussed this further, and came to the realization that there's a cleaner way to do this: with #117, ComponentizeJS will only generate exports for symbols exported at the JS level, too. Based on that, we could simply say "if after treeshaking we still would export the symbol, then we'll do so. Otherwise, we'll leave it alone."

In effect, that'd mean that if the input StarlingMonkey already includes an export, and content doesn't specify that same export, ComponentizeJS will leave the original export alone.

@karthik2804, if that makes sense to you, then changing #117 accordingly should make it so that this issue isn't needed anymore.

[karthik2804]

I think I am roughly tracking it. Currently, #117 is purely for imports. I will take a stab at updating it and see if I understand things correctly and report back.