Add a basic mutex test

test/stdlib/Synchronization/Mutex/LockSingleConsumerStack.swift

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+// RUN: %target-run-simple-swift(%import-libdispatch)
+// REQUIRES: executable_test
+// REQUIRES: libdispatch
+// REQUIRES: synchronization
+
+import Synchronization
+import StdlibUnittest
+import Dispatch
+
+let suite = TestSuite("LockSingleConsumerStackTests")
+
+@available(SwiftStdlib 6.0, *)
+class LockSingleConsumerStack<Element> {
+  struct Node {
+    let value: Element
+    var next: UnsafeMutablePointer<Node>?
+  }
+  typealias NodePtr = UnsafeMutablePointer<Node>
+
+  private let _last = Mutex<NodePtr?>(nil)
+  private let _consumerCount = Mutex<Int>(0)
+  private var foo = 0
+
+  deinit {
+    // Discard remaining nodes
+    while let _ = pop() {}
+  }
+
+  // Push the given element to the top of the stack.
+  // It is okay to concurrently call this in an arbitrary number of threads.
+  func push(_ value: Element) {
+    let new = NodePtr.allocate(capacity: 1)
+    new.initialize(to: Node(value: value, next: nil))
+
+    _last.withLockUnchecked {
+      new.pointee.next = $0
+      $0 = new
+    }
+  }
+
+  // Pop and return the topmost element from the stack.
+  // This method does not support multiple overlapping concurrent calls.
+  func pop() -> Element? {
+    precondition(
+      _consumerCount.withLock {
+        let old = $0
+        $0 += 1
+        return old == 0
+      },
+      "Multiple consumers detected")
+
+    defer {
+      _consumerCount.withLock {
+        $0 -= 1
+      }
+    }
+
+    return _last.withLock { (c: inout NodePtr?) -> Element? in
+      guard let current = c else {
+        return nil
+      }
+
+      c = current.pointee.next
+
+      let result = current.move()
+      current.deallocate()
+      return result.value
+    }
+  }
+}
+
+if #available(SwiftStdlib 6.0, *) {
+
+suite.test("basics") {
+  let stack = LockSingleConsumerStack<Int>()
+  expectNil(stack.pop())
+  stack.push(0)
+  expectEqual(0, stack.pop())
+
+  stack.push(1)
+  stack.push(2)
+  stack.push(3)
+  stack.push(4)
+  expectEqual(4, stack.pop())
+  expectEqual(3, stack.pop())
+  expectEqual(2, stack.pop())
+  expectEqual(1, stack.pop())
+  expectNil(stack.pop())
+}
+
+suite.test("concurrentPushes") {
+  let stack = LockSingleConsumerStack<(thread: Int, value: Int)>()
+
+  let numThreads = 100
+  let numValues = 10_000
+  DispatchQueue.concurrentPerform(iterations: numThreads) { thread in
+    for value in 1 ... numValues {
+      stack.push((thread: thread, value: value))
+    }
+  }
+
+  var expected: [Int] = Array(repeating: numValues, count: numThreads)
+  while let (thread, value) = stack.pop() {
+    expectEqual(expected[thread], value)
+    expected[thread] -= 1
+  }
+  expectEqual(Array(repeating: 0, count: numThreads), expected)
+}
+
+suite.test("concurrentPushesAndPops") {
+  let stack = LockSingleConsumerStack<(thread: Int, value: Int)>()
+
+  let numThreads = 100
+  let numValues = 10_000
+
+  var perThreadSums: [Int] = Array(repeating: 0, count: numThreads)
+  let consumerQueue = DispatchQueue(label: "org.swift.background")
+  consumerQueue.async {
+    var count = 0
+    while count < numThreads * numValues {
+      // Note: busy wait
+      if let (thread, value) = stack.pop() {
+        perThreadSums[thread] += value
+        count += 1
+      }
+    }
+  }
+
+  DispatchQueue.concurrentPerform(iterations: numThreads + 1) { thread in
+    if thread < numThreads {
+      // Producers
+      for value in 0 ..< numValues {
+        stack.push((thread: thread, value: value))
+      }
+    }
+  }
+
+  consumerQueue.sync {
+    expectEqual(Array(repeating: numValues * (numValues - 1) / 2, count: numThreads), perThreadSums)
+  }
+}
+
+} // if #available(SwiftStdlib 6.0)
+
+runAllTests()