Cancel DispatchSource before closing socket (swiftlang#4791)

Extends socket lifetime enough to let DispatchSource cancel properly.
Also prevents from creating new DispatchSources while other are in the
middle of cancelling.

Also includes tests (see swiftlang#4854 for test details).

Author: lxbndr

CoreFoundation/URL.subproj/CFURLSessionInterface.c

@@ -111,6 +111,10 @@ CFURLSessionEasyCode CFURLSession_easy_setopt_tc(CFURLSessionEasyHandle _Nonnull
     return MakeEasyCode(curl_easy_setopt(curl,  option.value, a));
 }
 
+CFURLSessionEasyCode CFURLSession_easy_setopt_scl(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionOption option, CFURLSessionCloseSocketCallback * _Nullable a) {
+    return MakeEasyCode(curl_easy_setopt(curl, option.value, a));
+}
+
 CFURLSessionEasyCode CFURLSession_easy_getinfo_long(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionInfo info, long *_Nonnull a) {
     return MakeEasyCode(curl_easy_getinfo(curl, info.value, a));
 }

CoreFoundation/URL.subproj/CFURLSessionInterface.h

@@ -625,6 +625,8 @@ typedef int (CFURLSessionSeekCallback)(void *_Nullable userp, long long offset,
 CF_EXPORT CFURLSessionEasyCode CFURLSession_easy_setopt_seek(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionOption option, CFURLSessionSeekCallback * _Nullable a);
 typedef int (CFURLSessionTransferInfoCallback)(void *_Nullable userp, long long dltotal, long long dlnow, long long ultotal, long long ulnow);
 CF_EXPORT CFURLSessionEasyCode CFURLSession_easy_setopt_tc(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionOption option, CFURLSessionTransferInfoCallback * _Nullable a);
+typedef int (CFURLSessionCloseSocketCallback)(void *_Nullable clientp, CFURLSession_socket_t item);
+CF_EXPORT CFURLSessionEasyCode CFURLSession_easy_setopt_scl(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionOption option, CFURLSessionCloseSocketCallback * _Nullable a);
 
 CF_EXPORT CFURLSessionEasyCode CFURLSession_easy_getinfo_long(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionInfo info, long *_Nonnull a);
 CF_EXPORT CFURLSessionEasyCode CFURLSession_easy_getinfo_double(CFURLSessionEasyHandle _Nonnull curl, CFURLSessionInfo info, double *_Nonnull a);

Sources/FoundationNetworking/URLSession/libcurl/MultiHandle.swift

@@ -45,6 +45,7 @@ extension URLSession {
         let queue: DispatchQueue
         let group = DispatchGroup()
         fileprivate var easyHandles: [_EasyHandle] = []
+        fileprivate var socketReferences: [CFURLSession_socket_t: _SocketReference] = [:]
         fileprivate var timeoutSource: _TimeoutSource? = nil
         private var reentrantInUpdateTimeoutTimer = false
 
@@ -127,13 +128,14 @@ fileprivate extension URLSession._MultiHandle {
             if let opaque = socketSourcePtr {
                 Unmanaged<_SocketSources>.fromOpaque(opaque).release()
             }
+            socketSources?.tearDown(handle: self, socket: socket, queue: queue)
             socketSources = nil
         }
         if let ss = socketSources {
             let handler = DispatchWorkItem { [weak self] in
                 self?.performAction(for: socket)
             }
-            ss.createSources(with: action, socket: socket, queue: queue, handler: handler)
+            ss.createSources(with: action, handle: self, socket: socket, queue: queue, handler: handler)
         }
         return 0
     }
@@ -161,9 +163,104 @@ extension Collection where Element == _EasyHandle {
   }
 }
 
