Atomicity updates for Thread Sanitizer warnings (#237)

* Refactored Atomic implementation to avoid TSan warnings

* Additional reference

Author: bsneed

Sources/Segment/Utilities/Atomic.swift

@@ -7,29 +7,26 @@
 
 import Foundation
 
+// NOTE: Revised from previous implementation which used a struct and NSLock's.
+// Thread Sanitizer was *correctly* capturing this issue, which was a little obscure
+// given the property wrapper PLUS the semantics of a struct.  Moving to `class`
+// removes the semantics problem and lets TSan approve of what's happening.
+//
+// Additionally, moving to a lock free version is just desirable, so moved to a queue.
+//
+// Also see thread here: https://github.com/apple/swift-evolution/pull/1387
+
 @propertyWrapper
-public struct Atomic<T> {
-    var value: T
-    private let lock = NSLock()
+public class Atomic<T> {
+    private var value: T
+    private let queue = DispatchQueue(label: "com.segment.atomic.\(UUID().uuidString)")
 
     public init(wrappedValue value: T) {
         self.value = value
     }
 
     public var wrappedValue: T {
-      get { return load() }
-      set { store(newValue: newValue) }
-    }
-
-    func load() -> T {
-        lock.lock()
-        defer { lock.unlock() }
-        return value
-    }
-
-    mutating func store(newValue: T) {
-        lock.lock()
-        defer { lock.unlock() }
-        value = newValue
+        get { return queue.sync { return value } }
+        set { queue.sync { value = newValue } }
     }
 }

Sources/Segment/Utilities/Policies/CountBasedFlushPolicy.swift

@@ -10,7 +10,7 @@ import Foundation
 public class CountBasedFlushPolicy: FlushPolicy {
     public weak var analytics: Analytics?
     internal var desiredCount: Int?
-    internal var count: Int = 0
+    @Atomic internal var count: Int = 0
 
     init() { }