From fecaa6aae83a3ffa8f1fd41c5aa8275a1bfa7c9b Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Wed, 27 Apr 2016 23:44:57 -0700
Subject: [PATCH] Workaround Windows DLL unload issue with threads running

We don't know why it happens, so let's apply a workaround. See the
comment for more details.

Task-number: QTBUG-53031
Change-Id: Ifea6e497f11a461db432ffff144972e892fbbda5
Reviewed-by: Roland Winklmeier <Roland.M.Winklmeier@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/dbus/qdbusconnection.cpp | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/src/dbus/qdbusconnection.cpp b/src/dbus/qdbusconnection.cpp
index 34b3da7df38..7cdacc12849 100644
--- a/src/dbus/qdbusconnection.cpp
+++ b/src/dbus/qdbusconnection.cpp
@@ -62,6 +62,10 @@
 
 QT_BEGIN_NAMESPACE
 
+#ifdef Q_OS_WIN
+static void preventDllUnload();
+#endif
+
 Q_GLOBAL_STATIC(QDBusConnectionManager, _q_manager)
 
 // can be replaced with a lambda in Qt 5.7
@@ -150,6 +154,10 @@ QDBusConnectionManager::QDBusConnectionManager()
             this, &QDBusConnectionManager::createServer, Qt::BlockingQueuedConnection);
     moveToThread(this);         // ugly, don't do this in other projects
 
+#ifdef Q_OS_WIN
+    // prevent the library from being unloaded on Windows. See comments in the function.
+    preventDllUnload();
+#endif
     defaultBuses[0] = defaultBuses[1] = Q_NULLPTR;
     start();
 }
@@ -1275,4 +1283,31 @@ QT_END_NAMESPACE
 
 #include "qdbusconnection.moc"
 
+#ifdef Q_OS_WIN
+#  include <qt_windows.h>
+
+QT_BEGIN_NAMESPACE
+static void preventDllUnload()
+{
+    // Thread termination is really wacky on Windows. For some reason we don't
+    // understand, exiting from the thread may try to unload the DLL. Since the
+    // QDBusConnectionManager thread runs until the DLL is unloaded, we've got
+    // a deadlock: the main thread is waiting for the manager thread to exit,
+    // but the manager thread is attempting to acquire a lock to unload the DLL.
+    //
+    // We work around the issue by preventing the unload from happening in the
+    // first place.
+    //
+    // For this trick, see
+    // https://blogs.msdn.microsoft.com/oldnewthing/20131105-00/?p=2733
+
+    static HMODULE self;
+    GetModuleHandleEx(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS |
+                      GET_MODULE_HANDLE_EX_FLAG_PIN,
+                      reinterpret_cast<const wchar_t *>(&self), // any address in this DLL
+                      &self);
+}
+QT_END_NAMESPACE
+#endif
+
 #endif // QT_NO_DBUS