cmd/compile: add go:wasmexport directive

[johanbrandhorst]

Background

#38248 defined a new compiler directive, go:wasmimport, for interfacing with host defined functions. This allowed calling from Go code into host functions, but it’s still not possible to call from the WebAssembly (Wasm) host into Go code.

Some applications have adopted the practice of allowing them to be extended by calling into Wasm compiled code according to some well defined ABI. Examples include Envoy, Istio, VS Code and others. Go cannot support compiling code to these applications, as the only exported function in the module compiled by Go is _start, mapping to the main function in a main package.

Despite this, some users are designing custom plugin systems using this interface, utilizing standard in and standard out for communicating with the Wasm binary. This shows a desire for exporting Go functions in the community.

There have been historical discussions on implementing this before (including #42372, #25612 and #41715), but none of them have reached a consensus on a design and implementation. In particular, #42372 had a long discussion (and design doc) that never provided a satisfying answer for how to run executed functions in the Go runtime. Instead of reviving that discussion, this proposal will attempt to build on it and answer the questions posed. This proposal supersedes #42372.

Exporting functions to the wasm host is also a necessity for a hypothetical GOOS=wasip2 targeting preview 2 of the WASI specification. This could be implemented as a special case in the compiler but since this is a feature requested by users it could reuse that functionality (similar to go:wasmimport today).

Proposal

Repurpose the -buildmode build flag value c-shared for the wasip1 port. It now signals to the compiler to replace the _start function with an _initialize function, which performs runtime and package initialization.

Add a new compiler directive, go:wasmexport, which is used to signal to the compiler that a function should be exported using a Wasm export in the resulting Wasm binary. Using the compiler directive will result in a compilation failure unless the target GOOS is wasip1.

There is a single optional required parameter to the directive, defining the name of the exported function:
(UPDATE: make the parameter required, consistent with the //export pragma and easier to implement).

//go:wasmexport name

The directive is only allowed on functions, not methods.

Discussion

Parallel with -buildmode=c-shared and CGO

The proposed implementation is inspired by the implementation of C references to Go functions. When an exported function is called, a new goroutine (G) is created, which executes on a single thread (M), since Wasm is a single threaded architecture. The runtime will wake up and resume scheduling goroutines as necessary, with the exported function being one of the goroutines available for scheduling. Any other goroutines started during package initialization or left over from previous exported function executions will also be available for scheduling.

Why a "-buildmode" option?

The wasi_snapshot_preview1 documentation states that a _start function and an _initialize function are mutually exclusive. Additionally, at the end of the current _start functions as compiled by Go, proc_exit is called. At this point, the module is considered done, and cannot be interacted with. Given these conditions, we need some way for a user to declare that they want to build a binary especially for exporting one or more functions and to include the _initialize function for package and runtime initialization.

We also considered using a GOWASM option instead, but this feels wrong since that environment variable is used to specify options relating to the architecture (existing options are satconv and signext), while this export option is dependent on the behavior of the "OS" (what functions to export, what initialization pattern to expect).

What happens to func main when exports are involved?

Go code compiled to a wasip1 Wasm binary can be either a "Command", which includes the _start function, or a "Reactor/Library", which includes the _initialize function.

When using -buildmode=c-shared, the resulting Wasm binary will not contain a _start function, and will only contain the _initialize function and any exported functions. The Go main function will not be exported to the host. The user can choose to export it like any other function using the //go:wasmexport directive. The _initialize function will not automatically call main. The main function will not initialize the runtime.

When the -buildmode flag is unset, the _start function and any exported functions will be exported to the host. Using //go:wasmexport on the main function in this mode will result in a compilation error. In this mode, only _start will initialize the runtime, and so must be the first export called from the host. Any other exported functions may only be called through calling into host functions that call other exports during the execution of the _start function. Once the _start function has returned, no other exports may be called on the same instance.

Why not reuse //export?

//export is used to export Go functions to C when using buildmode=c-shared. Use of //export puts restrictions on the use of the file, namely that it cannot contain definitions, only declarations. It’s also something of an ugly duckling among compiler directives in that it doesn’t use the now established go: prefix. A new directive removes the need for users to define functions separately from the declaration, has a nice symmetry with go:wasmimport, and uses the well established go: prefix.

Handling Reentrant Calls and Panics

Reentrant calls happen when the Go application calls a host import, and that invocation calls back into an exported function. Reentrant calls are handled by creating a new goroutine. If a panic reaches the top-level of the go:wasmexport call, the program crashes because there are no mechanisms allowing the guest application to propagate the panic to the Wasm host.

