Add message append functionality

Adds C++ binding to sd_bus_message_append* class functions.  Uses
type deduction to identify all of the parameters passed to the
append function and, at compile-time, creates the appropriate dbus
type string.

Example:
    sdbusplus::message::append(m, "asdf"s);
Is equal to:
    sd_bus_message_append_basic(m, 's', ("asdf"s).c_str());

Rather than use a naive sequence of ..._append_basic calls, the
implementation will attempt to group multiple basic types into a
single sd_bus_message_append call.

Change-Id: I1341a0299573c61588fe225c07174b743cb3a282
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>

2 files changed, 400 insertions, 0 deletions

diff --git a/sdbusplus/message/append.hpp b/sdbusplus/message/append.hpp
new file mode 100644
index 0000000..7ce429b
--- /dev/null
+++ b/sdbusplus/message/append.hpp
@@ -0,0 +1,260 @@
+#pragma once
+
+#include <tuple>
+#include <sdbusplus/message/types.hpp>
+#include <sdbusplus/utility/type_traits.hpp>
+#include <sdbusplus/utility/tuple_to_array.hpp>
+#include <systemd/sd-bus.h>
+
+namespace sdbusplus
+{
+
+namespace message
+{
+
+/** @brief Append data into an sdbus message.
+ *
+ *  (This is an empty no-op function that is useful in some cases for
+ *   variadic template reasons.)
+ */
+inline void append(sd_bus_message* m) {};
+/** @brief Append data into an sdbus message.
+ *
+ *  @param[in] msg - The message to append to.
+ *  @tparam Args - C++ types of arguments to append to message.
+ *  @param[in] args - values to append to message.
+ *
+ *  This function will, at compile-time, deduce the DBus types of the passed
+ *  C++ values and call the sd_bus_message_append functions with the
+ *  appropriate type parameters.  It may also do conversions, where needed,
+ *  to convert C++ types into C representations (eg. string, vector).
+ */
+template <typename ...Args> void append(sd_bus_message* m, Args&&... args);
+
+namespace details
+{
+
+/** @struct can_append_multiple
+ *  @brief Utility to identify C++ types that may not be grouped into a
+ *         single sd_bus_message_append call and instead need special
+ *         handling.
+ *
+ *  @tparam T - Type for identification.
+ *
+ *  User-defined types are expected to inherit from std::false_type.
+ *
+ */
+template<typename T> struct can_append_multiple : std::true_type {};
+    // std::string needs a c_str() call.
+template<> struct can_append_multiple<std::string> : std::false_type {};
+
+/** @struct append_single
+ *  @brief Utility to append a single C++ element into a sd_bus_message.
+ *
+ *  User-defined types are expected to specialize this template in order to
+ *  get their functionality.
+ *
+ *  @tparam S - Type of element to append.
+ */
+template<typename S> struct append_single
+{
+    // Downcast
+    template<typename T>
+    using Td = types::details::type_id_downcast_t<T>;
+
+    /** @brief Do the operation to append element.
+     *
+     *  @tparam T - Type of element to append.
+     *
+     *  Template parameters T (function) and S (class) are different
+     *  to allow the function to be utilized for many varients of S:
+     *  S&, S&&, const S&, volatile S&, etc. The type_id_downcast is used
+     *  to ensure T and S are equivalent.  For 'char*', this also allows
+     *  use for 'char[N]' types.
+     *
+     *  @param[in] m - sd_bus_message to append into.
+     *  @param[in] t - The item to append.
+     */
+    template<typename T,
+             typename = std::enable_if_t<std::is_same<S, Td<T>>::value>>
+    static void op(sd_bus_message* m, T&& t)
+    {
+        // For this default implementation, we need to ensure that only
+        // basic types are used.
+        static_assert(std::is_fundamental<Td<T>>::value ||
+                      std::is_convertible<Td<T>, const char*>::value,
+                      "Non-basic types are not allowed.");
+
+        constexpr auto dbusType = std::get<0>(types::type_id<T>());
+        sd_bus_message_append_basic(m, dbusType, &t);
+    }
+};
+
+template<typename T> using append_single_t =
+        append_single<types::details::type_id_downcast_t<T>>;
+
+/** @brief Specialization of append_single for std::strings. */
+template <> struct append_single<std::string>
+{
+    template<typename T>
+    static void op(sd_bus_message* m, T&& s)
+    {
+        constexpr auto dbusType = std::get<0>(types::type_id<T>());
+        sd_bus_message_append_basic(m, dbusType, s.c_str());
+    }
+};
+
+/** @brief Append a tuple of content into the sd_bus_message.
+ *
+ *  @tparam Tuple - The tuple type to append.
+ *  @param[in] t - The tuple value to append.
+ *  @tparam I - The indexes of the tuple type Tuple.
+ *  @param[in] [unamed] - unused index_sequence for type deduction of I.
+ */
+template <typename Tuple, size_t... I>
