Fix rotating log makes dfdaemon.log file huge size

When the size of dfdaemon.log file large than 20M,
the size of log file will continuous growth up.

Use Mmap to copy the file which need be truncated.

Related to issue:
dragonflyoss/dragonfly#364

dfdaemon/initializer/initializer.go

@@ -18,7 +18,6 @@ package initializer
 
 import (
 	"fmt"
-	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -102,29 +101,33 @@ func cleanLocalRepo(options *options.Options) {
 }
 
 // rotateLog truncates the logs file by a certain amount bytes.
-func rotateLog(logFile *os.File, logFilePath string) {
+func rotateLog(logFile *os.File) {
 	logSizeLimit := int64(20 * 1024 * 1024)
 	for {
 		time.Sleep(time.Second * 60)
-		stat, err := os.Stat(logFilePath)
+		stat, err := logFile.Stat()
 		if err != nil {
-			log.Errorf("failed to stat %s: %s", logFilePath, err)
+			log.Errorf("failed to stat: %s", err)
 			continue
 		}
 		// if it exceeds the 20MB limitation
 		if stat.Size() > logSizeLimit {
 			log.SetOutput(ioutil.Discard)
 			logFile.Sync()
-			if transFile, err := os.Open(logFilePath); err == nil {
-				// move the pointer to be (end - 10MB)
-				transFile.Seek(-10*1024*1024, 2)
-				// move the pointer to head
-				logFile.Seek(0, 0)
-				count, _ := io.Copy(logFile, transFile)
-				logFile.Truncate(count)
-				log.SetOutput(logFile)
-				transFile.Close()
+			truncateSize := logSizeLimit/2 - 1
+			mem, err := syscall.Mmap(int(logFile.Fd()), 0, int(stat.Size()), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
+			if err != nil {
+				log.Errorf("failed to map file and memory: %s", err)
+				continue
+			}
+			copy(mem[0:], mem[truncateSize:])
+			if err := syscall.Munmap(mem); err != nil {
+				log.Errorf("failed to unmap file and memory: %s", err)
+				continue
 			}
+			logFile.Truncate(stat.Size() - truncateSize)
+			logFile.Seek(truncateSize, 0)
+			log.SetOutput(logFile)
 		}
 	}
 
@@ -148,7 +151,7 @@ func initLogger() {
 		if logFile, err := os.OpenFile(logFilePath, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0644); err == nil {
 			logFile.Seek(0, 2)
 			log.SetOutput(logFile)
-			go rotateLog(logFile, logFilePath)
+			go rotateLog(logFile)
 		}
 	}