pkg/chdelay: implement ChannelDelayer (#5373)

pkg/chdelay/channel_delayer.go

@@ -0,0 +1,174 @@
+// Copyright 2022 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package chdelay
+
+import (
+	"sync"
+	"time"
+)
+
+// ChannelDelayer provides a mechanism to inject delay in a channel.
+//
+// Algorithm sketch:
+// When an element arrives from the input channel,
+// attach the current timestamp and put it in a queue.
+// The top of the queue is checked, if the element
+// has stayed in the queue for more than `delayBy`,
+// then the element in popped and sent to the output channel.
+type ChannelDelayer[T any] struct {
+	inCh  <-chan T
+	outCh chan T
+
+	queue []entry[T]
+	size  int
+
+	closeCh chan struct{}
+	wg      sync.WaitGroup
+
+	delayBy time.Duration
+}
+
+type entry[T any] struct {
+	elem   T
+	inTime time.Time
+}
+
+// NewChannelDelayer creates a new ChannelDelayer.
+func NewChannelDelayer[T any](
+	delayBy time.Duration,
+	in chan T,
+	queueSize int,
+	outChSize int,
+) *ChannelDelayer[T] {
+	ret := &ChannelDelayer[T]{
+		inCh:    in,
+		outCh:   make(chan T, outChSize),
+		queue:   make([]entry[T], 0, queueSize),
+		size:    queueSize,
+		closeCh: make(chan struct{}),
+		delayBy: delayBy,
+	}
+
+	ret.wg.Add(1)
+	go func() {
+		ret.run()
+	}()
+
+	return ret
+}
+
+// Out returns the delayed channel. The downstream logic
+// should read from Out().
+func (d *ChannelDelayer[T]) Out() <-chan T {
+	return d.outCh
+}
+
+// Close closes the ChannelDelayer.
+func (d *ChannelDelayer[T]) Close() {
+	close(d.closeCh)
+	d.wg.Wait()
+}
+
+func (d *ChannelDelayer[T]) run() {
+	defer d.wg.Done()
+	defer close(d.outCh)
+
+	ticker := time.NewTicker(time.Millisecond)
+	defer ticker.Stop()
+
+	// currentTime is a timestamp cache to
+	// avoid having to read the system's
+	// clock frequently.
+	currentTime := time.Now()
+
+	// Returns the output channel if
+	// the first element in the queue
+	// is ready to be popped.
+	// Otherwise, nil is returned.
+	// Note that nil channels are ignored in
+	// a select statement, so it would disable
+	// a case block.
+	outChIfReady := func() chan<- T {
+		if len(d.queue) == 0 {
+			return nil
+		}
+		if currentTime.Sub(d.queue[0].inTime) >= d.delayBy {
+			return d.outCh
+		}
+		return nil
+	}
+
+	// dummyEntry provides a zero value entry.
+	var dummyEntry entry[T]
+
+	for {
+		var firstElem *T
+		if len(d.queue) > 0 {
+			firstElem = &d.queue[0].elem
+		} else {
+			// Must provide a valid pointer.
+			firstElem = &dummyEntry.elem
+		}
+
+		select {
+		case <-d.closeCh:
+			return
+		case <-ticker.C:
+			currentTime = time.Now()
+		case inElem, ok := <-d.inChIfSizeOk():
+			if !ok {
+				if len(d.queue) == 0 {
+					return
+				}
+				continue
+			}
+			d.queue = append(d.queue, entry[T]{
+				elem:   inElem,
+				inTime: time.Now(),
+			})
+		case outChIfReady() <- *firstElem:
+			// Cleans any reference to *T if T is a pointer,
+			// to prompt a timely GC.
+			d.queue[0] = dummyEntry
+			d.queue = d.queue[1:]
+
+		LOOP:
+			// Drain the queue as much as possible.
+			for {
+				if len(d.queue) == 0 {
+					break LOOP
+				}
+				if currentTime.Sub(d.queue[0].inTime) < d.delayBy {
+					break
+				}
+
+				select {
+				case d.outCh <- d.queue[0].elem:
+					// Cleans any reference to *T if T is a pointer
+					d.queue[0] = dummyEntry
+					d.queue = d.queue[1:]
+				default:
+					break LOOP
+				}
+			}
+		}
+	}
+}
+
+func (d *ChannelDelayer[T]) inChIfSizeOk() <-chan T {
+	if len(d.queue) < d.size {
+		return d.inCh
+	}
+	return nil
+}

pkg/chdelay/channel_delayer_test.go

@@ -0,0 +1,66 @@
+// Copyright 2022 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package chdelay
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestChannelDelayerShortDelay(t *testing.T) {
+	t.Parallel()
+
+	testChannelDelayer(t, time.Millisecond*100, 10000)
+}
+
+func TestChannelDelayerLongDelay(t *testing.T) {
+	t.Parallel()
+
+	testChannelDelayer(t, time.Second*2, 100)
+}
+
+func testChannelDelayer(t *testing.T, delayBy time.Duration, count int) {
+	inCh := make(chan time.Time, 16)
+	delayer := NewChannelDelayer(delayBy, inCh, 1024, 16)
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer close(inCh)
+
+		for i := 0; i < count; i++ {
+			inCh <- time.Now()
+		}
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		counter := 0
+		for ts := range delayer.Out() {
+			require.Greater(t, time.Since(ts), delayBy)
+			counter++
+		}
+
+		require.Equal(t, count, counter)
+	}()
+
+	wg.Wait()
+	delayer.Close()
+}