Add explicit context_id parameter to all hostcalls.

[PiotrSikora]

While there, add and consistently use UNKNOWN_RESOURCE_ID error
for when resources are accessed using an unknown ID.

[leonm1]

My understanding now is that the wasm code itself tells the host which context to use via the context_id parameter. Previously, it was implicit based on the last wasm callback the host invoked.

The context id is given to the wasm code during proxy_on_context_create, then is it the responsibility of the SDK to keep track of it, or is it on plugin authors? It would be helpful guidance to explain this in the description. Can you expand the PR description a bit to express what is changing with this spec change?

[PiotrSikora]

My understanding now is that the wasm code itself tells the host which context to use via the context_id parameter. Previously, it was implicit based on the last wasm callback the host invoked.

Correct (ignoring the proxy_set_effective_context hostcall).

The context id is given to the wasm code during proxy_on_context_create, then is it the responsibility of the SDK to keep track of it, or is it on plugin authors? It would be helpful guidance to explain this in the description. Can you expand the PR description a bit to express what is changing with this spec change?

Technically, they don't have to be tracked at all, since all callbacks already include resource ID (context_id, callout_id, kvstore_id, etc.)

Having said that, I assume that SDKs would provide objects that track the resource IDs and won't require users to operate directly on them.

As for the guidance, is a bit tricky, because ABI defines Host<>Wasm interface, and whatever happens on the Wasm side is not part of that contract. The plugin authors may or may not use any SDK at all. We could provide some guidance for the host implementations, but I think anything for the SDK users should be documented in the SDKs, since it may vary from SDK to SDK.

[PiotrSikora]

@mpwarres @leonm1 I believe all comments and discussion from the meetings have been addressed here (I left the comments open, so please resolve them if you're satisfied with the answers).

[leonm1]

Approved, with a few questions on hostcalls without context_id parameters.

[leonm1]

LGTM for merging.