asio: unpack in place

The existing code uses several different tuples to do the unpack, then
concatenates them with tuple_cat.  Simplify the code by generating a
tuple of references to the dbus args, and unpack directly to the main
struct, thus simplifying the flow.

To accomplish this the make_dbus_args_tuple helper method is added,
which strips the non-dbus arguments from a callback so they can be
unpacked separately.  This is relatively complex because this code tries
to support

callback(boost::error_code, message_t, dbus_arg);
as well as
callback(boost::error_code, dbus_arg);

Dynamically dependent on which overload is provided.  There's likely
more cleanup that can happen there to separate these two, but this
should at least help.

Tested: OpenBMC launches and runs.  Redfish service validator (which
should hit the majority of these overloads) passes.

Change-Id: I6bb7ee960459a505eb8633b1cdb30cd2b3037466
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/test/meson.build b/test/meson.build
index 300c3bc..2b7bcfc 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -37,6 +37,7 @@
     'message/types',
     'timer',
     'unpack_properties',
+    'utility/make_dbus_args_tuple',
     'utility/tuple_to_array',
     'utility/type_traits',
 ]
diff --git a/test/utility/make_dbus_args_tuple.cpp b/test/utility/make_dbus_args_tuple.cpp
new file mode 100644
index 0000000..6ec2d71
--- /dev/null
+++ b/test/utility/make_dbus_args_tuple.cpp
@@ -0,0 +1,49 @@
+#include <boost/system/error_code.hpp>
+#include <sdbusplus/message.hpp>
+#include <sdbusplus/utility/make_dbus_args_tuple.hpp>
+
+#include <gtest/gtest.h>
+
+namespace sdbusplus
+{
+namespace utility
+{
+
+TEST(MakeDbusArgsTuple, MessageFirst)
+{
+    std::tuple<boost::system::error_code, sdbusplus::message_t, int>
+        input_tuple;
+    auto tuple_out = make_dbus_args_tuple(input_tuple);
+    static_assert(
+        std::is_same_v<std::tuple_element_t<0, decltype(tuple_out)>, int&>,
+        "Second type wasn't int");
+    static_assert(std::tuple_size_v<decltype(tuple_out)> == 1,
+                  "Size was wrong");
+    // Verify the output reference is now the first member, and references the 2
+    // index tuple arg
+    EXPECT_EQ(&std::get<2>(input_tuple), &std::get<0>(tuple_out));
+}
+TEST(MakeDbusArgsTuple, ArgFirst)
+{
+    std::tuple<boost::system::error_code, int> input_tuple{
+        boost::system::error_code(), 42};
+    auto tuple_out = make_dbus_args_tuple(input_tuple);
+    static_assert(
+        std::is_same_v<std::tuple_element_t<0, decltype(tuple_out)>, int&>,
+        "Second type wasn't int");
+    static_assert(std::tuple_size_v<decltype(tuple_out)> == 1,
+                  "Size was wrong");
+    // Verify the output reference is now the first member, and references the 1
+    // index tuple arg
+    EXPECT_EQ(&std::get<1>(input_tuple), &std::get<0>(tuple_out));
+    EXPECT_EQ(std::get<0>(tuple_out), 42);
+}
+TEST(MakeDbusArgsTuple, NoArgs)
+{
+    std::tuple<boost::system::error_code> input_tuple;
+    auto tuple_out = make_dbus_args_tuple(input_tuple);
+    static_assert(std::tuple_size_v<decltype(tuple_out)> == 0,
+                  "Size was wrong");
+}
+} // namespace utility
+} // namespace sdbusplus