Bug Reproduction

During development of an animated webassembly feature a function that takes and returns a large typed array started failing occasionally with an error similar to:

index.ts:125 Error: The JSClosure has been already released by Swift side. The closure is created at

This repository reproduces that issue.

Swift:

import Foundation

import JavaScriptKit

func reverseArray(bytes: [Float32]) -> [Float32] {
    return [Float32](bytes.reversed())
}

let jsClosure = JSClosure { (input: [JSValue]) in
    let bytes: [Float32] = try! JSValueDecoder().decode(from: input[0])

    return reverseArray(bytes: bytes).jsValue
}

@_cdecl("main")
func main(_ i: Int32, _ j: Int32) -> Int32 {
    JSObject.global.reverseFloat32Array = .object(jsClosure)

    return 0
}

Installation

1. Clone this repo
2. Run yarn
3. Run ./build-wasm.sh - designed for mac, might not work on your system
4. Run yarn dev

Expected Output

A page with numbers similar to:

fps: 69.50
ok: 47%

0.6486879587173462
0.7697053551673889
0.6013171076774597
0.9578807353973389
0.7919120192527771
...

FPS represents the fps of the resulting wasm execution

OK represents the number of times the wasm successfully executes

The console will be filling with errors

Investigation and hacky fix

Substantial time was spent poking JSClosure.swift

This change "fixes" the issue by cloning the JSClosure.sharedClosures dictionary. The key is present, the value is present, the memory address changes between several values, but the guard sometimes returns nil.

/// Returns true if the host function has been already released, otherwise false.
@_cdecl("_call_host_function_impl")
func _call_host_function_impl(
    _ hostFuncRef: JavaScriptHostFuncRef,
    _ argv: UnsafePointer<RawJSValue>, _ argc: Int32,
    _ callbackFuncRef: JavaScriptObjectRef
) -> Bool {
    // TODO: This is some sort of horrible hack due to some sort of horrible wasm thing
    // Otherwise the sharedClone SOMETIMES fails
    let sharedClone = Dictionary(uniqueKeysWithValues: zip(JSClosure.sharedClosures.keys, JSClosure.sharedClosures.values))

    guard let (_, hostFunc) = sharedClone[hostFuncRef] else {
        return true
    }
    let arguments = UnsafeBufferPointer(start: argv, count: Int(argc)).map(\.jsValue)
    let result = hostFunc(arguments)
    let callbackFuncRef = JSFunction(id: callbackFuncRef)
    _ = callbackFuncRef(result)
    return false
}