DRTVWR-476: Enrich LLExceptions thrown by LLTHROW() with stack trace.

The LLTHROW() abstraction allows us to enrich the subject exception with a
boost::stacktrace -- without having to propagate the boost/stacktrace.hpp
header throughout the code base.

To my delight, our existing use of
boost::current_exception_diagnostic_information() already reports the newly
added boost::stacktrace information -- we don't have to query it specifically!

indra/llcommon/llexception.cpp

@@ -18,10 +18,23 @@
 #include <typeinfo>
 // external library headers
 #include <boost/exception/diagnostic_information.hpp>
+#include <boost/exception/error_info.hpp>
+// On Mac, got:
+// #error "Boost.Stacktrace requires `_Unwind_Backtrace` function. Define
+// `_GNU_SOURCE` macro or `BOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED` if
+// _Unwind_Backtrace is available without `_GNU_SOURCE`."
+#define BOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED
+#include <boost/stacktrace.hpp>
 // other Linden headers
 #include "llerror.h"
 #include "llerrorcontrol.h"
 
+// used to attach and extract stacktrace information to/from boost::exception,
+// see https://www.boost.org/doc/libs/release/doc/html/stacktrace/getting_started.html#stacktrace.getting_started.exceptions_with_stacktrace
+// apparently the struct passed as the first template param needs no definition?
+typedef boost::error_info<struct errinfo_stacktrace_, boost::stacktrace::stacktrace>
+        errinfo_stacktrace;
+
 namespace {
 // used by crash_on_unhandled_exception_() and log_unhandled_exception_()
 void log_unhandled_exception_(LLError::ELevel level,
@@ -53,3 +66,17 @@ void log_unhandled_exception_(const char* file, int line, const char* pretty_fun
     // routinely, but we DO expect to return from this function.
     log_unhandled_exception_(LLError::LEVEL_WARN, file, line, pretty_function, context);
 }
+
+void annotate_exception_(boost::exception& exc)
+{
+    // https://www.boost.org/doc/libs/release/libs/exception/doc/tutorial_transporting_data.html
+    // "Adding of Arbitrary Data to Active Exception Objects"
+    // Given a boost::exception&, we can add boost::error_info items to it
+    // without knowing its leaf type.
+    // The stacktrace constructor that lets us skip a level -- and why would
+    // we always include annotate_exception_()? -- also requires a max depth.
+    // For the nullary constructor, the stacktrace class declaration itself
+    // passes static_cast<std::size_t>(-1), but that's kind of dubious.
+    // Anyway, which of us is really going to examine more than 100 frames?
+    exc << errinfo_stacktrace(boost::stacktrace::stacktrace(1, 100));
+}

indra/llcommon/llexception.h

@@ -67,9 +67,29 @@ struct LLContinueError: public LLException
  * 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)
+#define LLTHROW(x)                                                      \
+do {                                                                    \
+    /* Capture the exception object 'x' by value. (Exceptions must */   \
+    /* be copyable.) It might seem simpler to use                  */   \
+    /* BOOST_THROW_EXCEPTION(annotate_exception_(x)) instead of    */   \
+    /* three separate statements, but:                             */   \
+    /* - We want to throw 'x' with its original type, not just a   */   \
+    /*   reference to boost::exception.                            */   \
+    /* - To return x's original type, annotate_exception_() would  */   \
+    /*   have to be a template function.                           */   \
+    /* - We want annotate_exception_() to be opaque.               */   \
+    /* We also might consider embedding BOOST_THROW_EXCEPTION() in */   \
+    /* our helper function, but we want the filename and line info */   \
+    /* embedded by BOOST_THROW_EXCEPTION() to be the throw point   */   \
+    /* rather than always indicating the same line in              */   \
+    /* llexception.cpp.                                            */   \
+    auto exc{x};                                                        \
+    annotate_exception_(exc);                                           \
+    BOOST_THROW_EXCEPTION(exc);                                         \
+    /* Use the classic 'do { ... } while (0)' macro trick to wrap  */   \
+    /* our multiple statements.                                    */   \
+} while (0)
+void annotate_exception_(boost::exception& exc);
 
 /// Call this macro from a catch (...) clause
 #define CRASH_ON_UNHANDLED_EXCEPTION(CONTEXT) \