Middleware: injecting host function imports

[uint]

Summary

I'm trying to write a piece of middleware for profiling Wasm code blocks. For this, I'm trying to inject a couple function imports and a little Wasm code that would call the imported functions before a block and after it.

What would be a good way to inject those imports? So far I haven't been able to make it happen.

Additional details

I've been trying to mutate ModuleInfo to include the new imports, but that seems to confuse existing local functions in the module. I didn't dig too deep, but I'm guessing things get tricky because normally imported functions would be indexed first, and local ones later?

Here's some code I stripped down to a sort-of MWE:

https://gist.github.com/uint/be1d83734a76d8cb79cc09c2b0b2a753

Here's the output:

failures:

---- tests::middleware_test stdout ----
start measuring
thread 'tests::middleware_test' panicked at 'assertion failed: `(left == right)`
  left: `I32(0)`,
 right: `I32(43)`', src/lib.rs:110:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note the start measuring print. This means that instead of the expected local function being called my import got confused here and is being called.

Edit: Also note that if you comment out the middleware code, the test passes.

[Hywan]

Hello,

I believe it can be much “simpler” and more versatile to trace the VM execution rather than instrumenting the Wasm module (see #2152). Otherwise, I don't believe it's a good idea to instrument the Wasm module at runtime, it should be done as part of a pre-processor (see #1210) maybe.

Since we can also compile Wasm modules to native objects, we can probably use gdb or stuff like that.

I think we should come with a solution to cover most usecases. cc @wchaudry