Add QLocalServer::socketDescriptor

Adds a function to return the native socket descriptor. It allows
threaded or forked applications to reuse a previously created socket.

[ChangeLog][QtNetwork][QLocalServer] Added a function to retrieve
the socket descriptor.

Task-number: QTBUG-55043
Change-Id: I556e97000d2c02ad2bdd636984de6c7564381c6a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/network/socket/qlocalserver.cpp

@@ -41,6 +41,10 @@
 #include "qlocalserver_p.h"
 #include "qlocalsocket.h"
 
+#if defined(Q_OS_WIN) && !defined(QT_LOCALSOCKET_TCP)
+#include <QtCore/qt_windows.h>
+#endif
+
 QT_BEGIN_NAMESPACE
 
 #ifndef QT_NO_LOCALSERVER
@@ -182,6 +186,42 @@ QLocalServer::SocketOptions QLocalServer::socketOptions() const
     return d->socketOptions;
 }
 
+/*!
+    \since 5.10
+    Returns the native socket descriptor the server uses to listen
+    for incoming instructions, or -1 if the server is not listening.
+
+    The type of the descriptor depends on the platform:
+    \list
+        \li On Windows, the returned value is a
+        \l{https://msdn.microsoft.com/en-us/library/windows/desktop/ms740522(v=vs.85).aspx}
+        {Winsock 2 Socket Handle}.
+
+        \li With WinRT and on INTEGRITY, the returned value is the
+        QTcpServer socket descriptor and the type is defined by
+        \l{QTcpServer::socketDescriptor}{socketDescriptor}.
+
+        \li On all other UNIX-like operating systems, the type is
+        a file descriptor representing a listening socket.
+    \endlist
+
+    \sa listen()
+*/
+qintptr QLocalServer::socketDescriptor() const
+{
+    Q_D(const QLocalServer);
+    if (!isListening())
+        return -1;
+#if defined(QT_LOCALSOCKET_TCP)
+    return d->tcpServer.socketDescriptor();
+#elif defined(Q_OS_WIN)
+    const auto handle = d->connectionEventNotifier->handle();
+    return handle != INVALID_HANDLE_VALUE ? qintptr(handle) : -1;
+#else
+    return d->socketNotifier->socket();
+#endif
+}
+
 /*!
     Stop listening for incoming connections.  Existing connections are not
     affected, but any new connections will be refused.

src/network/socket/qlocalserver.h

@@ -93,6 +93,8 @@ public:
     void setSocketOptions(SocketOptions options);
     SocketOptions socketOptions() const;
 
+    qintptr socketDescriptor() const;
+
 protected:
     virtual void incomingConnection(quintptr socketDescriptor);
 

tests/auto/network/socket/qlocalsocket/tst_qlocalsocket.cpp

@@ -306,9 +306,11 @@ void tst_QLocalSocket::listen()
         // already isListening
         QTest::ignoreMessage(QtWarningMsg, "QLocalServer::listen() called when already listening");
         QVERIFY(!server.listen(name));
+        QVERIFY(server.socketDescriptor() != -1);
     } else {
         QVERIFY(!server.errorString().isEmpty());
         QCOMPARE(server.serverError(), QAbstractSocket::HostNotFoundError);
+        QCOMPARE(server.socketDescriptor(), -1);
     }
     QCOMPARE(server.maxPendingConnections(), 30);
     bool timedOut = false;