The Go Blog

WASI support in Go

Johan Brandhorst-Satzkorn, Julien Fabre, Damian Gryski, Evan Phoenix, and Achille Roussel
13 September 2023

Go 1.21 adds a new port targeting the WASI preview 1 syscall API through the new GOOS value wasip1. This port builds on the existing WebAssembly port introduced in Go 1.11.

What is WebAssembly?

WebAssembly (Wasm) is a binary instruction format originally designed for the web. It represents a standard that allows developers to run high-performance, low-level code directly in web browsers at near-native speeds.

Go first added support for compiling to Wasm in the 1.11 release, through the js/wasm port. This allowed Go code compiled using the Go compiler to be executed in web browsers, but it required a JavaScript execution environment.

As the use of Wasm has grown, so have use cases outside of the browser. Many cloud providers are now offering services that allow the user to execute Wasm executables directly, leveraging the new WebAssembly System Interface (WASI) syscall API.

The WebAssembly System Interface

WASI defines a syscall API for Wasm executables, allowing them to interact with system resources such as the filesystem, the system clock, random data utilities, and more. The latest release of the WASI spec is called wasi_snapshot_preview1, from which we derive the GOOS name wasip1. New versions of the API are being developed, and supporting them in the Go compiler in the future will likely mean adding a new GOOS.

The creation of WASI has allowed a number of Wasm runtimes (hosts) to standardize their syscall API around it. Examples of Wasm/WASI hosts include Wasmtime, Wazero, WasmEdge, Wasmer, and NodeJS. There are also a number of cloud providers offering hosting of Wasm/WASI executables.

How can we use it with Go?

Make sure that you have installed at least version 1.21 of Go. For this demo, we’ll use the Wasmtime host to execute our binary. Let’s start with a simple main.go:

package main

import "fmt"

func main() {
    fmt.Println("Hello world!")
}

We can build it for wasip1 using the command:

$ GOOS=wasip1 GOARCH=wasm go build -o main.wasm main.go

This will produce a file, main.wasm which we can execute with wasmtime:

$ wasmtime main.wasm
Hello world!

That’s all it takes to get started with Wasm/WASI! You can expect almost all the features of Go to just work with wasip1. To learn more about the details of how WASI works with Go, please see the proposal.

Running go tests with wasip1

Building and running a binary is easy, but sometimes we want to be able to run go test directly without having to build and execute the binary manually. Similar to the js/wasm port, the standard library distribution included in your Go installation comes with a file that makes this very easy. Add the misc/wasm directory to your PATH when running Go tests and it will run the tests using the Wasm host of your choice. This works by go test automatically executing misc/wasm/go_wasip1_wasm_exec when it finds this file in the PATH.

$ export PATH=$PATH:$(go env GOROOT)/misc/wasm
$ GOOS=wasip1 GOARCH=wasm go test ./...

This will run go test using Wasmtime. The Wasm host used can be controlled using the environment variable GOWASIRUNTIME. Currently supported values for this variable are wazero, wasmedge, wasmtime, and wasmer. This script is subject to breaking changes between Go versions. Note that Go wasip1 binaries don’t execute perfectly on all hosts yet (see #59907 and #60097).

This functionality also works when using go run:

$ GOOS=wasip1 GOARCH=wasm go run ./main.go
Hello world!

Wrapping Wasm functions in Go with go:wasmimport

In addition to the new wasip1/wasm port, Go 1.21 introduces a new compiler directive: go:wasmimport. It instructs the compiler to translate calls to the annotated function into a call to the function specified by the host module name and function name. This new compiler functionality is what allowed us to define the wasip1 syscall API in Go to support the new port, but it isn’t limited to being used in the standard library.

For example, the wasip1 syscall API defines the random_get function, and it is exposed to the Go standard library through a function wrapper defined in the runtime package. It looks like this:

//go:wasmimport wasi_snapshot_preview1 random_get
//go:noescape
func random_get(buf unsafe.Pointer, bufLen size) errno

This function wrapper is then wrapped in a more ergonomic function for use in the standard library:

func getRandomData(r []byte) {
    if random_get(unsafe.Pointer(&r[0]), size(len(r))) != 0 {
        throw("random_get failed")
    }
}

This way, a user can call getRandomData with a byte slice and it will eventually make its way to the host-defined random_get function. In the same way, users can define their own wrappers for host functions.

To learn more about the intricacies of wrapping Wasm functions in Go, please see the go:wasmimport proposal.

Limitations

While the wasip1 port passes all standard library tests, there are some notable fundamental limitations of the Wasm architecture that may surprise users.

Wasm is a single threaded architecture with no parallelism. The scheduler can still schedule goroutines to run concurrently, and standard in/out/error is non-blocking, so a goroutine can execute while another reads or writes, but any host function calls (such as requesting random data using the example above) will cause all goroutines to block until the host function call has returned.

A notable missing feature in the wasip1 API is a full implementation of network sockets. wasip1 only defines functions that operate on already opened sockets, making it impossible to support some of the most popular features of the Go standard library, such as HTTP servers. Hosts like Wasmer and WasmEdge implement extensions to the wasip1 API, allowing the opening of network sockets. While these extensions are not implemented by the Go compiler, there exists a third party library, github.com/stealthrocket/net, which uses go:wasmimport to allow the use of net.Dial and net.Listen on supported Wasm hosts. This enables the creation of net/http servers and other network related functionality when using this package.

The future of Wasm in Go

The addition of the wasip1/wasm port is just the beginning of the Wasm capabilities we would like to bring to Go. Please keep an eye out on the issue tracker for proposals around exporting Go functions to Wasm (go:wasmexport), a 32-bit port and future WASI API compatibility.

Get involved

If you are experimenting with and want to contribute to Wasm and Go, please get involved! The Go issue tracker tracks all in-progress work and the #webassembly channel on the Gophers Slack is a great place to discuss Go and WebAssembly. We look forward to hearing from you!