improve threading correctness in URLSession

Foundation/URLSession/URLSession.swift

@@ -179,11 +179,13 @@ extension URLSession {
     }
 }
 
+fileprivate let globalVarSyncQ = DispatchQueue(label: "org.swift.Foundation.URLSession.GlobalVarSyncQ")
 fileprivate var sessionCounter = Int32(0)
 fileprivate func nextSessionIdentifier() -> Int32 {
-    //TODO: find an alternative for OSAtomicIncrement32Barrier() on Linux
-    sessionCounter += 1
-    return sessionCounter
+    return globalVarSyncQ.sync {
+        sessionCounter += 1
+        return sessionCounter
+    }
 }
 public let NSURLSessionTransferSizeUnknown: Int64 = -1
 

Foundation/URLSession/URLSessionTask.swift

@@ -34,13 +34,10 @@ open class URLSessionTask : NSObject, NSCopying {
     internal var session: URLSessionProtocol! //change to nil when task completes
     internal let body: _Body
     fileprivate var _protocol: URLProtocol! = nil
+    private let syncQ = DispatchQueue(label: "org.swift.URLSessionTask.SyncQ")
 
     /// All operations must run on this queue.
     internal let workQueue: DispatchQueue 
-    /// Using dispatch semaphore to make public attributes thread safe.
-    /// A semaphore is a simpler option against the usage of concurrent queue
-    /// as the critical sections are very short.
-    fileprivate let semaphore = DispatchSemaphore(value: 1)    
 
     public override init() {
         // Darwin Foundation oddly allows calling this initializer, even though
@@ -66,7 +63,8 @@ open class URLSessionTask : NSObject, NSCopying {
     }
     internal init(session: URLSession, request: URLRequest, taskIdentifier: Int, body: _Body) {
         self.session = session
-        self.workQueue = session.workQueue
+        /* make sure we're actually having a serial queue as it's used for synchronization */
+        self.workQueue = DispatchQueue.init(label: "org.swift.URLSessionTask.WorkQueue", target: session.workQueue)
         self.taskIdentifier = taskIdentifier
         self.originalRequest = request
         self.body = body
@@ -112,31 +110,19 @@ open class URLSessionTask : NSObject, NSCopying {
     /// May differ from originalRequest due to http server redirection
     /*@NSCopying*/ open internal(set) var currentRequest: URLRequest? {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._currentRequest
+            return self.syncQ.sync { return self._currentRequest }
         }
         set {
-            semaphore.wait()
-            self._currentRequest = newValue
-            semaphore.signal()
+            self.syncQ.sync { self._currentRequest = newValue }
         }
     }
     fileprivate var _currentRequest: URLRequest? = nil
     /*@NSCopying*/ open internal(set) var response: URLResponse? {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._response
+            return self.syncQ.sync { return self._response }
         }
         set {
-            semaphore.wait()
-            self._response = newValue
-            semaphore.signal()
+            self.syncQ.sync { self._response = newValue }
         }
     }
     fileprivate var _response: URLResponse? = nil
@@ -149,33 +135,21 @@ open class URLSessionTask : NSObject, NSCopying {
     /// Number of body bytes already received
     open internal(set) var countOfBytesReceived: Int64 {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._countOfBytesReceived
+            return self.syncQ.sync { return self._countOfBytesReceived }
         }
         set {
-            semaphore.wait()
-            self._countOfBytesReceived = newValue
-            semaphore.signal()
+            self.syncQ.sync { self._countOfBytesReceived = newValue }
         }
     }
     fileprivate var _countOfBytesReceived: Int64 = 0
 
     /// Number of body bytes already sent */
     open internal(set) var countOfBytesSent: Int64 {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._countOfBytesSent
+            return self.syncQ.sync { return self._countOfBytesSent }
         }
         set {
-            semaphore.wait()
-            self._countOfBytesSent = newValue
-            semaphore.signal()
+            self.syncQ.sync { self._countOfBytesSent = newValue }
         }
     }
 
@@ -215,16 +189,10 @@ open class URLSessionTask : NSObject, NSCopying {
      */
     open var state: URLSessionTask.State {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._state
+            return self.syncQ.sync { self._state }
         }
         set {
-            semaphore.wait()
-            self._state = newValue
-            semaphore.signal()
+            self.syncQ.sync { self._state = newValue }
         }
     }
     fileprivate var _state: URLSessionTask.State = .suspended
@@ -305,16 +273,10 @@ open class URLSessionTask : NSObject, NSCopying {
     /// URLSessionTask.highPriority, but use is not restricted to these.
     open var priority: Float {
         get {
-            semaphore.wait()
-            defer {
-                semaphore.signal()
-            }
-            return self._priority
+            return self.workQueue.sync { return self._priority }
         }
         set {
-            semaphore.wait()
-            self._priority = newValue
-            semaphore.signal()
+            self.workQueue.sync { self._priority = newValue }
         }
     }
     fileprivate var _priority: Float = URLSessionTask.defaultPriority
@@ -565,7 +527,9 @@ extension _ProtocolClient : URLProtocolClient {
             session.delegateQueue.addOperation {
                 delegate.urlSession(session, task: task, didCompleteWithError: nil)
                 task.state = .completed
-                session.taskRegistry.remove(task)
+                task.workQueue.async {
+                    session.taskRegistry.remove(task)
+                }
             }
         case .noDelegate:
             task.state = .completed
@@ -616,7 +580,9 @@ extension _ProtocolClient : URLProtocolClient {
             session.delegateQueue.addOperation {
                 delegate.urlSession(session, task: task, didCompleteWithError: error as Error)
                 task.state = .completed
-                session.taskRegistry.remove(task)
+                task.workQueue.async {
+                    session.taskRegistry.remove(task)
+                }
             }
         case .noDelegate:
             task.state = .completed
@@ -625,7 +591,9 @@ extension _ProtocolClient : URLProtocolClient {
             session.delegateQueue.addOperation {
                 completion(nil, nil, error)
                 task.state = .completed
-                session.taskRegistry.remove(task)
+                task.workQueue.async {
+                    session.taskRegistry.remove(task)
+                }
             }
         case .downloadCompletionHandler(let completion):
             session.delegateQueue.addOperation {

