QDBusConnection: Merge the dispatch and the watch-and-timeout locks

We don't need two anymore because they now protect the same thing: the
state of the DBusConnection. The difference existed when it was possible
for two threads to access the DBusConnection at the same time: one doing
dispatching and one doing something else. Unfortunately, even though
DBusConnection supports this, QtDBus doesn't.

From d47c05b1889bb4f06203bbc65f4660b8d0128954 (2008-10-08):
   Details:  if we're removing a timer or a watcher from our list,
   there's a race condition: one thread (not the QDBusConnection thread)
   could be asking for the removal (which causes an event to be sent),
   then deletes the pointer. In the meantime, QDBusConnection will
   process the timers and socket notifiers and could end up calling
   lidbus-1 with deleted pointers.

That commit fixed the race condition but introduced a deadlock.

Task-number: QTBUG-42189
Change-Id: I034038f763cbad3a67398909defd31a23c27c965
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Albert Astals Cid <albert.astals@canonical.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>

3 files changed, 16 insertions, 29 deletions

diff --git a/src/dbus/qdbusconnection_p.h b/src/dbus/qdbusconnection_p.h
index fd4ced078d..a75dabeeee 100644
--- a/src/dbus/qdbusconnection_p.h
+++ b/src/dbus/qdbusconnection_p.h
@@ -290,24 +290,18 @@ public:
     QStringList serverConnectionNames;
 
     ConnectionMode mode;
+    QDBusConnectionInterface *busService;
 
-    // members accessed in unlocked mode (except for deletion)
-    // connection and server provide their own locking mechanisms
-    // busService doesn't have state to be changed
+    // the dispatch lock protects everything related to the DBusConnection or DBusServer
+    // including the timeouts and watches
+    QMutex dispatchLock;
     DBusConnection *connection;
     DBusServer *server;
-    QDBusConnectionInterface *busService;
-
-    // watchers and timeouts are accessed from any thread
-    // but the corresponding timer and QSocketNotifier must be handled
-    // only in the object's thread
-    QMutex watchAndTimeoutLock;
     WatcherHash watchers;
     TimeoutHash timeouts;
     PendingTimeoutList timeoutsPendingAdd;
 
-    // members accessed through a lock
-    QMutex dispatchLock;
+    // the master lock protects our own internal state
     QReadWriteLock lock;
     QDBusError lastError;
 
diff --git a/src/dbus/qdbusintegrator.cpp b/src/dbus/qdbusintegrator.cpp
index b2f6635d1b..39ab797426 100644
--- a/src/dbus/qdbusintegrator.cpp
+++ b/src/dbus/qdbusintegrator.cpp
@@ -155,7 +155,7 @@ static dbus_bool_t qDBusAddTimeout(DBusTimeout *timeout, void *data)
     if (!q_dbus_timeout_get_enabled(timeout))
         return true;
 
-    QDBusWatchAndTimeoutLocker locker(AddTimeoutAction, d);
+    QDBusDispatchLocker locker(AddTimeoutAction, d);
     if (QCoreApplication::instance() && QThread::currentThread() == d->thread()) {
         // correct thread
         return qDBusRealAddTimeout(d, timeout, q_dbus_timeout_get_interval(timeout));
@@ -190,7 +190,7 @@ static void qDBusRemoveTimeout(DBusTimeout *timeout, void *data)
 
     QDBusConnectionPrivate *d = static_cast<QDBusConnectionPrivate *>(data);
 
-    QDBusWatchAndTimeoutLocker locker(RemoveTimeoutAction, d);
+    QDBusDispatchLocker locker(RemoveTimeoutAction, d);
 
     // is it pending addition?
     QDBusConnectionPrivate::PendingTimeoutList::iterator pit = d->timeoutsPendingAdd.begin();
@@ -263,7 +263,7 @@ static bool qDBusRealAddWatch(QDBusConnectionPrivate *d, DBusWatch *watch, int f
 {
     QDBusConnectionPrivate::Watcher watcher;
 
-    QDBusWatchAndTimeoutLocker locker(AddWatchAction, d);
+    QDBusDispatchLocker locker(AddWatchAction, d);
     if (flags & DBUS_WATCH_READABLE) {
         //qDebug("addReadWatch %d", fd);
         watcher.watch = watch;
@@ -297,7 +297,7 @@ static void qDBusRemoveWatch(DBusWatch *watch, void *data)
     QDBusConnectionPrivate *d = static_cast<QDBusConnectionPrivate *>(data);
     int fd = q_dbus_watch_get_unix_fd(watch);
 
