2 files changed, 56 insertions, 8 deletions
diff --git a/common/atomic.h b/common/atomic.h
index e34f35bb..5e9b04c6 100644
--- a/common/atomic.h
+++ b/common/atomic.h
@@ -6,14 +6,14 @@
 
 using RefCount = std::atomic<unsigned int>;
 
-inline void InitRef(RefCount *ptr, unsigned int value)
-{ ptr->store(value, std::memory_order_relaxed); }
-inline unsigned int ReadRef(RefCount *ptr)
-{ return ptr->load(std::memory_order_acquire); }
-inline unsigned int IncrementRef(RefCount *ptr)
-{ return ptr->fetch_add(1u, std::memory_order_acq_rel)+1u; }
-inline unsigned int DecrementRef(RefCount *ptr)
-{ return ptr->fetch_sub(1u, std::memory_order_acq_rel)-1u; }
+inline void InitRef(RefCount &ref, unsigned int value)
+{ ref.store(value, std::memory_order_relaxed); }
+inline unsigned int ReadRef(RefCount &ref)
+{ return ref.load(std::memory_order_acquire); }
+inline unsigned int IncrementRef(RefCount &ref)
+{ return ref.fetch_add(1u, std::memory_order_acq_rel)+1u; }
+inline unsigned int DecrementRef(RefCount &ref)
+{ return ref.fetch_sub(1u, std::memory_order_acq_rel)-1u; }
 
 
 /* WARNING: A livelock is theoretically possible if another thread keeps
diff --git a/common/intrusive_ptr.h b/common/intrusive_ptr.h
new file mode 100644
index 00000000..8f34aeac
--- /dev/null
+++ b/common/intrusive_ptr.h
@@ -0,0 +1,48 @@
+#ifndef INTRUSIVE_PTR_H
+#define INTRUSIVE_PTR_H
+
+#include "atomic.h"
+#include "opthelpers.h"
+
+
+namespace al {
+
+template<typename T>
+class intrusive_ref {
+    RefCount mRef{1u};
+
+public:
+    unsigned int add_ref() noexcept { return IncrementRef(mRef); }
+    unsigned int release() noexcept
+    {
+        auto ref = DecrementRef(mRef);
+        if(UNLIKELY(ref == 0))
+            delete static_cast<T*>(this);
+        return ref;
+    }
+
+    /**
+     * Release only if doing so would not bring the object to 0 references and
+     * delete it. Returns false if the object could not be released.
+     *
+     * NOTE: The caller is responsible for handling a failed release, as it
+     * means the object has no other references and needs to be be deleted
+     * somehow.
+     */
+    bool releaseIfNoDelete() noexcept
+    {
+        auto val = mRef.load(std::memory_order_acquire);
+        while(val > 1 && !mRef.compare_exchange_strong(val, val-1, std::memory_order_acq_rel))
+        {
+            /* val was updated with the current value on failure, so just try
+             * again.
+             */
+        }
+
+        return val >= 2;
+    }
+};
+
+} // namespace al
+
+#endif /* INTRUSIVE_PTR_H */