Added Mapped Diagnostic Context (MDC) support (#2907)

* Added Mapped Diagnostic Context (MDC) support

* Update include statement

* Optimize string creation

* Fix includes

* Fix padding rules in mdc empty case

* Add comment to describe the use of mdc formatter

include/spdlog/mdc.h

@@ -0,0 +1,31 @@
+#include <spdlog/common.h>
+#include <map>
+
+namespace spdlog {
+
+class SPDLOG_API mdc {
+public:
+    static void put(const std::string &key, const std::string &value) {
+        get_context()[key] = value;
+    }
+
+    static std::string get(const std::string &key) {
+        auto &context = get_context();
+        auto it = context.find(key);
+        if (it != context.end()) {
+            return it->second;
+        }
+        return "";
+    }
+
+    static void remove(const std::string &key) { get_context().erase(key); }
+
+    static void clear() { get_context().clear(); }
+
+    static std::map<std::string, std::string> &get_context() {
+        static thread_local std::map<std::string, std::string> context;
+        return context;
+    }
+};
+
+}  // namespace spdlog

include/spdlog/pattern_formatter-inl.h

@@ -10,6 +10,7 @@
 #include <spdlog/details/fmt_helper.h>
 #include <spdlog/details/log_msg.h>
 #include <spdlog/details/os.h>
+#include <spdlog/mdc.h>
 #include <spdlog/fmt/fmt.h>
 #include <spdlog/formatter.h>
 
@@ -867,6 +868,43 @@ private:
     memory_buf_t cached_datetime_;
 };
 
+// Class for formatting Mapped Diagnostic Context (MDC) in log messages.
+// Example: [logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message
+template <typename ScopedPadder>
+class mdc_formatter : public flag_formatter {
+public:
+    explicit mdc_formatter(padding_info padinfo)
+        : flag_formatter(padinfo) {}
+
+    void format(const details::log_msg &, const std::tm &, memory_buf_t &dest) override {
+        auto mdc_map = mdc::get_context();
+        if (mdc_map.empty()) {
+            ScopedPadder p(0, padinfo_, dest);
+            return;
+        } else {
+            auto last_element = --mdc_map.end();
+            for (auto it = mdc_map.begin(); it != mdc_map.end(); ++it) {
+                auto &pair = *it;
+                const auto &key = pair.first;
+                const auto &value = pair.second;
+                size_t content_size = key.size() + value.size() + 1;  // 1 for ':'
+
+                if (it != last_element) {
+                    content_size++;  // 1 for ' '
+                }
+
+                ScopedPadder p(content_size, padinfo_, dest);
+                fmt_helper::append_string_view(key, dest);
+                fmt_helper::append_string_view(":", dest);
+                fmt_helper::append_string_view(value, dest);
+                if (it != last_element) {
+                    fmt_helper::append_string_view(" ", dest);
+                }
+            }
+        }
+    }
+};
+
 }  // namespace details
 
 SPDLOG_INLINE pattern_formatter::pattern_formatter(std::string pattern,
@@ -1159,6 +1197,10 @@ SPDLOG_INLINE void pattern_formatter::handle_flag_(char flag, details::padding_i
                     padding));
             break;
 
+        case ('&'):
+            formatters_.push_back(details::make_unique<details::mdc_formatter<Padder>>(padding));
+            break;
+
         default:  // Unknown flag appears as is
             auto unknown_flag = details::make_unique<details::aggregate_formatter>();
 

tests/includes.h

@@ -26,6 +26,7 @@
 #include "spdlog/spdlog.h"
 #include "spdlog/async.h"
 #include "spdlog/details/fmt_helper.h"
+#include "spdlog/mdc.h"
 #include "spdlog/sinks/basic_file_sink.h"
 #include "spdlog/sinks/daily_file_sink.h"
 #include "spdlog/sinks/null_sink.h"