-    QDBusWatchAndTimeoutLocker locker(RemoveWatchAction, d);
+    QDBusDispatchLocker locker(RemoveWatchAction, d);
     QDBusConnectionPrivate::WatcherHash::iterator i = d->watchers.find(fd);
     while (i != d->watchers.end() && i.key() == fd) {
         if (i.value().watch == watch) {
@@ -341,7 +341,7 @@ static void qDBusToggleWatch(DBusWatch *watch, void *data)
 
 static void qDBusRealToggleWatch(QDBusConnectionPrivate *d, DBusWatch *watch, int fd)
 {
-    QDBusWatchAndTimeoutLocker locker(ToggleWatchAction, d);
+    QDBusDispatchLocker locker(ToggleWatchAction, d);
 
     QDBusConnectionPrivate::WatcherHash::iterator i = d->watchers.find(fd);
     while (i != d->watchers.end() && i.key() == fd) {
@@ -1016,8 +1016,8 @@ void QDBusConnectionPrivate::deliverCall(QObject *object, int /*flags*/, const Q
 extern bool qDBusInitThreads();
 
 QDBusConnectionPrivate::QDBusConnectionPrivate(QObject *p)
-    : QObject(p), ref(1), capabilities(0), mode(InvalidMode), connection(0), server(0), busService(0),
-      watchAndTimeoutLock(QMutex::Recursive), dispatchLock(QMutex::Recursive),
+    : QObject(p), ref(1), capabilities(0), mode(InvalidMode), busService(0),
+      dispatchLock(QMutex::Recursive), connection(0), server(0),
       rootNode(QString(QLatin1Char('/'))),
       anonymousAuthenticationAllowed(false)
 {
@@ -1127,7 +1127,7 @@ bool QDBusConnectionPrivate::handleError(const QDBusErrorInternal &error)
 void QDBusConnectionPrivate::timerEvent(QTimerEvent *e)
 {
     {
-        QDBusWatchAndTimeoutLocker locker(TimerEventAction, this);
+        QDBusDispatchLocker locker(TimerEventAction, this);
         DBusTimeout *timeout = timeouts.value(e->timerId(), 0);
         if (timeout)
             q_dbus_timeout_handle(timeout);
@@ -1146,7 +1146,7 @@ void QDBusConnectionPrivate::customEvent(QEvent *e)
     switch (ev->subtype)
     {
     case QDBusConnectionCallbackEvent::AddTimeout: {
-        QDBusWatchAndTimeoutLocker locker(RealAddTimeoutAction, this);
+        QDBusDispatchLocker locker(RealAddTimeoutAction, this);
         while (!timeoutsPendingAdd.isEmpty()) {
             QPair<DBusTimeout *, int> entry = timeoutsPendingAdd.takeFirst();
             qDBusRealAddTimeout(this, entry.first, entry.second);
@@ -1182,7 +1182,7 @@ void QDBusConnectionPrivate::socketRead(int fd)
     QVarLengthArray<DBusWatch *, 2> pendingWatches;
 
     {
-        QDBusWatchAndTimeoutLocker locker(SocketReadAction, this);
+        QDBusDispatchLocker locker(SocketReadAction, this);
         WatcherHash::ConstIterator it = watchers.constFind(fd);
         while (it != watchers.constEnd() && it.key() == fd) {
             if (it->watch && it->read && it->read->isEnabled())
@@ -1202,7 +1202,7 @@ void QDBusConnectionPrivate::socketWrite(int fd)
     QVarLengthArray<DBusWatch *, 2> pendingWatches;
 
     {
-        QDBusWatchAndTimeoutLocker locker(SocketWriteAction, this);
+        QDBusDispatchLocker locker(SocketWriteAction, this);
         WatcherHash::ConstIterator it = watchers.constFind(fd);
         while (it != watchers.constEnd() && it.key() == fd) {
             if (it->watch && it->write && it->write->isEnabled())
diff --git a/src/dbus/qdbusthreaddebug_p.h b/src/dbus/qdbusthreaddebug_p.h
index dcde99169c..726ab051d0 100644
--- a/src/dbus/qdbusthreaddebug_p.h
+++ b/src/dbus/qdbusthreaddebug_p.h
@@ -207,13 +207,6 @@ struct QDBusDispatchLocker: QDBusMutexLocker
     { }
 };
 
-struct QDBusWatchAndTimeoutLocker: QDBusMutexLocker
-{
-    inline QDBusWatchAndTimeoutLocker(ThreadAction a, QDBusConnectionPrivate *s)
-        : QDBusMutexLocker(a, s, &s->watchAndTimeoutLock)
-    { }
-};
-
 #if QDBUS_THREAD_DEBUG
 # define SEM_ACQUIRE(action, sem)                                       \
     do {                                                                \