TestFoundation/HTTPServer.swift

@@ -26,6 +26,8 @@ import Dispatch
 #endif
 
 public let globalDispatchQueue = DispatchQueue.global()
+public let dispatchQueueMake: (String) -> DispatchQueue = { DispatchQueue.init(label: $0) }
+public let dispatchGroupMake: () -> DispatchGroup = DispatchGroup.init
 
 struct _HTTPUtils {
     static let CRLF = "\r\n"

TestFoundation/TestURLSession.swift

@@ -43,6 +43,7 @@ class TestURLSession : LoopbackServerTest {
             ("test_illegalHTTPServerResponses", test_illegalHTTPServerResponses),
             ("test_dataTaskWithSharedDelegate", test_dataTaskWithSharedDelegate),
             ("test_simpleUploadWithDelegate", test_simpleUploadWithDelegate),
+            ("test_concurrentRequests", test_concurrentRequests),
         ]
     }
 
@@ -459,6 +460,34 @@ class TestURLSession : LoopbackServerTest {
         task.resume()
         waitForExpectations(timeout: 20)
     }
+
+    func test_concurrentRequests() {
+        let syncQ = dispatchQueueMake("test_dataTaskWithURL.syncQ")
+        var dataTasks: [DataTask] = []
+        let g = dispatchGroupMake()
+        for f in 0..<640 {
+            g.enter()
+            let urlString = "http://127.0.0.1:\(TestURLSession.serverPort)/Nepal"
+            let expectation = self.expectation(description: "GET \(urlString) [\(f)]: with a delegate")
+            globalDispatchQueue.async {
+                let url = URL(string: urlString)!
+                let d = DataTask(with: expectation)
+                d.run(with: url)
+                syncQ.async {
+                    dataTasks.append(d)
+                    g.leave()
+                }
+            }
+        }
+        waitForExpectations(timeout: 12)
+        g.wait()
+        for d in syncQ.sync(execute: {dataTasks}) {
+            if !d.error {
+                XCTAssertEqual(d.capital, "Kathmandu", "test_dataTaskWithURLRequest returned an unexpected result")
+            }
+        }
+    }
+
 }
 
 class SharedDelegate: NSObject {
@@ -488,19 +517,73 @@ class SessionDelegate: NSObject, URLSessionDelegate {
 }
 
 class DataTask : NSObject {
+    let syncQ = dispatchQueueMake("org.swift.TestFoundation.TestURLSession.DataTask.syncQ")
     let dataTaskExpectation: XCTestExpectation!
-    var capital = "unknown"
-    var session: URLSession! = nil
-    var task: URLSessionDataTask! = nil
-    var cancelExpectation: XCTestExpectation?
-    var responseReceivedExpectation: XCTestExpectation?
-    var protocols: [AnyClass]?
+    let protocols: [AnyClass]?
+
+    /* all the following var _XYZ need to be synchronized on syncQ.
+       We can't just assert that we're on main thread here as we're modified in the URLSessionDataDelegate extension
+       for DataTask
+     */
+    var _capital = "unknown"
+    var capital: String {
+        get {
+            return self.syncQ.sync { self._capital }
+        }
+        set {
+            self.syncQ.sync { self._capital = newValue }
+        }
+    }
+    var _session: URLSession! = nil
+    var session: URLSession! {
+        get {
+            return self.syncQ.sync { self._session }
+        }
+        set {
+            self.syncQ.sync { self._session = newValue }
+        }
+    }
+    var _task: URLSessionDataTask! = nil
+    var task: URLSessionDataTask! {
+        get {
+            return self.syncQ.sync { self._task }
+        }
+        set {
+            self.syncQ.sync { self._task = newValue }
+        }
+    }
+    var _cancelExpectation: XCTestExpectation?
+    var cancelExpectation: XCTestExpectation? {
+        get {
+            return self.syncQ.sync { self._cancelExpectation }
+        }
+        set {
+            self.syncQ.sync { self._cancelExpectation = newValue }
+        }
+    }
+    var _responseReceivedExpectation: XCTestExpectation?
+    var responseReceivedExpectation: XCTestExpectation? {
+        get {
+            return self.syncQ.sync { self._responseReceivedExpectation }
+        }
+        set {
+            self.syncQ.sync { self._responseReceivedExpectation = newValue }
+        }
+    }
 
-    public var error = false
+    private var _error = false
+    public var error: Bool {
+        get {
+            return self.syncQ.sync { self._error }
+        }
+        set {
+            self.syncQ.sync { self._error = newValue }
+        }
+    }
 
     init(with expectation: XCTestExpectation, protocolClasses: [AnyClass]? = nil) {
         dataTaskExpectation = expectation
-	protocols = protocolClasses
+        protocols = protocolClasses
     }
 
     func run(with request: URLRequest) {