handling of function imports during module instantiation

[sjamesr]

Hi all, I was just wondering why module imports are treated specially during module instantiation (compared to table, global and memories). If a linked global, table or memory cannot be found, check_linked_symbol will return false, e.g. see https://github.com/bytecodealliance/wasm-micro-runtime/blob/main/core/iwasm/interpreter/wasm_runtime.c#L1914.

On the other hand, a missing function will log a warning, but will not otherwise prevent module instantiation. Unless I'm misunderstanding, this seems to contradict Rule 3 in https://webassembly.github.io/spec/core/exec/modules.html#exec-instantiation.

I came across this while writing my own spec test runner. In imports.wast spec test, we have

(assert_unlinkable
  (module (import "test" "unknown" (func)))
  "unknown import"
)

WAMR's spec test runner checks to see that there is a linker failure in the STDOUT or STDERR of the iwasm repl, which there is. But this failure does not actually prevent module instantiation like the spec says it should.

I'm curious about WAMR's behavior in this scenario. It seems like a deliberate choice, but I'd like to understand why. Thanks!

[TianlongLiang]

I think the choice of warning over error is that this import function may not be used. If it's actually used, it will report an exception error in runtime.