聊聊zerolog的diode.Writer

序

本文主要研究一下zerolog的diode.Writer

diode.Writer

github.com/rs/zerolog@v1.20.0/diode/diode.go

// Writer is a io.Writer wrapper that uses a diode to make Write lock-free,
// non-blocking and thread safe.
type Writer struct {
    w    io.Writer
    d    diodeFetcher
    c    context.CancelFunc
    done chan struct{}
}

func NewWriter(w io.Writer, size int, pollInterval time.Duration, f Alerter) Writer {
    ctx, cancel := context.WithCancel(context.Background())
    dw := Writer{
        w:    w,
        c:    cancel,
        done: make(chan struct{}),
    }
    if f == nil {
        f = func(int) {}
    }
    d := diodes.NewManyToOne(size, diodes.AlertFunc(f))
    if pollInterval > 0 {
        dw.d = diodes.NewPoller(d,
            diodes.WithPollingInterval(pollInterval),
            diodes.WithPollingContext(ctx))
    } else {
        dw.d = diodes.NewWaiter(d,
            diodes.WithWaiterContext(ctx))
    }
    go dw.poll()
    return dw
}

diode.Writer是一个lock-free,non-blocking及thread safe的Writer；它借助了diodes来实现；NewWriter会创建diode.Writer，并启动dw.poll()

poll

github.com/rs/zerolog@v1.20.0/diode/diode.go

func (dw Writer) poll() {
    defer close(dw.done)
    for {
        d := dw.d.Next()
        if d == nil {
            return
        }
        p := *(*[]byte)(d)
        dw.w.Write(p)

        // Proper usage of a sync.Pool requires each entry to have approximately
        // the same memory cost. To obtain this property when the stored type
        // contains a variably-sized buffer, we add a hard limit on the maximum buffer
        // to place back in the pool.
        //
        // See https://golang.org/issue/23199
        const maxSize = 1 << 16 // 64KiB
        if cap(p) <= maxSize {
            bufPool.Put(p[:0])
        }
    }
}

poll方法使用for循环执行dw.d.Next()及dw.w.Write(p)

diodeFetcher

github.com/rs/zerolog@v1.20.0/diode/diode.go

type diodeFetcher interface {
    diodes.Diode
    Next() diodes.GenericDataType
}

// Diode is any implementation of a diode.
type Diode interface {
    Set(GenericDataType)
    TryNext() (GenericDataType, bool)
}

diodeFetcher接口内嵌了Diode接口，定义了Next方法

Next

github.com/rs/zerolog@v1.20.0/diode/internal/diodes/poller.go

// Next polls the diode until data is available or until the context is done.
// If the context is done, then nil will be returned.
func (p *Poller) Next() GenericDataType {
    for {
        data, ok := p.Diode.TryNext()
        if !ok {
            if p.isDone() {
                return nil
            }

            time.Sleep(p.interval)
            continue
        }
        return data
    }
}

Poller实现了diodeFetcher接口的Next方法，它使用for循环，不断通过p.Diode.TryNext()来获取data

ManyToOne

github.com/rs/zerolog@v1.20.0/diode/internal/diodes/many_to_one.go

// ManyToOne diode is optimal for many writers (go-routines B-n) and a single
// reader (go-routine A). It is not thread safe for multiple readers.
type ManyToOne struct {
    writeIndex uint64
    readIndex  uint64
    buffer     []unsafe.Pointer
    alerter    Alerter
}

// Set sets the data in the next slot of the ring buffer.
func (d *ManyToOne) Set(data GenericDataType) {
    for {
        writeIndex := atomic.AddUint64(&d.writeIndex, 1)
        idx := writeIndex % uint64(len(d.buffer))
        old := atomic.LoadPointer(&d.buffer[idx])

        if old != nil &&
            (*bucket)(old) != nil &&
            (*bucket)(old).seq > writeIndex-uint64(len(d.buffer)) {
            log.Println("Diode set collision: consider using a larger diode")
            continue
        }

        newBucket := &bucket{
            data: data,
            seq:  writeIndex,
        }

        if !atomic.CompareAndSwapPointer(&d.buffer[idx], old, unsafe.Pointer(newBucket)) {
            log.Println("Diode set collision: consider using a larger diode")
            continue
        }

        return
    }
}

// TryNext will attempt to read from the next slot of the ring buffer.
// If there is not data available, it will return (nil, false).
func (d *ManyToOne) TryNext() (data GenericDataType, ok bool) {
    // Read a value from the ring buffer based on the readIndex.
    idx := d.readIndex % uint64(len(d.buffer))
    result := (*bucket)(atomic.SwapPointer(&d.buffer[idx], nil))

    // When the result is nil that means the writer has not had the
    // opportunity to write a value into the diode. This value must be ignored
    // and the read head must not increment.
    if result == nil {
        return nil, false
    }

    // When the seq value is less than the current read index that means a
    // value was read from idx that was previously written but has since has
    // been dropped. This value must be ignored and the read head must not
    // increment.
    //
    // The simulation for this scenario assumes the fast forward occurred as
    // detailed below.
    //
    // 5. The reader reads again getting seq 5. It then reads again expecting
    //    seq 6 but gets seq 2. This is a read of a stale value that was
    //    effectively "dropped" so the read fails and the read head stays put.
    //    `| 4 | 5 | 2 | 3 |` r: 7, w: 6
    //
    if result.seq < d.readIndex {
        return nil, false
    }

    // When the seq value is greater than the current read index that means a
    // value was read from idx that overwrote the value that was expected to
    // be at this idx. This happens when the writer has lapped the reader. The
    // reader needs to catch up to the writer so it moves its write head to
    // the new seq, effectively dropping the messages that were not read in
    // between the two values.
    //
    // Here is a simulation of this scenario:
    //
    // 1. Both the read and write heads start at 0.
    //    `| nil | nil | nil | nil |` r: 0, w: 0
    // 2. The writer fills the buffer.
    //    `| 0 | 1 | 2 | 3 |` r: 0, w: 4
    // 3. The writer laps the read head.
    //    `| 4 | 5 | 2 | 3 |` r: 0, w: 6
    // 4. The reader reads the first value, expecting a seq of 0 but reads 4,
    //    this forces the reader to fast forward to 5.
    //    `| 4 | 5 | 2 | 3 |` r: 5, w: 6
    //
    if result.seq > d.readIndex {
        dropped := result.seq - d.readIndex
        d.readIndex = result.seq
        d.alerter.Alert(int(dropped))
    }

    // Only increment read index if a regular read occurred (where seq was
    // equal to readIndex) or a value was read that caused a fast forward
    // (where seq was greater than readIndex).
    //
    d.readIndex++
    return result.data, true
}

ManyToOne实现了Diode接口的Set和TryNext方法

实例

func diodeDemo() {
    wr := diode.NewWriter(os.Stdout, 1000, 10*time.Millisecond, func(missed int) {
        fmt.Printf("Logger Dropped %d messages", missed)
    })
    log := zerolog.New(wr)
    log.Print("test")

    time.Sleep(1 * time.Second)
}

输出

{"level":"debug","message":"test"}

小结

zerolog借助diodes提供了一个lock-free,non-blocking及thread safe的diode.Writer

doc

• zerolog