+void append_tuple(sd_bus_message* m, Tuple&& t, std::index_sequence<I...>)
+{
+    auto dbusTypes = utility::tuple_to_array(
+            types::type_id<decltype(std::get<I>(t))...>());
+
+    sd_bus_message_append(m, dbusTypes.data(), std::get<I>(t)...);
+}
+
+/** @brief Append a tuple of 2 or more entries into the sd_bus_message.
+ *
+ *  @tparam Tuple - The tuple type to append.
+ *  @param[in] t - The tuple to append.
+ *
+ *  A tuple of 2 or more entries can be added as a set with
+ *  sd_bus_message_append.
+ */
+template <typename Tuple> std::enable_if_t<2 <= std::tuple_size<Tuple>::value>
+append_tuple(sd_bus_message* m, Tuple&& t)
+{
+    append_tuple(m, std::move(t),
+                 std::make_index_sequence<std::tuple_size<Tuple>::value>());
+}
+
+/** @brief Append a tuple of exactly 1 entry into the sd_bus_message.
+ *
+ *  @tparam Tuple - The tuple type to append.
+ *  @param[in] t - The tuple to append.
+ *
+ *  A tuple of 1 entry can be added with sd_bus_message_append_basic.
+ *
+ *  Note: Some 1-entry tuples may need special handling due to
+ *  can_append_multiple::value == false.
+ */
+template <typename Tuple> std::enable_if_t<1 == std::tuple_size<Tuple>::value>
+append_tuple(sd_bus_message* m, Tuple&& t)
+{
+    using itemType = decltype(std::get<0>(t));
+    append_single_t<itemType>::op(m, std::forward<itemType>(std::get<0>(t)));
+}
+
+/** @brief Append a tuple of 0 entries - no-op.
+ *
+ *  This a no-op function that is useful due to variadic templates.
+ */
+template <typename Tuple> std::enable_if_t<0 == std::tuple_size<Tuple>::value>
+inline append_tuple(sd_bus_message* m, Tuple&& t) {}
+
+/** @brief Group a sequence of C++ types for appending into an sd_bus_message.
+ *  @tparam Tuple - A tuple of previously analyzed types.
+ *  @tparam Arg - The argument to analyze for grouping.
+ *
+ *  Specialization for when can_append_multiple<Arg> is true.
+ */
+template <typename Tuple, typename Arg> std::enable_if_t<
+        can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg);
+/** @brief Group a sequence of C++ types for appending into an sd_bus_message.
+ *  @tparam Tuple - A tuple of previously analyzed types.
+ *  @tparam Arg - The argument to analyze for grouping.
+ *
+ *  Specialization for when can_append_multiple<Arg> is false.
+ */
+template <typename Tuple, typename Arg> std::enable_if_t<
+        !can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg);
+/** @brief Group a sequence of C++ types for appending into an sd_bus_message.
+ *  @tparam Tuple - A tuple of previously analyzed types.
+ *  @tparam Arg - The argument to analyze for grouping.
+ *  @tparam Rest - The remaining arguments left to analyze.
+ *
+ *  Specialization for when can_append_multiple<Arg> is true.
+ */
+template <typename Tuple, typename Arg, typename ...Rest> std::enable_if_t<
+        can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg, Rest&&... rest);
+/** @brief Group a sequence of C++ types for appending into an sd_bus_message.
+ *  @tparam Tuple - A tuple of previously analyzed types.
+ *  @tparam Arg - The argument to analyze for grouping.
+ *  @tparam Rest - The remaining arguments left to analyze.
+ *
+ *  Specialization for when can_append_multiple<Arg> is false.
+ */
+template <typename Tuple, typename Arg, typename ...Rest> std::enable_if_t<
+        !can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg, Rest&&... rest);
+
+template <typename Tuple, typename Arg> std::enable_if_t<
+        can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg)
+{
+    // Last element of a sequence and can_append_multiple, so add it to
+    // the tuple and call append_tuple.
+
+    append_tuple(m, std::tuple_cat(std::forward<Tuple>(t),
+                                   std::forward_as_tuple(
+                                        std::forward<Arg>(arg))));
+}
+
+template <typename Tuple, typename Arg> std::enable_if_t<
+        !can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg)
+{
+    // Last element of a sequence but !can_append_multiple, so call
+    // append_tuple on the previous elements and separately this single
+    // element.
+
+    append_tuple(m, std::forward<Tuple>(t));
+    append_tuple(m, std::forward_as_tuple(std::forward<Arg>(arg)));
+}
+
+template <typename Tuple, typename Arg, typename ...Rest> std::enable_if_t<
+        can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg, Rest&&... rest)
+{
+    // Not the last element of a sequence and can_append_multiple, so add it
+    // to the tuple and keep grouping.
+
+    append_grouping(m, std::tuple_cat(std::forward<Tuple>(t),
+                                      std::forward_as_tuple(
+                                            std::forward<Arg>(arg))),
+                    std::forward<Rest>(rest)...);