+private extension URLSession._MultiHandle {
+    class _SocketReference {
+        let socket: CFURLSession_socket_t
+        var shouldClose: Bool
+        var workItem: DispatchWorkItem?
+        
+        init(socket: CFURLSession_socket_t) {
+            self.socket = socket
+            shouldClose = false
+        }
+        
+        deinit {
+            if shouldClose {
+                #if os(Windows)
+                closesocket(socket)
+                #else
+                close(socket)
+                #endif
+            }
+        }
+    }
+
+    /// Creates and stores socket reference. Reentrancy is not supported.
+    /// Trying to begin operation for same socket twice would mean something
+    /// went horribly wrong, or our assumptions about CURL register/unregister
+    /// action flow are nor correct.
+    func beginOperation(for socket: CFURLSession_socket_t) -> _SocketReference {
+        let reference = _SocketReference(socket: socket)
+        precondition(socketReferences.updateValue(reference, forKey: socket) == nil, "Reentrancy is not supported for socket operations")
+        return reference
+    }
+
+    /// Removes socket reference from the shared store. If there is work item scheduled,
+    /// executes it on the current thread.
+    func endOperation(for socketReference: _SocketReference) {
+        precondition(socketReferences.removeValue(forKey: socketReference.socket) != nil, "No operation associated with the socket")
+        if let workItem = socketReference.workItem, !workItem.isCancelled {
+            // CURL never asks for socket close without unregistering first, and
+            // we should cancel pending work when unregister action is requested.
+            precondition(!socketReference.shouldClose, "Socket close was scheduled, but there is some pending work left")
+            workItem.perform()
+        }
+    }
+
+    /// Marks this reference to close socket on deinit. This allows us
+    /// to extend socket lifecycle by keeping the reference alive.
+    func scheduleClose(for socket: CFURLSession_socket_t) {
+        let reference = socketReferences[socket] ?? _SocketReference(socket: socket)
+        reference.shouldClose = true
+    }
+
+    /// Schedules work to be performed when an operation ends for the socket,
+    /// or performs it immediately if there is no operation in progress.
+    /// 
+    /// We're using this to postpone Dispatch Source creation when
+    /// previous Dispatch Source is not cancelled yet.
+    func schedule(_ workItem: DispatchWorkItem, for socket: CFURLSession_socket_t) {
+        guard let socketReference = socketReferences[socket] else { 
+            workItem.perform()
+            return
+        }
+        // CURL never asks for register without pairing it with unregister later,
+        // and we're cancelling pending work item on unregister.
+        // But it is safe to just drop existing work item anyway,
+        // and replace it with the new one.
+        socketReference.workItem = workItem
+    }
+
+    /// Cancels pending work for socket operation. Does nothing if
+    /// there is no operation in progress or no pending work item.
+    /// 
+    /// CURL may become not interested in Dispatch Sources
+    /// we have planned to create. In this case we should just cancel
+    /// scheduled work. 
+    func cancelWorkItem(for socket: CFURLSession_socket_t) {
+        guard let socketReference = socketReferences[socket] else { 
+            return
+        }
+        socketReference.workItem?.cancel()
+        socketReference.workItem = nil
+    }
+
+}
+
 internal extension URLSession._MultiHandle {
     /// Add an easy handle -- start its transfer.
     func add(_ handle: _EasyHandle) {
+        // Set CLOSESOCKETFUNCTION. Note that while the option belongs to easy_handle,
+        // the connection cache is managed by CURL multi_handle, and sockets can actually
+        // outlive easy_handle (even after curl_easy_cleanup call). That's why
+        // socket management lives in _MultiHandle.
+        try! CFURLSession_easy_setopt_ptr(handle.rawHandle, CFURLSessionOptionCLOSESOCKETDATA, UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())).asError()
+        try! CFURLSession_easy_setopt_scl(handle.rawHandle, CFURLSessionOptionCLOSESOCKETFUNCTION) {  (clientp: UnsafeMutableRawPointer?, item: CFURLSession_socket_t) in
+            guard let handle = URLSession._MultiHandle.from(callbackUserData: clientp) else { fatalError() }
+            handle.scheduleClose(for: item)
+            return 0
+        }.asError()
+        
         // If this is the first handle being added, we need to `kick` the
         // underlying multi handle by calling `timeoutTimerFired` as
         // described in
@@ -448,25 +545,56 @@ fileprivate class _SocketSources {
         s.resume()
     }
 
-    func tearDown() {
-        if let s = readSource {
-            s.cancel()
+    func tearDown(handle: URLSession._MultiHandle, socket: CFURLSession_socket_t, queue: DispatchQueue) {
+        handle.cancelWorkItem(for: socket) // There could be pending register action which needs to be cancelled
+        
+        guard readSource != nil || writeSource != nil else {
+            // This means that we have posponed (and already abandoned)
+            // sources creation.
+            return
         }
-        readSource = nil
-        if let s = writeSource {
-            s.cancel()
+        
+        // Socket is guaranteed to not to be closed as long as we keeping
+        // the reference.
+        let socketReference = handle.beginOperation(for: socket)
+        let cancelHandlerGroup = DispatchGroup()
+        [readSource, writeSource].compactMap({ $0 }).forEach { source in
+            cancelHandlerGroup.enter()
+            source.setCancelHandler {   
+                cancelHandlerGroup.leave()
+            }
+            source.cancel()
+        }
+        cancelHandlerGroup.notify(queue: queue) {
+            handle.endOperation(for: socketReference)
         }
+        
+        readSource = nil
         writeSource = nil
     }
 }
 extension _SocketSources {
     /// Create a read and/or write source as specified by the action.
-    func createSources(with action: URLSession._MultiHandle._SocketRegisterAction, socket: CFURLSession_socket_t, queue: DispatchQueue, handler: DispatchWorkItem) {
-        if action.needsReadSource {
-            createReadSource(socket: socket, queue: queue, handler: handler)
+    func createSources(with action: URLSession._MultiHandle._SocketRegisterAction, handle: URLSession._MultiHandle, socket: CFURLSession_socket_t, queue: DispatchQueue, handler: DispatchWorkItem) {
+        // CURL casually requests to unregister and register handlers for same 
+        // socket in a row. There is (pretty low) chance of overlapping tear-down operation
+        // with "register" request. Bad things could happen if we create 
+        // a new Dispatch Source while other is being cancelled for the same socket.
+        // We're using `_MultiHandle.schedule(_:for:)` here to postpone sources creation until
+        // pending operation is finished (if there is none, submitted work item is performed
+        // immediately).
+        // Also, CURL may request unregister even before we perform any postponed work,
+        // so we have to cancel such work in such case. See 
+        let createSources = DispatchWorkItem {
+            if action.needsReadSource {
+                self.createReadSource(socket: socket, queue: queue, handler: handler)
+            }
+            if action.needsWriteSource {
+                self.createWriteSource(socket: socket, queue: queue, handler: handler)
+            }
         }
-        if action.needsWriteSource {
-            createWriteSource(socket: socket, queue: queue, handler: handler)
+        if (action.needsReadSource || action.needsWriteSource) {
+            handle.schedule(createSources, for: socket)
         }
     }
 }

