UserDefaults don't ~~persist~~ auto-synchronise on Linux

[Diggory]

Sample code:

import Foundation

print("User Defaults")

#if os(macOS)
print("macOS")
#elseif os(Linux)
print("Linux")
#endif

//	Keys
struct MyDefaultsKeys {
	static let aSampleString 	= "aSampleStringDefault"
	static let aSampleNumber 	= "aSampleNumberDefault"
	static let aSampleDate 		= "aSampleNumberDate"
}

//	Get singleton
let defaults = UserDefaults.standard

// At start, must register factory defaults.
let factoryDefaults = [
	MyDefaultsKeys.aSampleString : "Hello World!",
	MyDefaultsKeys.aSampleNumber : 42.0,
	MyDefaultsKeys.aSampleDate : Date.distantPast,
] as [String : Any]

defaults.register(defaults: factoryDefaults)

//	Read
let aString = defaults.string(forKey: MyDefaultsKeys.aSampleString)
print("aString: \(aString ?? "NO VALUE FOR \(MyDefaultsKeys.aSampleString) IN DEFAULTS")")

let aNumber = defaults.double(forKey: MyDefaultsKeys.aSampleNumber)
print("aNumber: \(aNumber)")

if let aDate = defaults.object(forKey: MyDefaultsKeys.aSampleDate) as? Date {
	print("aDate: \(aDate)")
} else {
	print("NO VALUE FOR \(MyDefaultsKeys.aSampleDate) IN DEFAULTS")
}

//	Write
defaults.set("I have changed!", forKey: MyDefaultsKeys.aSampleString)
defaults.set(1234567, forKey: MyDefaultsKeys.aSampleNumber)
defaults.set(Date(), forKey: MyDefaultsKeys.aSampleDate)

Output:

macOS:

First Run

User Defaults
macOS
aString: Hello World!
aNumber: 42.0
aDate: 0001-01-01 12:00:00 am +0000
Program ended with exit code: 0

Second Run

User Defaults
macOS
aString: I have changed!
aNumber: 1234567.0
aDate: 2023-10-29 7:34:56 am +0000
Program ended with exit code: 0

Linux

First Run

Build complete! (2.39s)
User Defaults
Linux
aString: Hello World!
aNumber: 42.0
aDate: 0000-12-30 00:00:00 +0000

Second Run

Build complete! (0.45s)
User Defaults
Linux
aString: Hello World!
aNumber: 42.0
aDate: 0000-12-30 00:00:00 +0000

[Diggory]

It turns-out that the issue is that synchronise() is not automatic on Linux. The user must manually call synchronize().

e.g. the sample code above behaves correctly if you alter the last section to this:

//	Write
defaults.set("I have changed!", forKey: MyDefaultsKeys.aSampleString)
defaults.set(1234567, forKey: MyDefaultsKeys.aSampleNumber)
defaults.set(Date(), forKey: MyDefaultsKeys.aSampleDate)

defaults.synchronize()