Add thread safety to Clock

Signed-off-by: Seweryn Plażuk <seweryn.plazuk@gmail.com>

Author: Seweryn Plażuk

Sources/Clock.swift

@@ -25,9 +25,20 @@ public typealias AnnotatedTime = (
 /// print(Clock.now)
 /// ```
 public struct Clock {
-    private static var stableTime: TimeFreeze? {
-        didSet {
-            self.storage.stableTime = self.stableTime
+    private static let syncQueue = DispatchQueue(label: "kronos.stable.time", attributes: .concurrent)
+
+    private static var _stableTime: TimeFreeze?
+    static var stableTime: TimeFreeze? {
+        get {
+            self.syncQueue.sync {
+                return self._stableTime
+            }
+        }
+        set {
+            self.syncQueue.async(flags: .barrier) {
+                self._stableTime = newValue
+                self.storage.stableTime = newValue
+            }
         }
     }
 

Sources/TimeFreeze.swift

@@ -7,7 +7,7 @@ private let kOffsetKey = "Offset"
 struct TimeFreeze {
     private let uptime: TimeInterval
     private let timestamp: TimeInterval
-    private let offset: TimeInterval
+    let offset: TimeInterval
 
     /// The stable timestamp adjusted by the most accurate offset known so far.
     var adjustedTimestamp: TimeInterval {

Tests/KronosTests/ClockTests.swift

@@ -38,4 +38,34 @@ final class ClockTests: XCTestCase {
 
         self.waitForExpectations(timeout: 20)
     }
+
+    func testStableTimeThreadSafety() {
+        let expectation = self.expectation(description: "Concurrent access to stableTime")
+        expectation.expectedFulfillmentCount = 10
+
+        let incrementBlock = {
+            for _ in 0..<1000 {
+                let newOffset = {
+                    if let offset = Clock.stableTime?.offset {
+                        return offset + 1
+                    } else {
+                        return 0
+                    }
+                }()
+                Clock.stableTime = TimeFreeze(offset: newOffset)
+            }
+        }
+
+        for _ in 0..<10 {
+            DispatchQueue.global().async {
+                incrementBlock()
+                expectation.fulfill()
+            }
+        }
+
+        self.waitForExpectations(timeout: 2)
+
+        XCTAssertEqual(Clock.stableTime?.offset, 1000)
+    }
+
 }