Regression in JSObjectSetPrototype for the global object in a context.

SetPrototype.swift

/*
swift SetPrototype.swift

On 10.13.4, the result is 'undefined', but it should be '5'.
*/

import Foundation
import JavaScriptCore

var definition:JSClassDefinition = kJSClassDefinitionEmpty
definition.attributes = JSClassAttributes(kJSClassAttributeNoAutomaticPrototype)
definition.className = ("Foo" as NSString).utf8String

let classRef = JSClassCreate(&definition)!

let ctx = JSGlobalContextCreate(classRef)
let originalGlobalObject = JSContextGetGlobalObject(ctx)!

let prototype = JSObjectMake(ctx, nil, nil)!
JSObjectSetPrototype(ctx, originalGlobalObject, prototype)

// In 10.13.3 and earlier, setting the prototype on the global object made the context return a new global object
let updatedGlobalObject = JSContextGetGlobalObject(ctx)!
if originalGlobalObject != updatedGlobalObject {
	print("global object changed from \(originalGlobalObject) to \(updatedGlobalObject))")
}

// Add a property to the prototype
let valueName = JSStringCreateWithCFString("value" as CFString);
do {
    var exception: JSValueRef? = nil
    JSObjectSetProperty(ctx, prototype, valueName, JSValueMakeNumber(ctx, 5.0), UInt32(kJSPropertyAttributeNone), &exception)
    guard exception == nil else {
        fatalError("Unable to set property")
    }
}

// Get the property via a script that should find the property via the prototype chain from the global object.
let scriptString = JSStringCreateWithCFString("this.value;" as CFString)
var scriptException: JSValueRef? = nil
guard let resultRef = JSEvaluateScript(ctx, scriptString, updatedGlobalObject, nil, 0, &scriptException) else {
    if let exc = scriptException {
        let excString = JSValueToStringCopy(ctx, exc, nil)
        fatalError("execution failed: \(JSStringCopyCFString(kCFAllocatorDefault, excString)!)")
    } else {
        fatalError("execution failed")
    }
}

// Should find the `5` in the prototype chain. Does before 10.13.4 beta.
if let resultString = JSValueToStringCopy(ctx, resultRef, nil) {
    print("resultString: \(JSStringCopyCFString(kCFAllocatorDefault, resultString)!)")
} else {
    print("failed to get resultString")
}