SL-20546: Merge branch 'DRTVWR-588-maint-W' into sl-20546.

1 files changed, 133 insertions, 17 deletions

diff --git a/indra/llcommon/llsdutil.h b/indra/llcommon/llsdutil.h
index 372278c51a..ad54d1b0be 100644
--- a/indra/llcommon/llsdutil.h
+++ b/indra/llcommon/llsdutil.h
@@ -29,8 +29,14 @@
 #ifndef LL_LLSDUTIL_H
 #define LL_LLSDUTIL_H
 
+#include "apply.h"                  // LL::invoke()
+#include "function_types.h"         // LL::function_arity
 #include "llsd.h"
 #include <boost/functional/hash.hpp>
+#include <cassert>
+#include <memory>                   // std::shared_ptr
+#include <type_traits>
+#include <vector>
 
 // U32
 LL_COMMON_API LLSD ll_sd_from_U32(const U32);
@@ -298,6 +304,11 @@ LLSD map(Ts&&... vs)
 /*****************************************************************************
 *   LLSDParam
 *****************************************************************************/
+struct LLSDParamBase
+{
+    virtual ~LLSDParamBase() {}
+};
+
 /**
  * LLSDParam is a customization point for passing LLSD values to function
  * parameters of more or less arbitrary type. LLSD provides a small set of
@@ -315,7 +326,7 @@ LLSD map(Ts&&... vs)
  * @endcode
  */
 template <typename T>
-class LLSDParam
+class LLSDParam: public LLSDParamBase
 {
 public:
     /**
@@ -323,13 +334,66 @@ public:
      * value for later retrieval
      */
     LLSDParam(const LLSD& value):
-        _value(value)
+        value_(value)
     {}
 
-    operator T() const { return _value; }
+    operator T() const { return value_; }
 
 private:
-    T _value;
+    T value_;
+};
+
+/**
+ * LLSDParam<LLSD> is for when you don't already have the target parameter
+ * type in hand. Instantiate LLSDParam<LLSD>(your LLSD object), and the
+ * templated conversion operator will try to select a more specific LLSDParam
+ * specialization.
+ */
+template <>
+class LLSDParam<LLSD>: public LLSDParamBase
+{
+private:
+    LLSD value_;
+    // LLSDParam<LLSD>::operator T() works by instantiating an LLSDParam<T> on
+    // demand. Returning that engages LLSDParam<T>::operator T(), producing
+    // the desired result. But LLSDParam<const char*> owns a std::string whose
+    // c_str() is returned by its operator const char*(). If we return a temp
+    // LLSDParam<const char*>, the compiler can destroy it right away, as soon
+    // as we've called operator const char*(). That's a problem! That
+    // invalidates the const char* we've just passed to the subject function.
+    // This LLSDParam<LLSD> is presumably guaranteed to survive until the
+    // subject function has returned, so we must ensure that any constructed
+    // LLSDParam<T> lives just as long as this LLSDParam<LLSD> does. Putting
+    // each LLSDParam<T> on the heap and capturing a smart pointer in a vector
+    // works. We would have liked to use std::unique_ptr, but vector entries
+    // must be copyable.
+    // (Alternatively we could assume that every instance of LLSDParam<LLSD>
+    // will be asked for at most ONE conversion. We could store a scalar
+    // std::unique_ptr and, when constructing an new LLSDParam<T>, assert that
+    // the unique_ptr is empty. But some future change in usage patterns, and
+    // consequent failure of that assertion, would be very mysterious. Instead
+    // of explaining how to fix it, just fix it now.)
+    mutable std::vector<std::shared_ptr<LLSDParamBase>> converters_;
+
+public:
+    LLSDParam(const LLSD& value): value_(value) {}
+
+    /// if we're literally being asked for an LLSD parameter, avoid infinite
+    /// recursion
+    operator LLSD() const { return value_; }
+
+    /// otherwise, instantiate a more specific LLSDParam<T> to convert; that
+    /// preserves the existing customization mechanism
+    template <typename T>
+    operator T() const
+    {
+        // capture 'ptr' with the specific subclass type because converters_
+        // only stores LLSDParamBase pointers
+        auto ptr{ std::make_shared<LLSDParam<std::decay_t<T>>>(value_) };
+        // keep the new converter alive until we ourselves are destroyed
+        converters_.push_back(ptr);
+        return *ptr;
+    }
 };
 
