Stream: general

Topic: wasm-tools compose example

Ryan Levick (rylev) (Apr 10 2023 at 15:41):

After I run the wasm-tools compose example, I cannot run wasm-tools component wit on the composed component. I get the following error:

Error: failed to decode WIT document

Caused by:
    0: failed to decode WIT from import `streams`
    1: unexpected unnamed type

Is this expected?

Ryan Levick (rylev) (Apr 10 2023 at 15:53):

If not I can look into tomorrow and see if there’s a possible fix.

Alex Crichton (Apr 10 2023 at 15:58):

That looks like a bug, possibly in wasm-tools compose, but I'd have to dig in to say for sure

Ryan Levick (rylev) (Apr 10 2023 at 16:06):

Ok cool. I’ll debug it tomorrow and try to push a fix.

Ryan Levick (rylev) (Apr 11 2023 at 12:26):

So the current error seems to be caused by an incorrect encoding of an instance during the composition:

Before composition

(component
 (type (;0;)
    (instance
      (type (;0;) u32)
      (export (;1;) "wall-clock" (type (eq 0)))
      (type (;2;) (record (field "seconds" u64) (field "nanoseconds" u32)))
      (export (;3;) "datetime" (type (eq 2)))
      (type (;4;) (func (param "this" 1) (result 3))) ;; NOTE: the result points to the "datetime" export
      (export (;0;) "now" (func (type 4)))
    )
  )

After composition:

(component
  (type (;0;)
    (instance
      (type (;0;) u32)
      (export (;1;) "wall-clock" (type (eq 0)))
      (type (;2;) (record (field "seconds" u64) (field "nanoseconds" u32)))
      (export (;3;) "datetime" (type (eq 2)))
      (type (;4;) (func (param "this" 0) (result 2))) ;; NOTE: the result type points directly to the record
      (export (;0;) "now" (func (type 4)))
    )
  )

Ryan Levick (rylev) (Apr 11 2023 at 12:29):

At some point this instance seems to have one of its functions changed so that its result type points directly to a record field (which is illegal) instead of pointing to a type.

Ryan Levick (rylev) (Apr 11 2023 at 12:43):

This particular instance is not itself being composed (i.e., it should be fine for the composer to just round trip).

Ryan Levick (rylev) (Apr 11 2023 at 12:43):

So I'm not sure if this is some mistake in how the compose code decodes instances or if its an issue with the encoder.

Alex Crichton (Apr 11 2023 at 14:31):

I suspect that this may be an issue with both the decoder and encoder perhaps? The constructs used here were added after the initial implementation of wasm-compose so it hasn't been updated to use exports in the same way. The decoder probably needs an update to read the structure of use-the-exported-item and the encoder needs to be updated to use items as they're exported. Shouldn't be too hard ideally as wit-component deals with similar problems as well, but if you'd like I think opening an issue for this would be good

Ryan Levick (rylev) (Apr 11 2023 at 15:17):

Done https://github.com/bytecodealliance/wasm-tools/issues/977

Last updated: Nov 22 2024 at 16:03 UTC