Tests/Foundation/HTTPServer.swift

@@ -99,7 +99,7 @@ class _TCPSocket: CustomStringConvertible {
         listening = false
     }
 
-    init(port: UInt16?) throws {
+    init(port: UInt16?, backlog: Int32) throws {
         listening = true
         self.port = 0
 
@@ -124,7 +124,7 @@ class _TCPSocket: CustomStringConvertible {
         try socketAddress.withMemoryRebound(to: sockaddr.self, capacity: MemoryLayout<sockaddr>.size, { 
             let addr = UnsafePointer<sockaddr>($0)
             _ = try attempt("bind", valid: isZero, bind(_socket, addr, socklen_t(MemoryLayout<sockaddr>.size)))
-            _ = try attempt("listen", valid: isZero, listen(_socket, SOMAXCONN))
+            _ = try attempt("listen", valid: isZero, listen(_socket, backlog))
         })
 
         var actualSA = sockaddr_in()
@@ -295,8 +295,8 @@ class _HTTPServer: CustomStringConvertible {
     let tcpSocket: _TCPSocket
     var port: UInt16 { tcpSocket.port }
 
-    init(port: UInt16?) throws {
-        tcpSocket = try _TCPSocket(port: port)
+    init(port: UInt16?, backlog: Int32 = SOMAXCONN) throws {
+        tcpSocket = try _TCPSocket(port: port, backlog: backlog)
     }
 
     init(socket: _TCPSocket) {
@@ -1094,15 +1094,32 @@ enum InternalServerError : Error {
     case badHeaders
 }
 
+extension LoopbackServerTest {
+    struct Options {
+        var serverBacklog: Int32
+        var isAsynchronous: Bool
+        
+        static let `default` = Options(serverBacklog: SOMAXCONN, isAsynchronous: true)
+    }
+}
 
 class LoopbackServerTest : XCTestCase {
     private static let staticSyncQ = DispatchQueue(label: "org.swift.TestFoundation.HTTPServer.StaticSyncQ")
 
     private static var _serverPort: Int = -1
     private static var _serverActive = false
     private static var testServer: _HTTPServer? = nil
-
-
+    private static var _options: Options = .default
+    
+    static var options: Options {
+        get {
+            return staticSyncQ.sync { _options }
+        }
+        set {
+            staticSyncQ.sync { _options = newValue }
+        }
+    }
+    
     static var serverPort: Int {
         get {
             return staticSyncQ.sync { _serverPort }
@@ -1119,27 +1136,42 @@ class LoopbackServerTest : XCTestCase {
 
     override class func setUp() {
         super.setUp()
+        Self.startServer()
+    }
 
+    override class func tearDown() {
+        Self.stopServer()
+        super.tearDown()
+    }
+    
+    static func startServer() {
         var _serverPort = 0
         let dispatchGroup = DispatchGroup()
 
         func runServer() throws {
-            testServer = try _HTTPServer(port: nil)
+            testServer = try _HTTPServer(port: nil, backlog: options.serverBacklog)
             _serverPort = Int(testServer!.port)
             serverActive = true
             dispatchGroup.leave()
 
             while serverActive {
                 do {
                     let httpServer = try testServer!.listen()
-                    globalDispatchQueue.async {
+                    
+                    func handleRequest() {
                         let subServer = TestURLSessionServer(httpServer: httpServer)
                         do {
                             try subServer.readAndRespond()
                         } catch {
                             NSLog("readAndRespond: \(error)")
                         }
                     }
+                    
+                    if options.isAsynchronous {
+                        globalDispatchQueue.async(execute: handleRequest)
+                    } else {
+                        handleRequest()
+                    }
                 } catch {
                     if (serverActive) { // Ignore errors thrown on shutdown
                         NSLog("httpServer: \(error)")
@@ -1165,11 +1197,11 @@ class LoopbackServerTest : XCTestCase {
             fatalError("Timedout waiting for server to be ready")
         }
         serverPort = _serverPort
+        debugLog("Listening on \(serverPort)")
     }
-
-    override class func tearDown() {
+    
+    static func stopServer() {
         serverActive = false
         try? testServer?.stop()
-        super.tearDown()
     }
 }