Mark mutex locking and unlocking functions with noexcept

Unlocking a mutex can never throw an exception. That doesn't make
sense and our code should make sure it can't happen. Right now,
provided that the system-level functions don't throw, we don't either.

Locking a mutex cannot throw on Linux because we use futexes
directly. A non-recursive mutex is just a futex, whereas a recursive
mutex uses a mutex (a futex) to manage a lock count.

However, on other platforms, due to the freelist, there can be memory
allocation, which means it might throw std::bad_alloc. Not because of
the freelist itself (that uses malloc and will just crash if malloc
fails) but because of Q_GLOBAL_STATIC. In 5.1, the global static will
be noexcept provided the type's constructor is so too (it is, in this
case).

Change-Id: I4c562383f48de1be7827b9afb512d73eaf0792d5
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qmutex.cpp

@@ -68,7 +68,6 @@ QT_BEGIN_NAMESPACE
     - Do not use tryLock with timeout > 0, else you can have a leak (see the ~QMutex destructor)
 */
 
-
 /*!
     \class QMutex
     \inmodule QtCore
@@ -179,7 +178,7 @@ QMutex::~QMutex()
 
     \sa unlock()
 */
-void QMutex::lock()
+void QMutex::lock() QT_MUTEX_LOCK_NOEXCEPT
 {
     QBasicMutex::lock();
 }
@@ -207,7 +206,7 @@ void QMutex::lock()
 
     \sa lock(), unlock()
 */
-bool QMutex::tryLock(int timeout)
+bool QMutex::tryLock(int timeout) QT_MUTEX_LOCK_NOEXCEPT
 {
     return QBasicMutex::tryLock(timeout);
 }
@@ -219,7 +218,7 @@ bool QMutex::tryLock(int timeout)
 
     \sa lock()
 */
-void QMutex::unlock()
+void QMutex::unlock() Q_DECL_NOTHROW
 {
     QBasicMutex::unlock();
 }
@@ -332,7 +331,7 @@ bool QBasicMutex::isRecursive() {
 /*!
     \internal helper for lock()
  */
-bool QBasicMutex::lockInternal(int timeout)
+bool QBasicMutex::lockInternal(int timeout) QT_MUTEX_LOCK_NOEXCEPT
 {
     while (!fastTryLock()) {
         QMutexData *copy = d_ptr.loadAcquire();
@@ -425,7 +424,7 @@ bool QBasicMutex::lockInternal(int timeout)
 /*!
     \internal
 */
-void QBasicMutex::unlockInternal()
+void QBasicMutex::unlockInternal() Q_DECL_NOTHROW
 {
     QMutexData *copy = d_ptr.loadAcquire();
     Q_ASSERT(copy); //we must be locked
@@ -493,7 +492,7 @@ void QMutexPrivate::release()
 }
 
 // atomically subtract "value" to the waiters, and remove the QMutexPrivate::BigNumber flag
-void QMutexPrivate::derefWaiters(int value)
+void QMutexPrivate::derefWaiters(int value) Q_DECL_NOTHROW
 {
     int old_waiters;
     int new_waiters;
@@ -511,7 +510,8 @@ void QMutexPrivate::derefWaiters(int value)
 /*!
    \internal
  */
-bool QRecursiveMutexPrivate::lock(int timeout) {
+bool QRecursiveMutexPrivate::lock(int timeout) QT_MUTEX_LOCK_NOEXCEPT
+{
     Qt::HANDLE self = QThread::currentThreadId();
     if (owner == self) {
         ++count;
@@ -533,7 +533,7 @@ bool QRecursiveMutexPrivate::lock(int timeout) {
 /*!
    \internal
  */
-void QRecursiveMutexPrivate::unlock()
+void QRecursiveMutexPrivate::unlock() Q_DECL_NOTHROW
 {
     if (count > 0) {
         count--;

src/corelib/thread/qmutex.h

@@ -53,34 +53,40 @@ QT_BEGIN_NAMESPACE
 
 #if !defined(QT_NO_THREAD) && !defined(qdoc)
 
+#ifdef Q_OS_LINUX
+# define QT_MUTEX_LOCK_NOEXCEPT Q_DECL_NOTHROW
+#else
+# define QT_MUTEX_LOCK_NOEXCEPT
+#endif
+
 class QMutexData;
 
 class Q_CORE_EXPORT QBasicMutex
 {
 public:
-    inline void lock() {
+    inline void lock() QT_MUTEX_LOCK_NOEXCEPT {
         if (!fastTryLock())
             lockInternal();
     }
 
-    inline void unlock() {
+    inline void unlock() Q_DECL_NOTHROW {
         Q_ASSERT(d_ptr.load()); //mutex must be locked
         if (!d_ptr.testAndSetRelease(dummyLocked(), 0))
             unlockInternal();
     }
 
-    bool tryLock(int timeout = 0) {
+    bool tryLock(int timeout = 0) QT_MUTEX_LOCK_NOEXCEPT {
         return fastTryLock() || lockInternal(timeout);
     }
 
     bool isRecursive();
 
 private:
-    inline bool fastTryLock() {
+    inline bool fastTryLock() Q_DECL_NOTHROW {
         return d_ptr.testAndSetAcquire(0, dummyLocked());
     }
-    bool lockInternal(int timeout = -1);
-    void unlockInternal();
+    bool lockInternal(int timeout = -1) QT_MUTEX_LOCK_NOEXCEPT;
+    void unlockInternal() Q_DECL_NOTHROW;
 
