feat: implement experimental BackScanner for reading files backward and add tests for various scenarios

internal/net/gphttp/accesslog/rotate.go

@@ -0,0 +1,119 @@
+package accesslog
+
+import (
+	"bytes"
+	"io"
+	"time"
+)
+
+func (l *AccessLogger) rotate() (err error) {
+	// Get retention configuration
+	config := l.Config().Retention
+	var shouldKeep func(t time.Time, lineCount int) bool
+
+	if config.Last > 0 {
+		shouldKeep = func(_ time.Time, lineCount int) bool {
+			return lineCount < int(config.Last)
+		}
+	} else if config.Days > 0 {
+		cutoff := time.Now().AddDate(0, 0, -int(config.Days))
+		shouldKeep = func(t time.Time, _ int) bool {
+			return !t.IsZero() && !t.Before(cutoff)
+		}
+	} else {
+		return nil // No retention policy set
+	}
+
+	s := NewBackScanner(l.io, defaultChunkSize)
+	nRead := 0
+	nLines := 0
+	for s.Scan() {
+		nRead += len(s.Bytes()) + 1
+		nLines++
+		t := ParseLogTime(s.Bytes())
+		if !shouldKeep(t, nLines) {
+			break
+		}
+	}
+	if s.Err() != nil {
+		return s.Err()
+	}
+
+	beg := int64(nRead)
+	if _, err := l.io.Seek(-beg, io.SeekEnd); err != nil {
+		return err
+	}
+	buf := make([]byte, nRead)
+	if _, err := l.io.Read(buf); err != nil {
+		return err
+	}
+
+	if err := l.writeTruncate(buf); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (l *AccessLogger) writeTruncate(buf []byte) (err error) {
+	// Seek to beginning and truncate
+	if _, err := l.io.Seek(0, 0); err != nil {
+		return err
+	}
+
+	// Write buffer back to file
+	nWritten, err := l.buffered.Write(buf)
+	if err != nil {
+		return err
+	}
+	if err = l.buffered.Flush(); err != nil {
+		return err
+	}
+
+	// Truncate file
+	if err = l.io.Truncate(int64(nWritten)); err != nil {
+		return err
+	}
+
+	// check bytes written == buffer size
+	if nWritten != len(buf) {
+		return io.ErrShortWrite
+	}
+	return
+}
+
+const timeLen = len(`"time":"`)
+
+var timeJSON = []byte(`"time":"`)
+
+func ParseLogTime(line []byte) (t time.Time) {
+	if len(line) == 0 {
+		return
+	}
+
+	if i := bytes.Index(line, timeJSON); i != -1 { // JSON format
+		var jsonStart = i + timeLen
+		var jsonEnd = i + timeLen + len(LogTimeFormat)
+		if len(line) < jsonEnd {
+			return
+		}
+		timeStr := line[jsonStart:jsonEnd]
+		t, _ = time.Parse(LogTimeFormat, string(timeStr))
+		return
+	}
+
+	// Common/Combined format
+	// Format: <virtual host> <host ip> - - [02/Jan/2006:15:04:05 -0700] ...
+	start := bytes.IndexByte(line, '[')
+	if start == -1 {
+		return
+	}
+	end := bytes.IndexByte(line[start:], ']')
+	if end == -1 {
+		return
+	}
+	end += start // adjust end position relative to full line
+
+	timeStr := line[start+1 : end]
+	t, _ = time.Parse(LogTimeFormat, string(timeStr)) // ignore error
+	return
+}