Unsafe Handling of DateFormatter

[sidepelican]

Problem

The ISO8601DateFormatter in swift-corelibs-foundation is not thread-safe.
However, URLEncodedFormDecoder and URLEncodedFormEncoder use this class as a singleton. This can cause a runtime crash, such as a segmentation fault.

Reference to the code:

hummingbird/Sources/HummingbirdFoundation/Codable/URLEncodedForm/URLEncodedForm.swift

Lines 51 to 55 in d7fb66e

	static var iso8601Formatter: ISO8601DateFormatter = {
	let formatter = ISO8601DateFormatter()
	formatter.formatOptions = .withInternetDateTime
	return formatter
	}()

Steps to Reproduce

The following code, when run multiple times in a Linux Swift environment, sometimes causes a crash:

import Foundation

let iso8601Formatter: ISO8601DateFormatter = {
  let formatter = ISO8601DateFormatter()
  formatter.formatOptions = .withInternetDateTime
  return formatter
}()

let now = Date()
for i in 0..<1000 {
  DispatchQueue.global().async {
    let date = now.addingTimeInterval(Double(i) * 60 * 60 * 24)
    print(iso8601Formatter.string(from: date))
  }
}

sleep(1)

Tested Environment

• Swift 5.9 (swift-5.9-RELEASE)
• Target: x86_64-unknown-linux-gnu

[adam-fowler]

I couldn't get it to crash, but I've still got rid of the global instance.
Fixed in #274
I thought this had been fixed in corelibs foundation, but looking through issues it doesn't look like it has. I doubt it will now and we'll have to wait for the new Foundation

[adam-fowler]

Fix in v1.9.1