Fixing Race Condition on multiple commits to saveReceipts

[brandonmaul]

This fix solves a relatively rare race condition that happens when multiple commits are made to MXFileStore in quick succession to readReceipts. It manifests in an error such as in #1810

To solve this, we copy the entire RoomThreadedReceiptsStore for a room, and it's sub-values. We then use that "copy" to actually write to the store, and passing it along to the NSKeyedArchiver.

If there is a better approach to solving this please let us know, we're more than happy to evaluate it and we understand that this may not be the most efficient way of solving this issue.

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off

Signed-off-by: Brandon Maul brandon.maul@powerhrg.com

[brandonmaul]

@pixlwave @stefanceriu @richvdh Asking for a review

[brandonmaul]

For more context, here is roughly how you can reproduce the bug (that this change fixes)

let mxSession: MXSession = ...
let creds: MXCredentials = ...
guard mxSession.store is MXFileStore else {
  print("⚠️ Session's store is not MXFileStore.")
  return
}

var receipts: [MXReceiptData] = []
for i in 0...1000 {
  let receipt = MXReceiptData()
  receipt.userId      = creds.userId! + String(describing: i)
  receipt.threadId    = kMXEventTimelineMain
  receipt.eventId     = "$crashRepro\(UUID().uuidString)"
  receipt.ts          = UInt64(Date().timeIntervalSince1970 * 1000)
  
  receipts.append(receipt)
}

for i in 0...1000 {
  let room = "!reproRoom:\(creds.homeServer!)" + String(describing: i)
  for j in 0...1000 {
    let receipt = receipts[j]
    mxSession.store.storeReceipt(receipt, inRoom: room)
    mxSession.store.commit?()
  }
}

[AlexandreHauber]

Following 👀!