Add AINRLock for debugging purposes.

This class can be used as RWLOCK parameter to AIThreadSafe to check that
data is only accessed by a single thread (like
AIThreadSafeSingleThreaded) AND is never write locked when a read or
write lock was already obtained (by the same thread). It doesn't
actually lock anything, it just keeps track if the "lock" was obtained
before. The use case is to check if STL containers aren't being used
(read or write locked) by a calling function when additional write
access is necessary, as write access might invalidate iterator that
point to the container (where the previous lock was taken).

indra/llcommon/llthread.h

@@ -397,6 +397,43 @@ public:
 #endif
 };
 
+#if LL_DEBUG
+class LL_COMMON_API AINRLock
+{
+private:
+	int read_locked;
+	int write_locked;
+
+	mutable bool mAccessed;
+	mutable AIThreadID mTheadID;
+
+	void accessed(void) const
+	{
+	  if (!mAccessed)
+	  {
+		mAccessed = true;
+		mTheadID.reset();
+	  }
+	  else
+	  {
+		llassert_always(mTheadID.equals_current_thread());
+	  }
+	}
+
+public:
+	AINRLock(void) : read_locked(false), write_locked(false), mAccessed(false) { }
+
+	bool isLocked() const { return read_locked || write_locked; }
+
+	void rdlock(bool high_priority = false) { accessed(); ++read_locked; }
+	void rdunlock() { --read_locked; }
+	void wrlock() { llassert(!isLocked()); accessed(); ++write_locked; }
+	void wrunlock() { --write_locked; }
+	void wr2rdlock() { llassert(false); }
+	void rd2wrlock() { llassert(false); }
+};
+#endif
+
 //============================================================================
 
 void LLThread::lockData()