Ownership model for streams is not clear.

[brendandburns]

When reading from a stream, it seems that the list that is returned from a stream is owned by the caller, but that is not clearly spelled out in the documentation.

[lukewagner]

The ownership of memory is not something specific to the wasi-http/wasi-io proposals, but rather defined by the Canonical ABI based purely on the types involved. For list<u8>, if you look at store_list in CanonicalABI.md, it describes how the core module's exported cabi_realloc function is called to allocate the memory, which is written into by the host, and then the pointer is returned to the core module. The C headers for the ownership semantics could be generated per-interface by wit-bindgen, though.

[brendandburns]

I was suggesting that the streams wit definition should add clarity in the comments about how this is expected to work.

[lukewagner]

Improving documentation is a good idea, but I don't know if the Wit document is the right place since this ends up being for many guest/host languages, a bindings-internal detail (e.g., JS will simply see a JS array pop out). But perhaps this could be usefully generated as part of the ABI documentation or, perhaps better, in the generated C header comments. @sunfishcode was thinking about this too recently, so that we're not having to send folks to read Canonical ABI all the time.

[brendandburns]

In garbage collected languages, the ownership is clearly not as important, but in non-gc languages like C it is a question that someone asks whenever they see a pointer as a return value.

[lukewagner]

Even for explicitly-managed languages like C++/Rust, the ownership transfer should be made explicit in the return types of the generated bindings (e.g., a std::vector return-by-value). Also, when wasm-gc is integrated by adding new canonopts, the core wasm signature won't even return an i32 pointer, but rather some sort of wasm-GC reference type. Thus, this does seem to be something most appropriate in a generated C header file or perhaps a generated ABI doc.