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diff --git a/indra/llcommon/llexception.h b/indra/llcommon/llexception.h
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+/**
+ * @file   llexception.h
+ * @author Nat Goodspeed
+ * @date   2016-06-29
+ * @brief  Types needed for generic exception handling
+ * 
+ * $LicenseInfo:firstyear=2016&license=viewerlgpl$
+ * Copyright (c) 2016, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_LLEXCEPTION_H)
+#define LL_LLEXCEPTION_H
+
+#include <stdexcept>
+#include <boost/exception/exception.hpp>
+#include <boost/throw_exception.hpp>
+#include <boost/current_function.hpp>
+
+// "Found someone who can comfort me
+//  But there are always exceptions..."
+//  - Empty Pages, Traffic, from John Barleycorn (1970)
+//    https://www.youtube.com/watch?v=dRH0CGVK7ic
+
+/**
+ * LLException is intended as the common base class from which all
+ * viewer-specific exceptions are derived. Rationale for why it's derived from
+ * both std::exception and boost::exception is explained in
+ * tests/llexception_test.cpp.
+ *
+ * boost::current_exception_diagnostic_information() is quite wonderful: if
+ * all we need to do with an exception is log it, in most places we should
+ * catch (...) and log boost::current_exception_diagnostic_information().
+ * See CRASH_ON_UNHANDLED_EXCEPTION() and LOG_UNHANDLED_EXCEPTION() below.
+ *
+ * There may be circumstances in which it would be valuable to distinguish an
+ * exception explicitly thrown by viewer code from an exception thrown by
+ * (say) a third-party library. Catching (const LLException&) supports such
+ * usage. However, most of the value of this base class is in the
+ * diagnostic_information() available via Boost.Exception.
+ */
+struct LLException:
+    public std::runtime_error,
+    public boost::exception
+{
+    LLException(const std::string& what):
+        std::runtime_error(what)
+    {}
+};
+
+/**
+ * The point of LLContinueError is to distinguish exceptions that need not
+ * terminate the whole viewer session. In general, an uncaught exception will
+ * be logged and will crash the viewer. However, though an uncaught exception
+ * derived from LLContinueError will still be logged, the viewer will attempt
+ * to continue processing.
+ */
+struct LLContinueError: public LLException
+{
+    LLContinueError(const std::string& what):
+        LLException(what)
+    {}
+};
+
+/**
+ * Please use LLTHROW() to throw viewer exceptions whenever possible. This
+ * enriches the exception's diagnostic_information() with the source file,
+ * line and containing function of the LLTHROW() macro.
+ */
+// Currently we implement that using BOOST_THROW_EXCEPTION(). Wrap it in
+// LLTHROW() in case we ever want to revisit that implementation decision.
+#define LLTHROW(x) BOOST_THROW_EXCEPTION(x)
+
+/// Call this macro from a catch (...) clause
+#define CRASH_ON_UNHANDLED_EXCEPTION(CONTEXT) \
+     crash_on_unhandled_exception_(__FILE__, __LINE__, BOOST_CURRENT_FUNCTION, CONTEXT)
+void crash_on_unhandled_exception_(const char*, int, const char*, const std::string&);
+
+/// Call this from a catch (const LLContinueError&) clause, or from a catch
+/// (...) clause in which you do NOT want the viewer to crash.
+#define LOG_UNHANDLED_EXCEPTION(CONTEXT) \
+     log_unhandled_exception_(__FILE__, __LINE__, BOOST_CURRENT_FUNCTION, CONTEXT)
+void log_unhandled_exception_(const char*, int, const char*, const std::string&);
+
+#endif /* ! defined(LL_LLEXCEPTION_H) */