Implement Statistic with atomics so it's thread safe

Prevents false positives in TSAN which obscure other real issues.

Author: jakepetroules

Sources/SWBUtil/Statistics.swift

@@ -10,21 +10,23 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Synchronization
+
 /// A scope to contain a group of statistics.
 public final class StatisticsGroup: Sendable {
     /// The name for this group.
     public let name: String
 
-    private let _statistics = LockedValue<[Statistic]>([])
+    private let _statistics = LockedValue<[any _StatisticBackend]>([])
 
     /// The list of statistics in the group.
-    public var statistics: [Statistic] { return _statistics.withLock { $0 } }
+    public var statistics: [any _StatisticBackend] { return _statistics.withLock { $0 } }
 
     public init(_ name: String) {
         self.name = name
     }
 
-    public func register(_ statistic: Statistic) {
+    public func register(_ statistic: any _StatisticBackend) {
         _statistics.withLock { $0.append(statistic) }
     }
 
@@ -40,20 +42,16 @@ public final class StatisticsGroup: Sendable {
 ///
 /// Currently statistics are always integers and are not thread safe (unless building in TSan mode); clients should implement their own locking if an accurate count is required.
 // FIXME: This should unconditionally be implemented using atomics, not conditionally be using a queue based on TSan...
-public final class Statistic: @unchecked Sendable {
+@available(macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0, visionOS 2, *)
+public final class _Statistic: @unchecked Sendable, _StatisticBackend {
     /// The name of the statistics.
     public let name: String
 
     /// The description of the statistics.
     public let description: String
 
-    #if ENABLE_THREAD_SANITIZER
-    /// Queue to serialize access to the statistic value.
-    let _queue = SWBQueue(label: "com.apple.dt.SWBUtil.Statistics")
-    #endif
-
     /// The value of the statistic.
-    var _value: Int = 0
+    private let _value = Atomic<Int>(0)
 
     public init(_ name: String, _ description: String, _ group: StatisticsGroup = allStatistics) {
         self.name = name
@@ -64,28 +62,19 @@ public final class Statistic: @unchecked Sendable {
 
     /// Get the current value of the statistic.
     public var value: Int {
-        return sync { _value }
+        return _value.load(ordering: .relaxed)
     }
 
     /// Increment the statistic.
     public func increment(_ n: Int = 1) {
-        sync { _value += n }
+        _value.wrappingAdd(n, ordering: .relaxed)
     }
 
     /// Zero all of the statistics.
     ///
     /// This is useful when using statistics to probe program behavior from within tests, and the test can guarantee no concurrent access.
     public func zero() {
-        sync { _value = 0 }
-    }
-
-    /// Helper method to execute a block on our serial queue (if it's enabled).
-    public func sync<T>(execute work: () throws -> T) rethrows -> T {
-        #if ENABLE_THREAD_SANITIZER
-        return try _queue.blocking_sync { try work() }
-        #else
-        return try work()
-        #endif
+        _value.store(0, ordering: .relaxed)
     }
 }
 
@@ -97,3 +86,44 @@ public let allStatistics = StatisticsGroup("swift-build")
 public func +=(statistic: Statistic, rhs: Int = 1) {
     statistic.increment(rhs)
 }
+
+// MARK: Back-deployment
+
+public final class Statistic: @unchecked Sendable, _StatisticBackend {
+    public let name: String
+    private let _statistic: (any _StatisticBackend)?
+
+    public init(_ name: String, _ description: String, _ group: StatisticsGroup = allStatistics) {
+        self.name = name
+        if #available(macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0, visionOS 2.0, *) {
+            _statistic = _Statistic(name, description, group)
+        } else {
+            _statistic = nil
+        }
+    }
+
+    public var value: Int {
+        _statistic?.value ?? 0
+    }
+
+    public func increment(_ n: Int) {
+        _statistic?.increment(n)
+    }
+
+    public func zero() {
+        _statistic?.zero()
+    }
+}
+
+public protocol _StatisticBackend {
+    var name: String { get }
+    var value: Int { get }
+    func increment(_ n: Int)
+    func zero()
+}
+
+extension _StatisticBackend {
+    public func increment() {
+        self.increment(1)
+    }
+}