How can I use WASI to open a file?

[jeschkies]

Hi,

maybe there is some documentation somewhere but I just cannot figure out how to run a WASM binary from Go with wasmtime.

I compiled a Rust program to wasm with cargo wasi build --release. I want to call a method from Golang and wrote the following simple example:

package main

import (
	"fmt"
	"github.com/bytecodealliance/wasmtime-go"
)

func main() {
	engine := wasmtime.NewEngine()
	store := wasmtime.NewStore(engine)

	linker := wasmtime.NewLinker(store)

	module, err := wasmtime.NewModuleFromFile(engine, "../target/wasm32-wasi/release/bench.wasm")
	check(err)

	for _, v := range module.Imports() {
		fmt.Printf("%v ", v.Name())
	}

	wasiConfig := wasmtime.NewWasiConfig()
	wasiConfig.InheritEnv()
	wasiConfig.PreopenDir(".", ".")
	wasi, err := wasmtime.NewWasiInstance(store, wasiConfig, "wasi_unstable")
	check(err)

	err = linker.DefineWasi(wasi)
	check(err)

	instance, err := linker.Instantiate(module)
	check(err)

	nom := instance.GetExport("run_nom").Func()
	_, err = nom.Call()
	check(err)
}

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

It prints the required imports fd_close fd_filestat_get fd_read fd_write path_open proc_exit environ_sizes_get environ_get and the fails with the error panic: unknown import: wasi_snapshot_preview1::fd_close has not been defined. Shouldn't PreopenDir define fd_close?

How can I define the imports?

Thanks a lot 🙂

PS: I am happy to contribute with an example in the docs.

[maxmcd]

Can you try changing this:

wasi, err := wasmtime.NewWasiInstance(store, wasiConfig, "wasi_unstable")

To this:

wasi, err := wasmtime.NewWasiInstance(store, wasiConfig, "wasi_snapshot_preview1")

I think that might do it.

[jeschkies]

Thanks. Does it mean the WASM binary was compiled against wasi_snapshot_preview1?

the changed helped but now I get this

wasm trap: unreachable
wasm backtrace:
  0: 0x9661 - <unknown>!std::panicking::rust_panic_with_hook::h3a0a1147e3efaaf2
  1: 0xd6c4 - <unknown>!rust_begin_unwind
  2: 0xd873 - <unknown>!core::panicking::panic_fmt::h6bf85cafed31336d
  3: 0xcb57 - <unknown>!core::option::expect_none_failed::h7dfd2381e84fa608
  4: 0x9bd1 - <unknown>!run_nom

[maxmcd]

Yes, I believe wasi_snapshot_preview1 is the current supported phase for rust. I think that is determined by this crate: https://github.com/bytecodealliance/wasi

Can you show your rust code? It looks like your code might be panicking on an option .unwrap with option::expect_none_failed? (or maybe something similar, not sure why it would be "expect_none".

[jeschkies]

Sure, this is the run_nom method. And I already see an .unwrap() 🙈

#[no_mangle]
pub extern "C" fn run_nom() {
    let data = std::fs::read_to_string("data/small_access.log").unwrap();
    let p = nom::CommonLogParser { input: &data };
    assert_eq!(p.count(), 161761);
}

EDIT: I changed the code to print the error failed to find a preopened file descriptor through which "data/small_access.log" could be opened.

PreopenDir has little documentation. What should the guest path be? I used . for both paths.

[maxmcd]

Maybe better docs here: http://docs.wasmtime.dev/c-api/wasi_8h.html#a6d738a3510c5f3aa4a6f49d7bb658cd1

If you click into the Go src you can usually find which function is being called in the wasmtime C api.

Maybe try wasiConfig.PreopenDir(".", "/"), the first path is the path on the host, the second path is where the directory is mounted for wasm.

[jeschkies]

Thanks for the link. Still no luck. I tried a few combinations such as wasiConfig.PreopenDir(".", "/") and wasiConfig.PreopenDir("./data", "/data"). I do get the same error when I try to use the wasmtime CLI wasmtime --dir=. target/wasm32-wasi/release/loki_bench.wasm --invoke run_nom 🤔

[maxmcd]

Can't get it working either. Maybe it's this issue? rust-lang/rust#79199

[jeschkies]

@maxmcd, thanks so much for your help. This seems to be it 🙂

[maxmcd]

btw, in case it's unclear. I was able to get your code working by just using the parse.wasm output. Since the parse wasm binary isn't a lib it seems to work if I update nom := instance.GetExport("run_nom").Func() to be nom := instance.GetExport("_start").Func()

That then runs this code: https://github.com/jeschkies/common-log-parser-bench/blob/karsten/go/src/bin/parse.rs#L5

And finds the right file

[jeschkies]

Thanks. That helped me with my original goal. I wanted to benchmark nom in WASM. How did you pass the argument to parse?

[alexcrichton]

To confirm, I think everything here's been figured out? Or are there lingering questions I could help resolve?

[jeschkies]

I'm not sure how to pass the main args to a WASM. However, that should be unrelated to this issue. I guess rust-lang/rust#79199 will solve this issue.

[alexcrichton]

Ah yeah I believe that'll solve those issues. In that case I'll go ahead and close this issue.

[maxmcd]

@jeschkies you might just want to use command line arguments or environment variables.