wasmparser behaviour in case of Nested Components

[thesuhas]

In the case of this nested component:

  (alias export 2 "incoming-request" (type (;45;)))
  (alias export 2 "response-outparam" (type (;46;)))
  (component (;0;)
    (import "import-type-incoming-request" (type (;0;) (sub resource)))
    (import "import-type-response-outparam" (type (;1;) (sub resource)))
    (import "import-type-incoming-request0" (type (;2;) (eq 0)))
    (type (;3;) (own 2))
    (import "import-type-response-outparam0" (type (;4;) (eq 1)))
    (type (;5;) (own 4))
    (type (;6;) (func (param "request" 3) (param "response-out" 5)))
    (import "import-func-handle" (func (;0;) (type 6)))
    (export (;7;) "incoming-request" (type 0))
    (export (;8;) "response-outparam" (type 1))
    (type (;9;) (own 7))
    (type (;10;) (own 8))
    (type (;11;) (func (param "request" 9) (param "response-out" 10)))
    (export (;1;) "handle" (func 0) (func (type 11)))
  )
  (instance (;14;) (instantiate 0
      (with "import-func-handle" (func 39))
      (with "import-type-incoming-request" (type 45))
      (with "import-type-response-outparam" (type 46))
      (with "import-type-incoming-request0" (type 40))
      (with "import-type-response-outparam0" (type 42))
    )
  )
  (export (;15;) "wasi:http/incoming-handler@0.2.0-rc-2023-10-18" (instance 14))

I see that wasmparser::Parser's payload is matched on Payload::ComponentSection which returns a new parser that I use to encode the nested component.
But, I also notice that the outer parser does not jump from:
(component (;0;) to (instance (;14;) (instantiate 0. Instead, it enters the nested component and starts parsing the contents.
Is this the expected behaviour?
For context, this is how I'm handling a nested Component section:

Payload::ComponentSection {
                    parser: comp_parser,
                    unchecked_range,
                } => {
                    let cmp =
                        Component::parse_comp(&wasm[unchecked_range], enable_multi_memory, comp_parser)?;
                    components.push(cmp.clone());
                    Self::add_to_sections(&mut sections, Section::Component, &mut num_sections, 1);
                }

This is where my entire parse function is. On encountering a ComponentSection, I recursively use the new child parser and encode the nested component and return to use the parent parser after it's done as mentioned here. I do not see what I'm doing wrong and was wondering if this was expected behaviour or if there was a bug.

I also see that I'm doing the same thing for a module:

Payload::ModuleSection {
                    parser,
                    unchecked_range,
                } => {
                    modules.push(Module::parse(
                        &wasm[unchecked_range],
                        enable_multi_memory,
                        parser,
                    )?);
                    Self::add_to_sections(&mut sections, Section::Module, &mut num_sections, 1);
                }

it works fine in this case.

[thesuhas]

I've also recreated the issue on this smaller binary:

(component
  (core type (;0;)
    (module)
  )
  (import "i1" (core module (;0;) (type 0)))
  (core module (;1;))
  (core module (;2;))
  (core module (;3;))
  (component (;1;)
    (type (;0;) (func (param "p" string)))
    (import "a" (func (;0;) (type 0)))
  )
  (export (;4;) "x" (core module 3))
)

Gives output:

(component
  (core type (;0;)
    (module)
  )
  (import "i1" (core module (;0;) (type 0)))
  (core module (;1;))
  (core module (;2;))
  (core module (;3;))
  (component (;0;)
    (type (;0;) (func (param "p" string)))
    (import "a" (func (;0;) (type 0)))
  )
  (type (;0;) (func (param "p" string)))
  (import "a" (func (;0;) (type 0)))
  (export (;4;) "x" (core module 3))
)

Because the outer parser does not jump to (export (;4;) "x" (core module 3)) after parsing (component (;0;)

[alexcrichton]

Ah when using the parse_all helper there's no need to switch to the parsers that are given since the parse_all helper handles that for you. You'll want to instead track the depth of which you're processing a component at yourself when the Version and End markers are seen.

[thesuhas]

Thanks!