Add support for WASM imports

[artur-ciocanu]

Motivation

I was trying to use wasmer-java to create a sample Java application that would be able to import and execute an Open Policy Agent policy compiled to WASM. More details about Open Policy Agent can be found here: https://www.openpolicyagent.org/docs/latest/wasm/.

An Open Policy Agent compiled WASM has a couple of imports, here is an excerpt from a policy.wasm using WASM text format:

  (import "env" "memory" (memory $env.memory 2))
  (import "env" "opa_abort" (func $env.opa_abort (type $t3)))
  (import "env" "opa_builtin0" (func $env.opa_builtin0 (type $t1)))
  (import "env" "opa_builtin1" (func $env.opa_builtin1 (type $t0)))
  (import "env" "opa_builtin2" (func $env.opa_builtin2 (type $t5)))
  (import "env" "opa_builtin3" (func $env.opa_builtin3 (type $t6)))
  (import "env" "opa_builtin4" (func $env.opa_builtin4 (type $t7)))

When trying to load use a policy.wasm from a simple Java app like the one below:

public class Application {

  public static void main(String[] args) throws IOException {
    byte[] wasmBytes = Files.readAllBytes(Paths.get("policy.wasm"));
    Instance instance = new Instance(wasmBytes);

    System.out.println(instance);
  }
}

I get the following exception:

Exception in thread "main" java.lang.RuntimeException: Failed to instantiate the module: 7 link errors: (1 of 7) Import not found, namespace: env, name: opa_abort (2 of 7) Import not found, namespace: env, name: opa_builtin0 (3 of 7) Import not found, namespace: env, name: opa_builtin1 (4 of 7) Import not found, namespace: env, name: opa_builtin2 (5 of 7) Import not found, namespace: env, name: opa_builtin3 (6 of 7) Import not found, namespace: env, name: opa_builtin4 (7 of 7) Import not found, namespace: env, name: memory
	at org.wasmer.Instance.nativeInstantiate(Native Method)
	at org.wasmer.Instance.<init>(Instance.java:45)
	at com.adobe.target.gw.Application.main(Application.java:12)

Proposed solution

As far as I can tell by looking here https://github.com/wasmerio/wasmer-java/blob/master/src/instance.rs#L26, we always pass an empty imports value. It would be great if we could pass some kind of object to Instance.nativeInstantiate(...) that would allow us to populate the "imports" before passing to wasmer runtime.

In the browser when loading WASM we can pass an import object like WebAssembly.instantiate(bufferSource, importObject), so it would be nice to have something similar in wasmer-java.

Alternatives

Not sure if there are other alternatives

Additional context

None

[Hywan]

Thanks for the issue! Duplicate of #11. I'm going to close this one :-).