Add TriggerLevelWriter.

See: #583

globals.go

@@ -132,6 +132,10 @@ var (
 		FatalLevel: "FTL",
 		PanicLevel: "PNC",
 	}
+
+	// TriggerLevelWriterBufferReuseLimit is a limit in bytes that a buffer is dropped
+	// from the TriggerLevelWriter buffer pool if the buffer grows above the limit.
+	TriggerLevelWriterBufferReuseLimit = 64 * 1024
 )
 
 var (

writer.go

@@ -180,3 +180,135 @@ func (w *FilteredLevelWriter) WriteLevel(level Level, p []byte) (int, error) {
 	}
 	return len(p), nil
 }
+
+var triggerWriterPool = &sync.Pool{
+	New: func() interface{} {
+		return bytes.NewBuffer(make([]byte, 0, 1024))
+	},
+}
+
+// TriggerLevelWriter buffers log lines at the ConditionalLevel or below
+// until a trigger level (or higher) line is emitted. Log lines with level
+// higher than ConditionalLevel are always written out to the destination
+// writer. If trigger never happens, buffered log lines are never written out.
+//
+// It can be used to configure "log level per request".
+type TriggerLevelWriter struct {
+	// Destination writer. If LevelWriter is provided (usually), its WriteLevel is used
+	// instead of Write.
+	io.Writer
+
+	// ConditionalLevel is the level (and below) at which lines are buffered until
+	// a trigger level (or higher) line is emitted. Usually this is set to DebugLevel.
+	ConditionalLevel Level
+
+	// TriggerLevel is the lowest level that triggers the sending of the conditional
+	// level lines. Usually this is set to ErrorLevel.
+	TriggerLevel Level
+
+	buf       *bytes.Buffer
+	triggered bool
+	mu        sync.Mutex
+}
+
+func (w *TriggerLevelWriter) WriteLevel(l Level, p []byte) (n int, err error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	// At first trigger level or above log line, we flush the buffer and change the
+	// trigger state to triggered.
+	if !w.triggered && l >= w.TriggerLevel {
+		err := w.trigger()
+		if err != nil {
+			return 0, err
+		}
+	}
+
+	// Unless triggered, we buffer everything at and below ConditionalLevel.
+	if !w.triggered && l <= w.ConditionalLevel {
+		if w.buf == nil {
+			w.buf = triggerWriterPool.Get().(*bytes.Buffer)
+		}
+
+		// We prefix each log line with a byte with the level.
+		// Hopefully we will never have a level value which equals a newline
+		// (which could interfere with reconstruction of log lines in the trigger method).
+		w.buf.WriteByte(byte(l))
+		w.buf.Write(p)
+		return len(p), nil
+	}
+
+	// Anything above ConditionalLevel is always passed through.
+	// Once triggered, everything is passed through.
+	if lw, ok := w.Writer.(LevelWriter); ok {
+		return lw.WriteLevel(l, p)
+	}
+	return w.Write(p)
+}
+
+// trigger expects lock to be held.
+func (w *TriggerLevelWriter) trigger() error {
+	if w.triggered {
+		return nil
+	}
+	w.triggered = true
+
+	if w.buf == nil {
+		return nil
+	}
+
+	p := w.buf.Bytes()
+	for len(p) > 0 {
+		// We do not use bufio.Scanner here because we already have full buffer
+		// in the memory and we do not want extra copying from the buffer to
+		// scanner's token slice, nor we want to hit scanner's token size limit,
+		// and we also want to preserve newlines.
+		i := bytes.IndexByte(p, '\n')
+		line := p[0 : i+1]
+		p = p[i+1:]
+		// We prefixed each log line with a byte with the level.
+		level := Level(line[0])
+		line = line[1:]
+		var err error
+		if lw, ok := w.Writer.(LevelWriter); ok {
+			_, err = lw.WriteLevel(level, line)
+		} else {
+			_, err = w.Write(line)
+		}
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Trigger forces flushing the buffer and change the trigger state to
+// triggered, if the writer has not already been triggered before.
+func (w *TriggerLevelWriter) Trigger() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	return w.trigger()
+}
+
+// Close closes the writer and returns the buffer to the pool.
+func (w *TriggerLevelWriter) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	if w.buf == nil {
+		return nil
+	}
+
+	// We return the buffer only if it has not grown above the limit.
+	// This prevents accumulation of large buffers in the pool just
+	// because occasionally a large buffer might be needed.
+	if w.buf.Cap() <= TriggerLevelWriterBufferReuseLimit {
+		w.buf.Reset()
+		triggerWriterPool.Put(w.buf)
+	}
+	w.buf = nil
+
+	return nil
+}

writer_test.go

@@ -195,3 +195,58 @@ func TestFilteredLevelWriter(t *testing.T) {
 		t.Errorf("Expected %q, got %q.", want, p)
 	}
 }
+
+type testWrite struct {
+	Level
+	Line []byte
+}
+
+func TestTriggerLevelWriter(t *testing.T) {
+	tests := []struct {
+		write []testWrite
+		want  []byte
+		all   []byte
+	}{{
+		[]testWrite{
+			{DebugLevel, []byte("no\n")},
+			{InfoLevel, []byte("yes\n")},
+		},
+		[]byte("yes\n"),
+		[]byte("yes\nno\n"),
+	}, {
+		[]testWrite{
+			{DebugLevel, []byte("yes1\n")},
+			{InfoLevel, []byte("yes2\n")},
+			{ErrorLevel, []byte("yes3\n")},
+			{DebugLevel, []byte("yes4\n")},
+		},
+		[]byte("yes2\nyes1\nyes3\nyes4\n"),
+		[]byte("yes2\nyes1\nyes3\nyes4\n"),
+	}}
+
+	for k, tt := range tests {
+		t.Run(fmt.Sprintf("case=%d", k), func(t *testing.T) {
+			buf := bytes.Buffer{}
+			writer := TriggerLevelWriter{Writer: LevelWriterAdapter{&buf}, ConditionalLevel: DebugLevel, TriggerLevel: ErrorLevel}
+			t.Cleanup(func() { writer.Close() })
+			for _, w := range tt.write {
+				_, err := writer.WriteLevel(w.Level, w.Line)
+				if err != nil {
+					t.Error(err)
+				}
+			}
+			p := buf.Bytes()
+			if want := tt.want; !bytes.Equal([]byte(want), p) {
+				t.Errorf("Expected %q, got %q.", want, p)
+			}
+			err := writer.Trigger()
+			if err != nil {
+				t.Error(err)
+			}
+			p = buf.Bytes()
+			if want := tt.all; !bytes.Equal([]byte(want), p) {
+				t.Errorf("Expected %q, got %q.", want, p)
+			}
+		})
+	}
+}