[core] Added a shared mutex class (Haivision#2981).

Co-authored-by: Maxim Sharabayko <maxlovic@gmail.com>

srtcore/sync.cpp

@@ -357,3 +357,98 @@ int srt::sync::genRandomInt(int minVal, int maxVal)
 #endif // HAVE_CXX11
 }
 
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Shared Mutex 
+//
+////////////////////////////////////////////////////////////////////////////////
+
+srt::sync::SharedMutex::SharedMutex()
+    : m_LockWriteCond()
+    , m_LockReadCond()
+    , m_Mutex()
+    , m_iCountRead(0)
+    , m_bWriterLocked(false)
+{
+    setupCond(m_LockReadCond, "SharedMutex::m_pLockReadCond");
+    setupCond(m_LockWriteCond, "SharedMutex::m_pLockWriteCond");
+    setupMutex(m_Mutex, "SharedMutex::m_pMutex");
+}
+
+srt::sync::SharedMutex::~SharedMutex()
+{
+    releaseMutex(m_Mutex);
+    releaseCond(m_LockWriteCond);
+    releaseCond(m_LockReadCond);
+}
+
+void srt::sync::SharedMutex::lock()
+{
+    UniqueLock l1(m_Mutex);
+    while (m_bWriterLocked)
+        m_LockWriteCond.wait(l1);
+
+    m_bWriterLocked = true;
+
+    while (m_iCountRead)
+        m_LockReadCond.wait(l1);
+}
+
+bool srt::sync::SharedMutex::try_lock()
+{
+    UniqueLock l1(m_Mutex);
+    if (m_bWriterLocked || m_iCountRead > 0)
+        return false;
+
+    m_bWriterLocked = true;
+    return true;
+}
+
+void srt::sync::SharedMutex::unlock()
+{
+    ScopedLock lk(m_Mutex);
+    m_bWriterLocked = false;
+
+    m_LockWriteCond.notify_all();
+}
+
+void srt::sync::SharedMutex::lock_shared()
+{
+    UniqueLock lk(m_Mutex);
+    while (m_bWriterLocked)
+        m_LockWriteCond.wait(lk);
+
+    m_iCountRead++;
+}
+
+bool srt::sync::SharedMutex::try_lock_shared()
+{
+    UniqueLock lk(m_Mutex);
+    if (m_bWriterLocked)
+        return false;
+
+    m_iCountRead++;
+    return true;
+}
+
+void srt::sync::SharedMutex::unlock_shared()
+{
+    ScopedLock lk(m_Mutex);
+
+    m_iCountRead--;
+
+    SRT_ASSERT(m_iCountRead >= 0);
+    if (m_iCountRead < 0)
+        m_iCountRead = 0;
+
+    if (m_bWriterLocked && m_iCountRead == 0)
+        m_LockReadCond.notify_one();
+
+}
+
+int srt::sync::SharedMutex::getReaderCount() const
+{
+    ScopedLock lk(m_Mutex);
+    return m_iCountRead;
+}

srtcore/sync.h

@@ -943,6 +943,43 @@ CUDTException& GetThreadLocalError();
 /// @param[in] maxVal maximum allowed value of the resulting random number.
 int genRandomInt(int minVal, int maxVal);
 
+
+/// Implementation of a read-write mutex. 
+/// This allows multiple readers at a time, or a single writer.
+/// TODO: The class can be improved if needed to give writer a preference
+/// by adding additional m_iWritersWaiting member variable (counter).
+/// TODO: The m_iCountRead could be made atomic to make unlok_shared() faster and lock-free.
+class SharedMutex
+{
+public:
+    SharedMutex();    
+    ~SharedMutex();
+
+private:
+    Condition  m_LockWriteCond;
+    Condition  m_LockReadCond;
+
+    mutable Mutex m_Mutex;
+
+    int m_iCountRead;
+    bool m_bWriterLocked;
+
+    /// Acquire the lock for writting purposes. Only one thread can acquire this lock at a time
+    /// Once it is locked, no reader can acquire it 
+public:
+    void lock();
+    bool try_lock();
+    void unlock();
+
+    /// Acquire the lock if no writter already has it. For read purpose only
+    /// Several readers can lock this at the same time. 
+    void lock_shared();
+    bool try_lock_shared();
+    void unlock_shared();
+
+    int getReaderCount() const;
+};
+
 } // namespace sync
 } // namespace srt
 

test/test_sync.cpp

@@ -609,6 +609,91 @@ TEST(SyncThread, Joinable)
     EXPECT_FALSE(foo.joinable());
 }
 
+/*****************************************************************************/
+/*
+ * SharedMutex
+ */
+ /*****************************************************************************/
+TEST(SharedMutex, LockWriteRead)
+{
+    SharedMutex mut;
+
+    mut.lock();
+    EXPECT_FALSE(mut.try_lock_shared());
+
+}
+
+TEST(SharedMutex, LockReadWrite)
+{
+    SharedMutex mut;
+
+    mut.lock_shared();
+    EXPECT_FALSE(mut.try_lock());
+
+}
+
+TEST(SharedMutex, LockReadTwice)
+{
+    SharedMutex mut;
+
+    mut.lock_shared();
+    mut.lock_shared();
+    EXPECT_TRUE(mut.try_lock_shared());
+}
+
+TEST(SharedMutex, LockWriteTwice)
+{
+    SharedMutex mut;
+
+    mut.lock();
+    EXPECT_FALSE(mut.try_lock());
+}
+
+TEST(SharedMutex, LockUnlockWrite)
+{
+    SharedMutex mut;
+    mut.lock();
+    EXPECT_FALSE(mut.try_lock());
+    mut.unlock();
+    EXPECT_TRUE(mut.try_lock());
+}
+
+TEST(SharedMutex, LockUnlockRead)
+{
+    SharedMutex mut;
+
+    mut.lock_shared();
+    EXPECT_FALSE(mut.try_lock());
+
+    mut.unlock_shared();
+    EXPECT_TRUE(mut.try_lock());
+}
+
+TEST(SharedMutex, LockedReadCount)
+{
+    SharedMutex mut;
+    int count = 0;
+
+    mut.lock_shared();
+    count++;
+    ASSERT_EQ(mut.getReaderCount(), count);
+
+    mut.lock_shared();
+    count++;
+    ASSERT_EQ(mut.getReaderCount(), count);
+
+    mut.unlock_shared();
+    count--;
+    ASSERT_EQ(mut.getReaderCount(), count);
+
+    mut.unlock_shared();
+    count--;
+    ASSERT_EQ(mut.getReaderCount(), count);
+
+    EXPECT_TRUE(mut.try_lock());
+}
+
+
 /*****************************************************************************/
 /*
  * FormatTime