Improve error message from MGS when attempting to set wrong active slot

[karencfv]

When attempting to set the wrong RoT bootloader slot as active, I get the following response from MGS

error: Error Response: status: 400 Bad Request; headers: {"content-type": "application/json", "x-request-id": "f843617f-4a97-4e01-8943-920f33b9c38f", "content-length": "257", "date": "Tue, 17 Jun 2025 00:42:33 GMT"}; value: Error { error_code: Some("RequestUnsupportedForComponent"), message: "error communicating with SP SpIdentifier { typ: Switch, slot: 0 }: Error response from SP: unsupported request for this SP component", request_id: "f843617f-4a97-4e01-8943-920f33b9c38f" }

Or from faux-mgs

$ ./target/debug/faux-mgs --sp-sim-addr [::1]:48938 component-active-slot stage0 --set 0 -p
Jun 17 01:08:53.054 INFO creating SP handle on to talk to SP simulator at [::1]:48938, component: faux-mgs
Jun 17 01:08:53.054 INFO initial discovery complete, addr: [::1]:48938, component: faux-mgs
Error: Error response from SP: unsupported request for this SP component

This message implies that you can't set the active slot for the RoT bootloader via MGS, when in reality you're just trying to set the wrong slot.

Working fine when you set the correct slot :)

$ ./target/debug/faux-mgs --sp-sim-addr [::1]:48938 component-active-slot stage0 --set 1 -p
Jun 17 01:09:09.211 INFO creating SP handle on to talk to SP simulator at [::1]:48938, component: faux-mgs
Jun 17 01:09:09.211 INFO initial discovery complete, addr: [::1]:48938, component: faux-mgs
set active slot for SpComponent { id: "stage0" } to 1

[karencfv]

This issue may belong in Hubris 🤔

[jgallagher]

I agree the error message is pretty sparse, but I don't think it's wrong, exactly, is it? Trying to set slot 0 is indeed not supported for the stage0 component (whereas it is supported for other components). It's not that the type of the request is not supported, it's that this particular request is not supported (because of one of its arguments).

I could see doing any of these things:

1. Do nothing; the error is a little confusing, but it's not an error we actually need to be able to detect at runtime and act on; it indicates the caller tried to do something that can never possibly work, which it just shouldn't ever try.
2. Add a more detailed kind of error response from the SP. This would require changes in both the MGS repo, to add the error variant, and hubris, to use it.
3. Add some special case handling for this error at the MGS layer (technically the gateway-sp-comms crate, which is used by both real MGS and faux-mgs) to convert the "unsupported request" error from the SP into something that explains why the request is unsupported. This would only require changes in the MGS repo, not hubris.

[karencfv]

I agree the error message is pretty sparse, but I don't think it's wrong, exactly, is it? Trying to set slot 0 is indeed not supported for the stage0 component (whereas it is supported for other components). It's not that the type of the request is not supported, it's that this particular request is not supported (because of one of its arguments).

Yeah, definitely not wrong. It is a little vague though. TBH I spent more time than I wish to admit trying to figure out why setting an active slot wasn't supported for stage0 until I realised that I had the wrong slot. If the error message had been a little more specific, I would have known what was going on right away. Basically, this is just a developer experience improvement.

Looks like the fix is a bit more convoluted than I anticipated. Perhaps this is an issue that doesn't need to be worked on now, but I'd like to keep it open for when I or someone has a few spare cycles for dev experience improvements :)