 /**
@@ -346,17 +410,17 @@ private:
  */
 #define LLSDParam_for(T, AS)                    \
 template <>                                     \
-class LLSDParam<T>                              \
+class LLSDParam<T>: public LLSDParamBase        \
 {                                               \
 public:                                         \
     LLSDParam(const LLSD& value):               \
-        _value((T)value.AS())                      \
+        value_((T)value.AS())                   \
     {}                                          \
                                                 \
-    operator T() const { return _value; }       \
+    operator T() const { return value_; }       \
                                                 \
 private:                                        \
-    T _value;                                   \
+    T value_;                                   \
 }
 
 LLSDParam_for(float,        asReal);
@@ -372,31 +436,31 @@ LLSDParam_for(LLSD::Binary, asBinary);
  * safely pass an LLSDParam<const char*>(yourLLSD).
  */
 template <>
-class LLSDParam<const char*>
+class LLSDParam<const char*>: public LLSDParamBase
 {
 private:
     // The difference here is that we store a std::string rather than a const
     // char*. It's important that the LLSDParam object own the std::string.
-    std::string _value;
+    std::string value_;
     // We don't bother storing the incoming LLSD object, but we do have to
-    // distinguish whether _value is an empty string because the LLSD object
+    // distinguish whether value_ is an empty string because the LLSD object
     // contains an empty string or because it's isUndefined().
-    bool _undefined;
+    bool undefined_;
 
 public:
     LLSDParam(const LLSD& value):
-        _value(value),
-        _undefined(value.isUndefined())
+        value_(value),
+        undefined_(value.isUndefined())
     {}
 
-    // The const char* we retrieve is for storage owned by our _value member.
+    // The const char* we retrieve is for storage owned by our value_ member.
     // That's how we guarantee that the const char* is valid for the lifetime
     // of this LLSDParam object. Constructing your LLSDParam in the argument
     // list should ensure that the LLSDParam object will persist for the
     // duration of the function call.
     operator const char*() const
     {
-        if (_undefined)
+        if (undefined_)
         {
             // By default, an isUndefined() LLSD object's asString() method
             // will produce an empty string. But for a function accepting
@@ -406,7 +470,7 @@ public:
             // case, though, no LLSD value could pass NULL.
             return NULL;
         }
-        return _value.c_str();
+        return value_.c_str();
     }
 };
 
@@ -555,4 +619,56 @@ struct hash<LLSD>
     }
 };
 }
+
+namespace LL
+{
+
+/*****************************************************************************
+*   apply(function, LLSD array)
+*****************************************************************************/
+// validate incoming LLSD blob, and return an LLSD array suitable to pass to
+// the function of interest
+LLSD apply_llsd_fix(size_t arity, const LLSD& args);
+
+// Derived from https://stackoverflow.com/a/20441189
+// and https://en.cppreference.com/w/cpp/utility/apply .
+// We can't simply make a tuple from the LLSD array and then apply() that
+// tuple to the function -- how would make_tuple() deduce the correct
+// parameter type for each entry? We must go directly to the target function.
+template <typename CALLABLE, std::size_t... I>
+auto apply_impl(CALLABLE&& func, const LLSD& array, std::index_sequence<I...>)
+{
+    // call func(unpacked args), using generic LLSDParam<LLSD> to convert each
+    // entry in 'array' to the target parameter type
+    return std::forward<CALLABLE>(func)(LLSDParam<LLSD>(array[I])...);
+}
+
+// use apply_n<ARITY>(function, LLSD) to call a specific arity of a variadic
+// function with (that many) items from the passed LLSD array
+template <size_t ARITY, typename CALLABLE>
+auto apply_n(CALLABLE&& func, const LLSD& args)
+{
+    return apply_impl(std::forward<CALLABLE>(func),
+                      apply_llsd_fix(ARITY, args),
+                      std::make_index_sequence<ARITY>());
+}
+
+/**
+ * apply(function, LLSD) goes beyond C++17 std::apply(). For this case
+ * @a function @emph cannot be variadic: the compiler must know at compile
+ * time how many arguments to pass. This isn't Python. (But see apply_n() to
+ * pass a specific number of args to a variadic function.)
+ */
+template <typename CALLABLE>
+auto apply(CALLABLE&& func, const LLSD& args)
+{
+    // infer arity from the definition of func
+    constexpr auto arity = function_arity<
+        typename std::remove_reference<CALLABLE>::type>::value;
+    // now that we have a compile-time arity, apply_n() works
+    return apply_n<arity>(std::forward<CALLABLE>(func), args);
+}
+
+} // namespace LL
+
 #endif // LL_LLSDUTIL_H