[library/cpp/logger] Support meta flags with TLogElement

Support opportunity to use TMetaFlags with TLogElement.

**Use example:**
`Log().With("Property", "Value").With("Code", "Code") << LogMessage`

Such interface allows to not parse required by TLogBackend  properties from raw string.

5 files changed, 76 insertions, 12 deletions

diff --git a/library/cpp/logger/element.cpp b/library/cpp/logger/element.cpp
index b510fe16e1..ecce365f1f 100644
--- a/library/cpp/logger/element.cpp
+++ b/library/cpp/logger/element.cpp
@@ -3,6 +3,7 @@
 
 #include <utility>
 
+
 TLogElement::TLogElement(const TLog* parent)
     : Parent_(parent)
     , Priority_(Parent_->DefaultPriority())
@@ -32,7 +33,7 @@ void TLogElement::DoFlush() {
     const size_t filled = Filled();
 
     if (filled) {
-        Parent_->Write(Priority_, Data(), filled);
+        Parent_->Write(Priority_, Data(), filled, std::move(Context_));
         Reset();
     }
 }
diff --git a/library/cpp/logger/element.h b/library/cpp/logger/element.h
index fc9bff851f..9ba27ab67e 100644
--- a/library/cpp/logger/element.h
+++ b/library/cpp/logger/element.h
@@ -1,18 +1,20 @@
 #pragma once
 
 #include "priority.h"
+#include "record.h"
 
+
+#include <util/string/cast.h>
 #include <util/stream/tempbuf.h>
 
+
 class TLog;
 
-/*
- * better do not use directly
- */
+/// @warning Better don't use directly.
 class TLogElement: public TTempBufOutput {
 public:
-    TLogElement(const TLog* parent);
-    TLogElement(const TLog* parent, ELogPriority priority);
+    explicit TLogElement(const TLog* parent);
+    explicit TLogElement(const TLog* parent, ELogPriority priority);
 
     TLogElement(TLogElement&&) noexcept = default;
     TLogElement& operator=(TLogElement&&) noexcept = default;
@@ -26,15 +28,20 @@ public:
         return *this;
     }
 
-    /*
-         * for pretty usage: logger << TLOG_ERROR << "Error description";
-         */
+    /// @note For pretty usage: logger << TLOG_ERROR << "Error description";
     inline TLogElement& operator<<(ELogPriority priority) {
         Flush();
         Priority_ = priority;
         return *this;
     }
 
+    template<typename T>
+    TLogElement& With(const TStringBuf key, const T value) {
+        Context_.emplace_back(key, ToString(value));
+
+        return *this;
+    }
+
     ELogPriority Priority() const noexcept {
         return Priority_;
     }
@@ -43,6 +50,7 @@ protected:
     void DoFlush() override;
 
 protected:
-    const TLog* Parent_;
+    const TLog* Parent_ = nullptr;
     ELogPriority Priority_;
+    TLogRecord::TMetaFlags Context_;
 };
diff --git a/library/cpp/logger/element_ut.cpp b/library/cpp/logger/element_ut.cpp
index 32edc52dfb..86303973d8 100644
--- a/library/cpp/logger/element_ut.cpp
+++ b/library/cpp/logger/element_ut.cpp
@@ -9,13 +9,16 @@
 
 #include <library/cpp/testing/unittest/registar.h>
 
+
 class TLogElementTest: public TTestBase {
     UNIT_TEST_SUITE(TLogElementTest);
     UNIT_TEST(TestMoveCtor);
+    UNIT_TEST(TestWith);
     UNIT_TEST_SUITE_END();
 
 public:
     void TestMoveCtor();
+    void TestWith();
 };
 
 UNIT_TEST_SUITE_REGISTRATION(TLogElementTest);
@@ -37,3 +40,16 @@ void TLogElementTest::TestMoveCtor() {
     dst.Destroy();
     UNIT_ASSERT(output.Str() == message);
 }
+
+void TLogElementTest::TestWith() {
+    TStringStream output;
+    TLog log(MakeHolder<TStreamWithContextLogBackend>(&output));
+
+    THolder<TLogElement> src = MakeHolder<TLogElement>(&log);
+
+    TString message = "Hello, World!";
+    (*src).With("Foo", "Bar").With("Foo", "Baz") << message;
+
+    src.Destroy();
+    UNIT_ASSERT(output.Str() == "Hello, World!; Foo=Bar; Foo=Baz; ");
+}
diff --git a/library/cpp/logger/stream.cpp b/library/cpp/logger/stream.cpp
index 96787ad94b..73cd5030ac 100644
--- a/library/cpp/logger/stream.cpp
+++ b/library/cpp/logger/stream.cpp
@@ -3,6 +3,7 @@
 
 #include <util/stream/output.h>
 
+
 TStreamLogBackend::TStreamLogBackend(IOutputStream* slave)
     : Slave_(slave)
 {
@@ -17,3 +18,23 @@ void TStreamLogBackend::WriteData(const TLogRecord& rec) {
 
 void TStreamLogBackend::ReopenLog() {
 }
+
+TStreamWithContextLogBackend::TStreamWithContextLogBackend(IOutputStream* slave)
+    : Slave_(slave)
+{
+}
+
+TStreamWithContextLogBackend::~TStreamWithContextLogBackend() {
+}
+
+void TStreamWithContextLogBackend::WriteData(const TLogRecord& rec) {
+    Slave_->Write(rec.Data, rec.Len);
+    Slave_->Write(DELIMITER);
+    for (const auto& [key, value] : rec.MetaFlags) {
+        Slave_->Write(TString::Join(key, "=", value));
+        Slave_->Write(DELIMITER);
+    }
+}
+
+void TStreamWithContextLogBackend::ReopenLog() {
+}
diff --git a/library/cpp/logger/stream.h b/library/cpp/logger/stream.h
index feb240afcb..a5eca9060e 100644
--- a/library/cpp/logger/stream.h
+++ b/library/cpp/logger/stream.h
@@ -2,11 +2,14 @@
 
 #include "backend.h"
 
+#include <util/generic/string.h>
+
+
 class IOutputStream;
 
-class TStreamLogBackend: public TLogBackend {
+class TStreamLogBackend : public TLogBackend {
 public:
-    TStreamLogBackend(IOutputStream* slave);
+    explicit TStreamLogBackend(IOutputStream* slave);
     ~TStreamLogBackend() override;
 
     void WriteData(const TLogRecord& rec) override;
@@ -15,3 +18,18 @@ public:
 private:
     IOutputStream* Slave_;
 };
+
+class TStreamWithContextLogBackend : public TLogBackend {
+private:
+    static constexpr TStringBuf DELIMITER = "; ";
+
+public:
+    explicit TStreamWithContextLogBackend(IOutputStream* slave);
+    ~TStreamWithContextLogBackend() override;
+
+    void WriteData(const TLogRecord& rec) override;
+    void ReopenLog() override;
+
+private:
+    IOutputStream* Slave_;
+};