Cannot create, read and write using a wasm module

[jmcad]

I've been trying to create a file and write data to it by using a WebAssembly module written in Rust:

#[no_mangle]
pub fn write() -> Result<(), io::Error> {
    println!("Trying to create a file");
    let mut file = File::create("./foo.txt")?;
    // let mut file = File::open("foo.txt")?;
    println!("I created a file");
    file.write_all(b"Hello, world!")?;
    Ok(())
}

#[no_mangle]
pub extern "C" fn waWrite() -> bool {
    let err = write();
    println!("I am in the waWrite function");
    match err {
        Ok(()) => {
            print!("GOOD");
            return true;
        }
        Err(_e) => {
            print!("GOOD");
            return false;
        }
    }
}

However this does not seem to work at all. Are there any solutions/workarounds for this problem or am I missing something obvious?

Here is my Go code:

func main() {
    // stdout to print WASI text
    dir, err := ioutil.TempDir("", "out")
    check(err)
    defer os.RemoveAll(dir)
    stdoutPath := filepath.Join(dir, "stdout")

    engine := wasmtime.NewEngine()
    store := wasmtime.NewStore(engine)
    linker := wasmtime.NewLinker(store)

    // configure WASI imports to write stdout into a file.
    wasiConfig := wasmtime.NewWasiConfig()
    wasiConfig.SetStdoutFile(stdoutPath)
    err = wasiConfig.PreopenDir(".", ".")
    check(err)

    args := []string{"foo.txt"} // file names
    wasiConfig.SetArgv(args)

    // set the version to the same as in the WAT.
    wasi, err := wasmtime.NewWasiInstance(store, wasiConfig, "wasi_snapshot_preview1")
    check(err)

    // link WASI
    err = linker.DefineWasi(wasi)
    check(err)

    // create the WebAssembly-module
    module, err := wasmtime.NewModuleFromFile(store.Engine, "rust_write.wasm")
    check(err)
    instance, err := linker.Instantiate(module)
    check(err)

    // export functions and memory from the WebAssembly module

    write := instance.GetExport("waWrite").Func()

    res, err := write.Call()
    check(err)
    if res == true {
        println("wrote to file")
    } else {
        println("didn't write to file")
    }

    out, err := ioutil.ReadFile(stdoutPath)
    check(err)
    fmt.Print(string(out))
}

func check(err error) {
    if err != nil {
        panic(err)
    }
}

[alexcrichton]

I think that this is likely due to rust-lang/rust#79199, can you perhaps try recompiling with the latest Rust nightly and see if that fixes the issue for you?

[AndreaEsposit]

I get the same problem with the latest Rust nightly,
How do wasiConfig.PreopenDir(".", ".") and wasiConfig.SetArgv(args) work exactly?

[alexcrichton]

SetArgv effectively configures the return value of env::args in Rust, and PreopenDir opens the first parameter as a host directory and the second parameter as the name of the guest directory.

[AndreaEsposit]

Oh, thank you for the explanation. Do you then have any idea why It doesn't work for me as well?
I tried using the same wasm module and wasmtime CLI like this wasmtime --dir=. --invoke waWrite rust_write.wasm
And get "failed to find a pre-opened file descriptor through which "foo.txt" could be opened"

[alexcrichton]

Hm if it doesn't work with the wasmtime CLI then it may be that issue I linked? Are you sure you're using the latest nightly to compile the wasm?

[AndreaEsposit]

(I am using rustc 1.51.0-nightly) @alexcrichton
Alright, I implemented a very simple Wasm module that has just a main function that creates a file and writes "hello" to it. It works perfectly in wasmtime CLI just using the --mapdir flag, Unfortunately the code written in the "library" style (jmcad's crate compiled as cdylib) still doesn't work, is it a bug? Or is it just not meant to work with library-style binaries?

Could it be that the issue is that _start is not called when you directly --invoke the waWrite function?

[AndreaEsposit]

I found a solution here: rust-lang/rust#79997 (comment)

[alexcrichton]

Ah oops sorry! I forgot that the new -Z flag was also needed. I'm glad it works for you though! I'm going to close this in that case then.