add stream.forward and future.forward builtins - #709
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The Explainer specifies that a component may call `future.drop-readable` before reading a value to signal loss of interest, in which case a blocked `future.write` completes with `future-write-result.dropped`. But `SharedFutureImpl.drop` asserted that any pending buffer is a `WritableBuffer`, i.e. a pending *read*, while the only pending buffer actually possible at this point is the buffer of a pending *write*: a reader cannot drop its end while its own read is pending (`CopyEnd.drop` traps while copying) and a writer cannot drop its end before the write resolves (`WritableFutureEnd.drop` traps unless DONE). Exercising this spec-legal path failed the assert instead of notifying the writer with DROPPED. Flip the assert to expect a `ReadableBuffer` (the source buffer of the pending write), make `ReadableBufferGuestImpl` actually subclass `ReadableBuffer` (it previously only derived `BufferGuestImpl`, unlike `WritableBufferGuestImpl`, which does mix in `WritableBuffer`), and add a test reproducing the scenario. Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net> Assisted-by: claude:claude-fable-5
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Introduce the ⏩-gated `{stream,future}.forward` built-ins for zero-copy
forwarding: given the readable end of a source stream (resp., future) and
the writable end of a destination of the same type, both ends are
transferred out of the calling component and the destination is fused
with the source, delegating all subsequent reads, cancels and drops to
it. Elements rendezvous directly between the outer producer and consumer
with end-to-end backpressure and no read-ahead, and end-of-stream and
drops propagate in both directions, allowing hosts to remove the
calling component from the copy path entirely. Forwards may be chained;
a forward that would (transitively) make a stream or future its own
source traps.
In the binary format, `stream.forward` is assigned opcode 0x2e and
`future.forward` opcode 0x2f. The binary decode test is left to a
follow-up commit so that test/binary/binary.wast keeps passing against
engines that do not decode these opcodes yet.
Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net>
Assisted-by: claude:claude-fable-5
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refs #658
tests blocked on #708