Add thread safety unit test for LazyCache

Tests/TSCBasicTests/LazyCacheTests.swift

@@ -13,20 +13,19 @@ import XCTest
 import TSCBasic
 
 class LazyCacheTests: XCTestCase {
-    func testBasics() {
-        class Foo {
-            var numCalls = 0
-
-            var bar: Int { return barCache.getValue(self) }
-            var barCache = LazyCache<Foo, Int>(someExpensiveMethod)
-            func someExpensiveMethod() -> Int {
-                numCalls += 1
-                return 42
-            }
-
+    private class Foo {
+        var numCalls = 0
+
+        var bar: Int { return barCache.getValue(self) }
+        var barCache = LazyCache<Foo, Int>(someExpensiveMethod)
+        func someExpensiveMethod() -> Int {
+            numCalls += 1
+            return 42
         }
 
-        // FIXME: Make this a more interesting test once we have concurrency primitives.
+    }
+
+    func testBasics() {
         for _ in 0..<10 {
             let foo = Foo()
             XCTAssertEqual(foo.numCalls, 0)
@@ -36,4 +35,27 @@ class LazyCacheTests: XCTestCase {
             }
         }
     }
+
+    func testThreadSafety() {
+        let dispatchGroup = DispatchGroup()
+        let exp = expectation(description: "multi thread")
+        for _ in 0..<10 {
+            let foo = Foo()
+            for _ in 0..<10 {
+                dispatchGroup.enter()
+                DispatchQueue.global().async {
+                    XCTAssertEqual(foo.bar, 42)
+                    dispatchGroup.leave()
+
+                    XCTAssertEqual(foo.numCalls, 1)
+                }
+            }
+        }
+
+        dispatchGroup.notify(queue: .main) {
+            exp.fulfill()
+        }
+
+        wait(for: [exp], timeout: 0.2)
+    }
 }