     QBasicAtomicPointer<QMutexData> d_ptr;
     static inline QMutexData *dummyLocked() {
@@ -97,9 +103,9 @@ public:
     explicit QMutex(RecursionMode mode = NonRecursive);
     ~QMutex();
 
-    void lock();
-    bool tryLock(int timeout = 0);
-    void unlock();
+    void lock() QT_MUTEX_LOCK_NOEXCEPT;
+    bool tryLock(int timeout = 0) QT_MUTEX_LOCK_NOEXCEPT;
+    void unlock() Q_DECL_NOTHROW;
 
     using QBasicMutex::isRecursive;
 
@@ -111,7 +117,7 @@ private:
 class Q_CORE_EXPORT QMutexLocker
 {
 public:
-    inline explicit QMutexLocker(QBasicMutex *m)
+    inline explicit QMutexLocker(QBasicMutex *m) QT_MUTEX_LOCK_NOEXCEPT
     {
         Q_ASSERT_X((reinterpret_cast<quintptr>(m) & quintptr(1u)) == quintptr(0),
                    "QMutexLocker", "QMutex pointer is misaligned");
@@ -124,7 +130,7 @@ public:
     }
     inline ~QMutexLocker() { unlock(); }
 
-    inline void unlock()
+    inline void unlock() Q_DECL_NOTHROW
     {
         if ((val & quintptr(1u)) == quintptr(1u)) {
             val &= ~quintptr(1u);
@@ -132,7 +138,7 @@ public:
         }
     }
 
-    inline void relock()
+    inline void relock() QT_MUTEX_LOCK_NOEXCEPT
     {
         if (val) {
             if ((val & quintptr(1u)) == quintptr(0u)) {
@@ -162,8 +168,6 @@ private:
     quintptr val;
 };
 
-
-
 #else // QT_NO_THREAD or qdoc
 
 class Q_CORE_EXPORT QMutex

src/corelib/thread/qmutex_linux.cpp

@@ -87,7 +87,7 @@ static inline int futexFlags()
     return value;
 }
 
-static inline int _q_futex(void *addr, int op, int val, const struct timespec *timeout)
+static inline int _q_futex(void *addr, int op, int val, const struct timespec *timeout) Q_DECL_NOTHROW
 {
     volatile int *int_addr = reinterpret_cast<volatile int *>(addr);
 #if Q_BYTE_ORDER == Q_BIG_ENDIAN && QT_POINTER_SIZE == 8
@@ -106,7 +106,7 @@ static inline QMutexData *dummyFutexValue()
     return reinterpret_cast<QMutexData *>(quintptr(3));
 }
 
-bool QBasicMutex::lockInternal(int timeout)
+bool QBasicMutex::lockInternal(int timeout) Q_DECL_NOTHROW
 {
     QElapsedTimer elapsedTimer;
     if (timeout >= 1)
@@ -147,7 +147,7 @@ bool QBasicMutex::lockInternal(int timeout)
     return true;
 }
 
-void QBasicMutex::unlockInternal()
+void QBasicMutex::unlockInternal() Q_DECL_NOTHROW
 {
     QMutexData *d = d_ptr.load();
     Q_ASSERT(d); //we must be locked

src/corelib/thread/qmutex_mac.cpp

@@ -84,7 +84,7 @@ bool QMutexPrivate::wait(int timeout)
     return (r == KERN_SUCCESS);
 }
 
-void QMutexPrivate::wakeUp()
+void QMutexPrivate::wakeUp() Q_DECL_NOTHROW
 {
     semaphore_signal(mach_semaphore);
 }

src/corelib/thread/qmutex_p.h

@@ -86,7 +86,7 @@ public:
     QMutexPrivate();
 
     bool wait(int timeout = -1);
-    void wakeUp();
+    void wakeUp() Q_DECL_NOTHROW;
 
     // Conrol the lifetime of the privates
     QAtomicInt refCount;
@@ -115,7 +115,7 @@ public:
     QAtomicInt waiters; //number of thread waiting
     QAtomicInt possiblyUnlocked; //bool saying that a timed wait timed out
     enum { BigNumber = 0x100000 }; //Must be bigger than the possible number of waiters (number of threads)
-    void derefWaiters(int value);
+    void derefWaiters(int value) Q_DECL_NOTHROW;
 
     //platform specific stuff
 #if defined(Q_OS_MAC)
@@ -139,8 +139,8 @@ public:
     uint count;
     QMutex mutex;
 
-    bool lock(int timeout);
-    void unlock();
+    bool lock(int timeout) QT_MUTEX_LOCK_NOEXCEPT;
+    void unlock() Q_DECL_NOTHROW;
 };
 
 QT_END_NAMESPACE

src/corelib/thread/qmutex_unix.cpp

@@ -104,7 +104,7 @@ bool QMutexPrivate::wait(int timeout)
     return ret;
 }
 
-void QMutexPrivate::wakeUp()
+void QMutexPrivate::wakeUp() Q_DECL_NOTHROW
 {
     report_error(pthread_mutex_lock(&mutex), "QMutex::unlock", "mutex lock");
     wakeup = true;

src/corelib/thread/qmutex_win.cpp

@@ -61,7 +61,7 @@ bool QMutexPrivate::wait(int timeout)
     return (WaitForSingleObject(event, timeout < 0 ? INFINITE : timeout) ==  WAIT_OBJECT_0);
 }
 
-void QMutexPrivate::wakeUp()
+void QMutexPrivate::wakeUp() Q_DECL_NOTHROW
 { SetEvent(event); }
 
 QT_END_NAMESPACE