Goruntine 是 Go 語言裡面一個很重要的功能，它是一個輕量化的執行緒，由 Go runtime 負責去排程執行，據官方說法，同時開啟好幾千個也沒問題。
使用 Golang 開發專案時或多或少都會使用到 Goruntine，我們也很習慣在 Goruntine 裡面直接使用 fmt package，但 fmt 並不保證 concurrency safe。
最近我在專案中使用了 GIN 和 logrotate，發現在某些情況下會導致 data race，底下便是我精簡過的程式碼。
package main
import (
"bufio"
"errors"
"path/filepath"
"fmt"
"log"
"os"
"sync"
"time"
)
type Options struct {
Directory string
FileNameFunc func() string
}
type Writer struct {
logger *log.Logger
opts Options
f *os.File
bw *bufio.Writer
bytesWritten int64
queue chan []byte
pending sync.WaitGroup
closing chan struct{}
done chan struct{}
}
func (w *Writer) Write(p []byte) (n int, err error) {
//p := make([]byte, len(b))
//copy(p, b)
select {
case <-w.closing:
return 0, errors.New("writer is closing")
default:
w.pending.Add(1)
defer w.pending.Done()
}
w.queue <- p
return len(p), nil
}
func (w *Writer) Close() error {
close(w.closing)
w.pending.Wait()
close(w.queue)
<-w.done
if w.f != nil {
if err := w.closeCurrentFile(); err != nil {
return err
}
}
return nil
}
func (w *Writer) listen() {
for b := range w.queue {
if w.f == nil {
if err := w.rotate(); err != nil {
w.logger.Println("Failed to create log file", err)
}
}
size := int64(len(b))
if _, err := w.bw.Write(b); err != nil {
w.logger.Println("Failed to write to file.", err)
}
w.bytesWritten += size
}
close(w.done)
}
func (w *Writer) closeCurrentFile() error {
if err := w.bw.Flush(); err != nil {
return errors.New("failed to flush buffered writer")
}
if err := w.f.Sync(); err != nil {
return errors.New("failed to sync current log file")
}
if err := w.f.Close(); err != nil {
return errors.New("failed to close current log file")
}
w.bytesWritten = 0
return nil
}
func (w *Writer) rotate() error {
if w.f != nil {
if err := w.closeCurrentFile(); err != nil {
return err
}
}
path := filepath.Join(w.opts.Directory, w.opts.FileNameFunc())
f, err := newFile(path)
if err != nil {
return errors.New("failed to create new file")
}
w.bw = bufio.NewWriter(f)
w.f = f
w.bytesWritten = 0
return nil
}
func New(logger *log.Logger, opts Options) (*Writer, error) {
if _, err := os.Stat(opts.Directory); os.IsNotExist(err) {
if err := os.MkdirAll(opts.Directory, os.ModePerm); err != nil {
return nil, errors.New("directory does not exist and could not be created")
}
}
w := &Writer{
logger: logger,
opts: opts,
queue: make(chan []byte, 2000),
closing: make(chan struct{}),
done: make(chan struct{}),
}
go w.listen()
return w, nil
}
func newFile(path string) (*os.File, error) {
return os.OpenFile(path, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0666)
}
func test(wg *sync.WaitGroup, w *Writer) {
i := 0
var s2 string
for i < 1 {
s2 = "abcdefg"
fmt.Println(s2)
fmt.Fprint(w, s2)
i++
}
wg.Done()
}
func FileNameFunc() (string) {
t := time.Now()
return fmt.Sprintf("%04d%02d%02d_%02d%02d%02d.log",
t.Year(),
t.Month(),
t.Day(),
t.Hour(),
t.Minute(),
t.Second(),
)
}
func main() {
var wg sync.WaitGroup
logger := log.New(os.Stderr, "logrotate", 0)
dir, err := os.Getwd()
if err != nil {
fmt.Println(err)
return
}
opts := Options{
Directory: dir,
FileNameFunc: FileNameFunc,
}
writer, err := New(logger, opts)
if err != nil {
logger.Println(err)
return
}
max := 2000
i := 0
for i < max {
wg.Add(1)
go test(&wg, writer)
i++
}
wg.Wait()
}
這是測試的結果。
==================
WARNING: DATA RACE
Read at 0x00c00000a523 by goroutine 7:
runtime.slicecopy()
c:/go/src/runtime/slice.go:197 +0x0
bufio.(*Writer).Write()
c:/go/src/bufio/bufio.go:635 +0x3e5
main.(*Writer).listen()
E:/test.go:77 +0x1ab
Previous write at 0x00c00000a523 by goroutine 185:
runtime.slicestringcopy()
c:/go/src/runtime/slice.go:232 +0x0
fmt.(*buffer).writeString()
c:/go/src/fmt/print.go:82 +0x107
fmt.(*fmt).padString()
c:/go/src/fmt/format.go:110 +0x6f
fmt.(*fmt).fmtS()
c:/go/src/fmt/format.go:359 +0x75
fmt.(*pp).fmtString()
c:/go/src/fmt/print.go:447 +0x1ba
fmt.(*pp).printArg()
c:/go/src/fmt/print.go:698 +0xdcf
fmt.(*pp).doPrint()
c:/go/src/fmt/print.go:1161 +0x12c
fmt.Fprint()
c:/go/src/fmt/print.go:232 +0x6c
main.test()
E:/test.go:154 +0x130
Goroutine 7 (running) created at:
main.New()
E:/test.go:138 +0x29b
main.main()
E:/test.go:190 +0x271
Goroutine 185 (finished) created at:
main.main()
E:/test.go:200 +0x2e0
==================
==================
WARNING: DATA RACE
Read at 0x00c00008b1bd by goroutine 7:
runtime.slicecopy()
c:/go/src/runtime/slice.go:197 +0x0
bufio.(*Writer).Write()
c:/go/src/bufio/bufio.go:635 +0x3e5
main.(*Writer).listen()
E:/test.go:77 +0x1ab
Previous write at 0x00c00008b1bd by goroutine 702:
runtime.slicestringcopy()
c:/go/src/runtime/slice.go:232 +0x0
fmt.(*buffer).writeString()
c:/go/src/fmt/print.go:82 +0x107
fmt.(*fmt).padString()
c:/go/src/fmt/format.go:110 +0x6f
fmt.(*fmt).fmtS()
c:/go/src/fmt/format.go:359 +0x75
fmt.(*pp).fmtString()
c:/go/src/fmt/print.go:447 +0x1ba
fmt.(*pp).printArg()
c:/go/src/fmt/print.go:698 +0xdcf
fmt.(*pp).doPrintln()
c:/go/src/fmt/print.go:1173 +0xb4
fmt.Fprintln()
c:/go/src/fmt/print.go:264 +0x6c
fmt.Println()
c:/go/src/fmt/print.go:274 +0xc0
main.test()
E:/test.go:153 +0x42
Goroutine 7 (running) created at:
main.New()
E:/test.go:138 +0x29b
main.main()
E:/test.go:190 +0x271
Goroutine 702 (running) created at:
main.main()
E:/test.go:200 +0x2e0
==================
Found 2 data race(s)
exit status 66
從上面我們可以看到，不管是 fmt.Println 或是 fmt.Fprintf，都有機會引起 data race！
目前我的解決方式是把 buffer 先複製起來，再傳進 logrorate 的 channel 裡。
p := make([]byte, len(b))
copy(p, b)
copy(p, b)
解題靈感來自
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