[AltServer] Fixes potential race condition crash when managing connections

Author: rileytestut

Shared/Connections/ConnectionManager.swift

@@ -41,6 +41,7 @@ public class ConnectionManager<RequestHandlerType: RequestHandler>
     public var isStarted = false
 
     private var connections = [Connection]()
+    private let connectionsLock = NSLock()
 
     public init(requestHandler: RequestHandlerType, connectionHandlers: [ConnectionHandler])
     {
@@ -88,6 +89,9 @@ private extension ConnectionManager
 {
     func prepare(_ connection: Connection)
     {
+        self.connectionsLock.lock()
+        defer { self.connectionsLock.unlock() }
+        
         guard !self.connections.contains(where: { $0 === connection }) else { return }
         self.connections.append(connection)
 
@@ -96,6 +100,9 @@ private extension ConnectionManager
 
     func disconnect(_ connection: Connection)
     {
+        self.connectionsLock.lock()
+        defer { self.connectionsLock.unlock() }
+        
         guard let index = self.connections.firstIndex(where: { $0 === connection }) else { return }
         self.connections.remove(at: index)
     }