fix(wasm): wasi:sockets TCP cannot bind — finish-connect and accept return too many values

[xepozz]

The wasi:sockets TCP host cannot be bound by any guest. Two methods return their payload as several bare values, which the canonical-ABI lowering has no way to accept, so every component that imports them fails to load.

The failure

A WASI Preview 2 component importing wasi:sockets/tcp fails at host binding:

[host] registration: register wasi:sockets/tcp@0.2.8#[method]tcp-socket.finish-connect
  (caused by: handler validation: result count mismatch: expected 1, got 3)

accept reports expected 1, got 4. Reproduced with a Rust wasm32-wasip2 component and, independently, with a Go-authored one — the guest cannot influence this.

Why it can never bind

CanonRegistry.processLower caps the lowered results at one (lower.Results = []wit.Type{result}), which is what the canonical ABI requires: a result that does not fit one flat value comes back through a retptr.

LowerWrapper.ValidateHandler then accepts exactly two arities — an exact match, or the (payload, error) pair it can fold through foldResultValue:

if numOut != expectedResults {
    if expectedResults == 1 && numOut == 2 && w.hasResultType() {
    } else if expectedResults == 0 && numOut == 0 {
    } else {
        return fmt.Errorf("result count mismatch: expected %d, got %d", expectedResults, numOut)
    }
}

The two methods match neither:

MethodTCPSocketFinishConnect(ctx, self uint32) (uint32, uint32, *NetworkError)          // 3
MethodTCPSocketAccept(ctx, self uint32)        (uint32, uint32, uint32, *NetworkError)  // 4

This is not a partial outage. NewTCPInputStreamResource is reached only from those two methods, so there is no other route to a socket's input and output streams: a guest can create a socket and start a connection, but can never read or write. UDP and DNS are unaffected.

The change

Both methods now return a record plus the error — the shape the address getters in the same file already use (MethodTCPSocketLocalAddress returns (*IPSocketAddress, *NetworkError)), so the fold applies and the payload lifts as a record:

type TCPStreams  struct { Input, Output uint32 }
type TCPAccepted struct { Socket, Input, Output uint32 }

Four call sites in tcp_test.go follow the new shape. No behaviour changes beyond the return shape, and no dependency is touched.

Test

boot/components/runtime/wasm/sockets_bind_test.go loads a real component that imports finish-connect and fails if any sockets method cannot bind. It is gated on WASM_SOCKETS_PROBE pointing at such a component, and skips otherwise, because the repository carries no component fixture — happy to add one if you would prefer the test to stand alone.

Against the component I built it fails on main with expected 1, got 3 and passes here.

Note on the WIT shape

ip-socket-address is implemented as a record (IPSocketAddress{Address string; Port uint16}) rather than the variant { ipv4, ipv6 } the published WASI interface declares, so a guest has to declare it the same way to bind. That is out of scope here, but worth knowing if the intent is to accept stock wasi:sockets guests.

[wolfy-j]

Superseded by #543. The canonical-ABI tuple fix and a committed stock Rust WASI sockets binding fixture are included there.