Naming exports

When the name of the Go function matches that of the desired Wasm export, the name parameter can be omitted.

For example:

//go:wasmexport add
func add(x, y int) int {
    return x + y
}

Is equivalent to

//go:wasmexport
func add(x, y int) int {
    return x + y
}

The names _start and _initialize are reserved and not available for user exported functions.

Third-party libraries

Third-party libraries will need to be able to define exports, as WASI functionality such as wasi-http requires calling into exported functions, which would be provided by the third party library in a user-friendly wrapper. Any exports defined in third party libraries are compiled to exported Wasm functions.

Module names

The current Wasm architecture doesn’t define a module name of the compiled module, and this proposal does not suggest adding one. Module names are useful to namespace different compiled Wasm binaries, but it can usually be configured by the runtime or using post-processing tools on the binaries. Future proposals may suggest some way to build this into the Go build system, but this proposal suggests not naming it for simplicity.

Conflicting exports

If the compiler detects multiple exports using the same name, a compile error will occur and warn the user that multiple definitions are in conflict. This may have to happen at link time. If this happens in third-party libraries the user has no recourse but to avoid using one of the libraries.

Supported Types

The go:wasmimport directive allows the declaration of host imports by naming the module and function that the application depends on. The directive applies restrictions on the types that can be used in the function signatures, limiting to fixed-size integers and floats, and unsafe.Pointer, which allows simple mapping rules between the Go and Wasm types. The go:wasmexport directive will use the same type restrictions. Any future relaxing of this restriction will be subject to a separate proposal.

Spawning Goroutines from go:wasmexport functions

The proposal considers scenarios where the go:wasmexport call spawns new goroutines. In the absence of threading or stack switching capability in Wasm, the simplest option is to document that all goroutines still running when the invocation of the go:wasmexport function returns will be paused until the control flow re-enters the Go application.

In the future, we anticipate that Wasm will gain the ability to either spawn threads or integrate with the event loop of the host runtime (e.g., via stack-switching) to drive background goroutines to completion after the invocation of a go:wasmexport function has returned.

Blocking in go:wasmexport functions

When the goroutine running the exported function blocks for any reason, the function will yield to the Go runtime. The Go runtime will schedule other goroutines as necessary. If there are no other goroutines, the application will crash with a deadlock, as there is no way to proceed, and Wasm code cannot block.

Authors

@johanbrandhorst, @achille-roussel, @Pryz, @dgryski, @evanphx, @neelance, @mdlayher

Acknowledgements

Thanks to all participants in the go:wasmexport discussion at the Go contributor summit at GopherCon 2023, without which this proposal would not have been possible.

CC @golang/wasm @cherrymui

[ydnar]

Thanks for putting this together—this is exciting.

Generating a module that can act as a reactor and a command sounds like a great idea. I noticed this might conflict with how Node interprets a module. If a module exports both _start and _initialize, it will throw an exception: https://nodejs.org/api/wasi.html

1. One could argue this is undesired behavior, and Node could change.
2. What happens if a host detects and calls both _initialize and _start?
3. Exporting one or the other, but not both, implies some kind of configuration or detection.
   • In this TinyGo PR I experimented with detecting lack of main.main as the trigger for "reactor" mode with _initialize as the entry point: runtime, builder: WebAssembly reactor mode tinygo-org/tinygo#4082

[ydnar]

The directive is only allowed on functions, not methods.

Using //go:wasmimport on methods has been helpful for mapping Component Model resource methods in WASI Preview 2:

From https://github.com/ydnar/wasm-tools-go/blob/9b4707e054a8b528b27240cba6c05557c4e26a53/wasi/io/error/error.wit.go:

// ToDebugString represents the method "wasi:io/error.error#to-debug-string".
//
// Returns a string that is suitable to assist humans in debugging
// this error.
//
// WARNING: The returned string should not be consumed mechanically!
// It may change across platforms, hosts, or other implementation
// details. Parsing this string is a major platform-compatibility
// hazard.
func (self Error) ToDebugString() string {
	var ret string
	self.to_debug_string(&ret)
	return ret
}

//go:wasmimport wasi:io/error@0.2.0-rc-2023-11-10 [method]error.to-debug-string
func (self Error) to_debug_string(ret *string)

Subjectively, using methods seems better aligned with the Component Model semantics than the equivalent:

//go:wasmimport wasi:io/error@0.2.0-rc-2023-11-10 [method]error.to-debug-string
func error__to_debug_string(self Error, ret *string)

Given that resources are opaque i32 handles, the same could be true for implementing exported methods via //go:wasmexport.

