feat: add support for dumping variant to string (#526)

This allows Message and Variant contents serialization to a string -- based on `sd_bus_message_dump` sd-bus API function.

.clang-tidy

@@ -55,7 +55,7 @@ CheckOptions:
   - key:             hicpp-move-const-arg.CheckTriviallyCopyableMove
     value:           '0'
   - key:             misc-include-cleaner.IgnoreHeaders
-    value:           'systemd/.*|sdbus-c\+\+/.*|gtest/.*|gmock/.*|bits/chrono.h|bits/basic_string.h|time.h|poll.h|stdlib.h'
+    value:           'systemd/.*|sdbus-c\+\+/.*|gtest/.*|gmock/.*|bits/chrono.h|bits/basic_string.h|time.h|poll.h|stdlib.h|stdio.h'
   - key:             readability-simplify-boolean-expr.IgnoreMacros
     value:           '1'
   - key:             cppcoreguidelines-rvalue-reference-param-not-moved.IgnoreUnnamedParams

include/sdbus-c++/Message.h

@@ -228,6 +228,15 @@ namespace sdbus {
         void seal();
         void rewind(bool complete);
 
+        enum class DumpFlags : uint64_t // NOLINT(performance-enum-size): using size from sd-bus
+        {
+            Default               = 0ULL,
+            WithHeader            = 1ULL << 0,
+            SubtreeOnly           = 1ULL << 1,
+            SubtreeOnlyWithHeader = WithHeader | SubtreeOnly
+        };
+        [[nodiscard]] std::string dumpToString(DumpFlags flags) const;
+
         pid_t getCredsPid() const;
         uid_t getCredsUid() const;
         uid_t getCredsEuid() const;
@@ -260,7 +269,6 @@ namespace sdbus {
 
         friend Factory;
 
-    
         void* msg_{};
         internal::IConnection* connection_{};
         mutable bool ok_{true};

include/sdbus-c++/Types.h

@@ -103,6 +103,13 @@ namespace sdbus {
             return val;
         }
 
+        [[nodiscard]] std::string dumpToString() const
+        {
+            msg_.rewind(false);
+
+            return msg_.dumpToString(Message::DumpFlags::SubtreeOnly);
+        }
+
         // Only allow conversion operator for true D-Bus type representations in C++
         // NOLINTNEXTLINE(modernize-use-constraints): TODO for future: Use `requires signature_of<_ValueType>::is_valid` (when we stop supporting C++17 in public API)
         template <typename ValueType, typename = std::enable_if_t<signature_of<ValueType>::is_valid>>

src/Message.cpp

@@ -35,7 +35,9 @@
 #include "ScopeGuard.h"
 
 #include <cassert>
+#include <cerrno>
 #include <cstdint> // int16_t, uint64_t, ...
+#include <cstdio>
 #include <cstdlib> // atexit
 #include <cstring>
 #include <string>
@@ -628,6 +630,28 @@ void Message::rewind(bool complete)
     SDBUS_THROW_ERROR_IF(r < 0, "Failed to rewind the message", -r);
 }
 
+std::string Message::dumpToString(DumpFlags flags) const
+{
+#if LIBSYSTEMD_VERSION>=245 && !defined(SDBUS_basu)
+    char* buffer{};
+    SCOPE_EXIT{ free(buffer); }; // NOLINT(cppcoreguidelines-no-malloc,hicpp-no-malloc,cppcoreguidelines-owning-memory)
+    size_t size{};
+
+    const std::unique_ptr<FILE, int(*)(FILE*)> stream{open_memstream(&buffer, &size), fclose};
+    SDBUS_THROW_ERROR_IF(!stream, "Failed to open memory stream", errno);
+
+    auto r = sd_bus_message_dump(static_cast<sd_bus_message*>(msg_), stream.get(), static_cast<uint64_t>(flags));
+    SDBUS_THROW_ERROR_IF(r < 0, "Failed to dump the message", -r);
+
+    (void)fflush(stream.get());
+
+    return {buffer, size};
+#else
+    (void)flags;
+    throw Error(Error::Name{SD_BUS_ERROR_NOT_SUPPORTED}, "Dumping sd-bus message not supported by underlying version of libsystemd");
+#endif
+}
+
 const char* Message::getInterfaceName() const
 {
     return sd_bus_message_get_interface(static_cast<sd_bus_message*>(msg_));

tests/unittests/Types_test.cpp

@@ -45,6 +45,7 @@
 
 using ::testing::Eq;
 using ::testing::Gt;
+using ::testing::HasSubstr;
 using namespace std::string_literals;
 
 namespace
@@ -148,6 +149,24 @@ TEST(ASimpleVariant, ReturnsTheSimpleValueWhenAsked)
     ASSERT_THAT(variant.get<int>(), Eq(value));
 }
 
+#ifndef SDBUS_basu // Dumping message or variant to a string is not supported on basu backend
+TEST(ASimpleVariant, CanBeDumpedToAString)
+{
+    const int value = 5;
+    const sdbus::Variant variant(value);
+
+    // This should produce something like:
+    // VARIANT "i" {
+    //     INT32 5;
+    // };
+    const auto str = variant.dumpToString();
+
+    EXPECT_THAT(str, ::HasSubstr("VARIANT \"i\""));
+    EXPECT_THAT(str, ::HasSubstr("INT32"));
+    EXPECT_THAT(str, ::HasSubstr("5"));
+}
+#endif // SDBUS_basu
+
 TEST(AComplexVariant, ReturnsTheComplexValueWhenAsked)
 {
     using ComplexType = std::map<uint64_t, std::vector<sdbus::Struct<std::string, double>>>;
@@ -158,6 +177,39 @@ TEST(AComplexVariant, ReturnsTheComplexValueWhenAsked)
     ASSERT_THAT(variant.get<ComplexType>(), Eq(value));
 }
 
+#ifndef SDBUS_basu // Dumping message or variant to a string is not supported on basu backend
+TEST(AComplexVariant, CanBeDumpedToAString)
+{
+    using ComplexType = std::map<uint64_t, std::vector<sdbus::Struct<std::string, double>>>;
+    const ComplexType value{ {ANY_UINT64, ComplexType::mapped_type{{"hello"s, ANY_DOUBLE}, {"world"s, ANY_DOUBLE}}} };
+    const sdbus::Variant variant(value);
+
+    // This should produce something like:
+    // VARIANT "a{ta(sd)}" {
+    //     ARRAY "{ta(sd)}" {
+    //         DICT_ENTRY "ta(sd)" {
+    //             UINT64 84578348354;
+    //             ARRAY "(sd)" {
+    //                 STRUCT "sd" {
+    //                     STRING "hello";
+    //                     DOUBLE 3.14;
+    //                 };
+    //                 STRUCT "sd" {
+    //                     STRING "world";
+    //                     DOUBLE 3.14;
+    //                 };
+    //             };
+    //         };
+    //     };
+    // };
+    const auto str = variant.dumpToString();
+
+    EXPECT_THAT(str, ::HasSubstr("VARIANT \"a{ta(sd)}\""));
+    EXPECT_THAT(str, ::HasSubstr("hello"));
+    EXPECT_THAT(str, ::HasSubstr("world"));
+}
+#endif // SDBUS_basu
+
 TEST(AVariant, HasConceptuallyNonmutableGetMethodWhichCanBeCalledXTimes)
 {
     const std::string value{"I am a string"};