tests/test_pattern_formatter.cpp

@@ -500,3 +500,131 @@ TEST_CASE("override need_localtime", "[pattern_formatter]") {
         REQUIRE(to_string_view(formatted) == oss.str());
     }
 }
+
+TEST_CASE("mdc formatter test-1", "[pattern_formatter]") {
+    spdlog::mdc::put("mdc_key_1", "mdc_value_1");
+    spdlog::mdc::put("mdc_key_2", "mdc_value_2");
+
+    auto formatter = std::make_shared<spdlog::pattern_formatter>();
+    formatter->set_pattern("[%n] [%l] [%&] %v");
+
+    memory_buf_t formatted;
+    spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info,
+                                 "some message");
+    formatter->format(msg, formatted);
+
+    auto expected = spdlog::fmt_lib::format(
+        "[logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message{}",
+        spdlog::details::os::default_eol);
+    REQUIRE(to_string_view(formatted) == expected);
+
+    SECTION("Tear down") { spdlog::mdc::clear(); }
+}
+
+TEST_CASE("mdc formatter value update", "[pattern_formatter]") {
+    spdlog::mdc::put("mdc_key_1", "mdc_value_1");
+    spdlog::mdc::put("mdc_key_2", "mdc_value_2");
+
+    auto formatter = std::make_shared<spdlog::pattern_formatter>();
+    formatter->set_pattern("[%n] [%l] [%&] %v");
+
+    memory_buf_t formatted_1;
+    spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info,
+                                 "some message");
+    formatter->format(msg, formatted_1);
+
+    auto expected = spdlog::fmt_lib::format(
+        "[logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message{}",
+        spdlog::details::os::default_eol);
+
+    REQUIRE(to_string_view(formatted_1) == expected);
+
+    spdlog::mdc::put("mdc_key_1", "new_mdc_value_1");
+    memory_buf_t formatted_2;
+    formatter->format(msg, formatted_2);
+    expected = spdlog::fmt_lib::format(
+        "[logger-name] [info] [mdc_key_1:new_mdc_value_1 mdc_key_2:mdc_value_2] some message{}",
+        spdlog::details::os::default_eol);
+
+    REQUIRE(to_string_view(formatted_2) == expected);
+
+    SECTION("Tear down") { spdlog::mdc::clear(); }
+}
+
+TEST_CASE("mdc different threads", "[pattern_formatter]") {
+    auto formatter = std::make_shared<spdlog::pattern_formatter>();
+    formatter->set_pattern("[%n] [%l] [%&] %v");
+    spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info,
+                                 "some message");
+
+    memory_buf_t formatted_2;
+
+    auto lambda_1 = [formatter, msg]() {
+        spdlog::mdc::put("mdc_key", "thread_1_id");
+        memory_buf_t formatted;
+        formatter->format(msg, formatted);
+
+        auto expected =
+            spdlog::fmt_lib::format("[logger-name] [info] [mdc_key:thread_1_id] some message{}",
+                                    spdlog::details::os::default_eol);
+
+        REQUIRE(to_string_view(formatted) == expected);
+    };
+
+    auto lambda_2 = [formatter, msg]() {
+        spdlog::mdc::put("mdc_key", "thread_2_id");
+        memory_buf_t formatted;
+        formatter->format(msg, formatted);
+
+        auto expected =
+            spdlog::fmt_lib::format("[logger-name] [info] [mdc_key:thread_2_id] some message{}",
+                                    spdlog::details::os::default_eol);
+
+        REQUIRE(to_string_view(formatted) == expected);
+    };
+
+    std::thread thread_1(lambda_1);
+    std::thread thread_2(lambda_2);
+
+    thread_1.join();
+    thread_2.join();
+
+    SECTION("Tear down") { spdlog::mdc::clear(); }
+}
+
+TEST_CASE("mdc remove key", "[pattern_formatter]") {
+    spdlog::mdc::put("mdc_key_1", "mdc_value_1");
+    spdlog::mdc::put("mdc_key_2", "mdc_value_2");
+    spdlog::mdc::remove("mdc_key_1");
+
+    auto formatter = std::make_shared<spdlog::pattern_formatter>();
+    formatter->set_pattern("[%n] [%l] [%&] %v");
+
+    memory_buf_t formatted;
+    spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info,
+                                 "some message");
+    formatter->format(msg, formatted);
+
+    auto expected =
+        spdlog::fmt_lib::format("[logger-name] [info] [mdc_key_2:mdc_value_2] some message{}",
+                                spdlog::details::os::default_eol);
+    REQUIRE(to_string_view(formatted) == expected);
+
+    SECTION("Tear down") { spdlog::mdc::clear(); }
+}
+
+TEST_CASE("mdc empty", "[pattern_formatter]") {
+    auto formatter = std::make_shared<spdlog::pattern_formatter>();
+    formatter->set_pattern("[%n] [%l] [%&] %v");
+
+    memory_buf_t formatted;
+    spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info,
+                                 "some message");
+    formatter->format(msg, formatted);
+
+    auto expected = spdlog::fmt_lib::format("[logger-name] [info] [] some message{}",
+                                            spdlog::details::os::default_eol);
+    REQUIRE(to_string_view(formatted) == expected);
+
+    SECTION("Tear down") { spdlog::mdc::clear(); }
+}