fix: improves handling on concurrent accesses to codableMap (#64)

* fix: improves handling on concurrent accesses to codableMap

* added Dispatch import for <Swift4.1

* fixed string interpolation issue, whitespacing

Sources/SwiftKueryORM/TableInfo.swift

@@ -18,23 +18,35 @@ import KituraContracts
 import SwiftKuery
 import Foundation
 import TypeDecoder
+#if os(Linux)
+    import Dispatch
+#endif
 
 /// Class caching the tables for the models of the application
 
 public class TableInfo {
   private var codableMap = [String: (info: TypeInfo, table: Table)]()
+  private var codableMapLock = DispatchSemaphore(value: 1)
 
   /// Get the table for a model
   func getTable<T: Decodable>(_ idColumn: (name: String, type: SQLDataType.Type), _ tableName: String, for type: T.Type) throws -> Table {
     return try getInfo(idColumn, tableName, type).table
   }
 
   func getInfo<T: Decodable>(_ idColumn: (name: String, type: SQLDataType.Type), _ tableName: String, _ type: T.Type) throws -> (info: TypeInfo, table: Table) {
-    if codableMap["\(type)"] == nil {
-      let typeInfo = try TypeDecoder.decode(type)
-      codableMap["\(type)"] = (info: typeInfo, table: try constructTable(idColumn, tableName, typeInfo))
+    let typeString = "\(type)"
+    if let result = codableMap[typeString] {
+        return result
     }
-    return codableMap["\(type)"]!
+
+    codableMapLock.wait()
+    defer { codableMapLock.signal() }
+
+    let typeInfo = try TypeDecoder.decode(type)
+    let result = (info: typeInfo, table: try constructTable(idColumn, tableName, typeInfo))
+    codableMap[typeString] = result
+
+    return result
   }
 
   /// Construct the table for a Model