+}
+
+template <typename Tuple, typename Arg, typename ...Rest> std::enable_if_t<
+        !can_append_multiple<types::details::type_id_downcast_t<Arg>>::value>
+append_grouping(sd_bus_message* m, Tuple&& t, Arg&& arg, Rest&&... rest)
+{
+    // Not the last element of a sequence but !can_append_multiple, so call
+    // append_tuple on the previous elements and separately this single
+    // element and then group the remaining elements.
+
+    append_tuple(m, std::forward<Tuple>(t));
+    append_tuple(m, std::forward_as_tuple(std::forward<Arg>(arg)));
+    append_grouping(m, std::make_tuple(), std::forward<Rest>(rest)...);
+}
+
+} // namespace details
+
+template <typename ...Args> void append(sd_bus_message* m, Args&&... args)
+{
+    details::append_grouping(m, std::make_tuple(), std::forward<Args>(args)...);
+}
+
+} // namespace message
+
+} // namespace sdbusplus
diff --git a/test/message/append.cpp b/test/message/append.cpp
new file mode 100644
index 0000000..1b0f70b
--- /dev/null
+++ b/test/message/append.cpp
@@ -0,0 +1,140 @@
+#include <iostream>
+#include <sdbusplus/message/append.hpp>
+#include <cassert>
+
+// Global to share the dbus type string between client and server.
+static std::string verifyTypeString;
+
+static constexpr auto SERVICE = "sdbusplus.test";
+static constexpr auto INTERFACE = SERVICE;
+static constexpr auto TEST_METHOD = "test";
+static constexpr auto QUIT_METHOD = "quit";
+
+// Open up the sdbus and claim SERVICE name.
+void serverInit(sd_bus** b)
+{
+    assert(0 <= sd_bus_open(b));
+    assert(0 <= sd_bus_request_name(*b, SERVICE, 0));
+}
+
+// Thread to run the dbus server.
+void* server(void* b)
+{
+    auto bus = reinterpret_cast<sd_bus*>(b);
+
+    while(1)
+    {
+        // Wait for messages.
+        sd_bus_message *m = nullptr;
+        if (0 == sd_bus_process(bus, &m))
+        {
+            sd_bus_wait(bus, 0);
+            continue;
+        }
+
+        if(!m)
+        {
+            continue;
+        }
+
+        if (sd_bus_message_is_method_call(m, INTERFACE, TEST_METHOD))
+        {
+            // Verify the message type matches what the test expects.
+            // TODO: It would be nice to verify content here as well.
+            assert(verifyTypeString == sd_bus_message_get_signature(m, true));
+            // Reply to client.
+            sd_bus_reply_method_return(m, nullptr);
+        }
+        else if (sd_bus_message_is_method_call(m, INTERFACE, QUIT_METHOD))
+        {
+            // Reply and exit.
+            sd_bus_reply_method_return(m, nullptr);
+            break;
+        }
+    }
+}
+
+void newMethodCall__test(sd_bus* b, sd_bus_message** m)
+{
+    // Allocate a method-call message for INTERFACE,TEST_METHOD.
+    assert(0 <= sd_bus_message_new_method_call(b, m, SERVICE, "/", INTERFACE,
+                                               TEST_METHOD));
+    sd_bus_message_set_expect_reply(*m, true);
+}
+
+void runTests()
+{
+    using namespace std::literals;
+
+    sd_bus_message* m = nullptr;
+    sd_bus* b = nullptr;
+
+    // Connect to dbus.
+    assert(0 <= sd_bus_open(&b));
+
+    // Test r-value int.
+    {
+        newMethodCall__test(b, &m);
+        sdbusplus::message::append(m, 1);
+        verifyTypeString = "i";
+        sd_bus_call(b, m, 0, nullptr, nullptr);
+    }
+
+    // Test l-value int.
+    {
+        newMethodCall__test(b, &m);
+        int a = 1;
+        sdbusplus::message::append(m, a, a);
+        verifyTypeString = "ii";
+        sd_bus_call(b, m, 0, nullptr, nullptr);
+    }
+
+    // Test multiple ints.
+    {
+        newMethodCall__test(b, &m);
+        sdbusplus::message::append(m, 1, 2, 3, 4, 5);
+        verifyTypeString = "iiiii";
+        sd_bus_call(b, m, 0, nullptr, nullptr);
+    }
+
+    // Test r-value string.
+    {
+        newMethodCall__test(b, &m);
+        sdbusplus::message::append(m, "asdf"s);
+        verifyTypeString = "s";
+        sd_bus_call(b, m, 0, nullptr, nullptr);
+    }
+
+    // Test multiple strings, various forms.
+    {
+        newMethodCall__test(b, &m);
+        auto str = "jkl;"s;
+        auto str2 = "JKL:"s;
+        sdbusplus::message::append(m, 1, "asdf", "ASDF"s, str,
+                                   std::move(str2), 5);
+        verifyTypeString = "issssi";
+        sd_bus_call(b, m, 0, nullptr, nullptr);
+    }
+
+    // Shutdown server.
+    sd_bus_call_method(b, SERVICE, "/", INTERFACE, QUIT_METHOD,
+                       nullptr, nullptr, nullptr);
+}
+
+int main()
+{
+    // Initialize and start server thread.
+    pthread_t t;
+    {
+        sd_bus* b;
+        serverInit(&b);
+        pthread_create(&t, NULL, server, b);
+    }
+
+    runTests();
+
+    // Wait for server thread to exit.
+    pthread_join(t, NULL);
+
+    return 0;
+}