Use block scope thread_local static for NSProgress progress stack (#1912

)

* Use block scope thread_local static for NSProgress progress stack:

Works around issue with file_scope thread_local in current clang. Leak check unit test added.

* cr feedback: collapse initialization

* switch to auto and return type deduction

Frameworks/Foundation/NSProgress.mm

@@ -46,16 +46,10 @@
     bool childCreated;
 };
 
-// Stack of NSProgress objects that have becomeCurrent
-thread_local static std::shared_ptr<std::stack<CurrentProgress>> s_currentProgressStack;
-
 // Returns the stack of NSProgress objects that have becomeCurrent on the current thread, initializing it if necessary
-static decltype(s_currentProgressStack) & _getProgressStackForCurrentThread() {
-    if (!s_currentProgressStack) {
-        s_currentProgressStack = std::make_shared<std::stack<CurrentProgress>>();
-    }
-
-    return s_currentProgressStack;
+static auto& _getProgressStackForCurrentThread() {
+    thread_local static auto tlsCurrentProgressStack = std::make_shared<std::stack<CurrentProgress>>();
+    return tlsCurrentProgressStack;
 }
 
 @implementation NSProgress {
@@ -105,8 +99,8 @@ - (instancetype)initWithParent:(NSProgress*)parentProgressOrNil userInfo:(NSDict
                 // Assign parent
                 _parent = parentProgressOrNil;
 
-                // s_currentProgressStack must not be empty to have reached here
-                CurrentProgress& current = s_currentProgressStack->top();
+                // Progress stack must not be empty to have reached here
+                CurrentProgress& current = _getProgressStackForCurrentThread()->top();
 
                 // Keep track of the pendingUnitCount to increment in the parent
                 _parentPendingUnitCount = current.pendingUnitCountToAssign;

tests/unittests/Foundation/NSProgressTests.mm

@@ -583,3 +583,16 @@ - (void)addParentTo:(NSProgress*)child {
         andExpectChangeCallbacks:nil];
     EXPECT_EQ(1, kvoListener.hits);
 }
+
+TEST(NSProgress, CurrentProgressLeak) {
+    NSProgress* progress = [NSProgress progressWithTotalUnitCount:100];
+
+    std::thread t([&progress]() {
+        [progress becomeCurrentWithPendingUnitCount:50];
+    });
+
+    t.join();
+
+    // Progress unfinished upon thread exit. Ensure we're not keeping progress alive:
+    EXPECT_EQ(1, [progress retainCount]);
+}