[johanbrandhorst]

Thanks for putting this together—this is exciting.

Generating a module that can act as a reactor and a command sounds like a great idea. I noticed this might conflict with how Node interprets a module. If a module exports both _start and _initialize, it will throw an exception: https://nodejs.org/api/wasi.html

1. One could argue this is undesired behavior, and Node could change.

2. What happens if a host detects and calls both _initialize and _start?

3. Exporting one or the other, but not both, implies some kind of configuration or detection.
   
   * In this TinyGo PR I experimented with detecting lack of main.main as the trigger for "reactor" mode with _initialize as the entry point: [runtime, builder: WebAssembly reactor mode tinygo-org/tinygo#4082](https://github.com/tinygo-org/tinygo/pull/4082)

Thank you for the information about Node's behavior here, I wasn't aware. That is certainly troubling. I will try to see what if any other precedent there is for this behavior in the ecosystem to see whether we or Node are in the wrong.

If a host calls both _initialize and _start, it will run initialization once (initialization has to be protected with something like a sync.Once to be idempotent) and then run func main(). Just calling _start will accomplish the same thing.

Indeed, if we do need some way to allow users to choose whether to build a command (executing func main()) or library (just initializating and exporting functions), this proposal would need to add some way for users to turn that knob. I don't want to prejudice that discussion until we know if we need it.

[johanbrandhorst]

Given that resources are opaque i32 handles, the same could be true for implementing exported methods via //go:wasmexport.

This may be true, but I still think this proposal serves as an MVP that we can enhance with method support in a subsequent proposal once the initial hurdles have been overcome.

[ydnar]

Thank you for the information about Node's behavior here, I wasn't aware. That is certainly troubling. I will try to see what if any other precedent there is for this behavior in the ecosystem to see whether we or Node are in the wrong.

If a host calls both _initialize and _start, it will run initialization once (initialization has to be protected with something like a sync.Once to be idempotent) and then run func main(). Just calling _start will accomplish the same thing.

Maybe it’s a bigger question about what is defined behavior. Is having both _initialize and _start valid, or undefined? Having only one entry point is less ambiguous, e.g. the host can only call one, but not both (or choose, which could be contrary to the user’s expectation).

[ydnar]

Indeed, if we do need some way to allow users to choose whether to build a command (executing func main()) or library (just initializating and exporting functions), this proposal would need to add some way for users to turn that knob. I don't want to prejudice that discussion until we know if we need it.

Have had previous discussions about -buildmode=wasm-reactor to mirror -buildmode=c-shared.

[johanbrandhorst]

I created an issue to ask the NodeJS devs for the source of this design decision: nodejs/node#51544

[cjihrig]

Hey. Node developer that implemented that design decision here. 👋

That change was nearly four years ago, and I have since forgotten the exact motivation. However, I was able to dig this up: WebAssembly/WASI@d8b286c. At that point in time, WASI commands had a _start() function, and WASI reactors had an _initialize() function. Commands and reactors were mutually exclusive.

WASI has changed a good bit since then. I no longer work on WASI, so I don't know if that design decision is still valid or not. I would recommend checking with the folks in the WASI repos.

[zetaab]

WebAssembly/wasi-http#95 contains discussion to use _initialize func. So if that is not possible to golang, it would be difficult

[johanbrandhorst]

Any user created func init() would be run in _initialize, is this not sufficient?

[johanbrandhorst]

Hey. Node developer that implemented that design decision here. 👋

That change was nearly four years ago, and I have since forgotten the exact motivation. However, I was able to dig this up: WebAssembly/WASI@d8b286c. At that point in time, WASI commands had a _start() function, and WASI reactors had an _initialize() function. Commands and reactors were mutually exclusive.

WASI has changed a good bit since then. I no longer work on WASI, so I don't know if that design decision is still valid or not. I would recommend checking with the folks in the WASI repos.

Thanks so much for providing your input and this reference. It seems this doc now lives at https://github.com/WebAssembly/WASI/blob/a7be582112b35e281058f1df7d8628bb30a69c3f/legacy/application-abi.md. I wonder, given that this is now under the legacy heading, whether this statement is still true:

These kinds are mutually exclusive; implementations should report an error if asked to instantiate a module containing exports which declare it to be of multiple kinds.

If so, this design would need to change to allow the user to choose whether to compile a Command or a Library (Reactor). @sunfishcode perhaps you could provide some guidance here?