panic: send on closed channel

[Liang Zheng]

~/go/src/deeproute.ai/rosedb (main ✗) go run examples/watch/main.go
The db is closed, so the watch channel is closed.
panic: send on closed channel

goroutine 6 [running]:
github.com/rosedblabs/rosedb/v2.(*Watcher).sendEvent(0x0?, 0x0?)
	/root/go/src/deeproute.ai/rosedb/watch.go:70 +0x45
created by github.com/rosedblabs/rosedb/v2.Open
	/root/go/src/deeproute.ai/rosedb/db.go:123 +0x5cd
exit status 2

How to reproduce:

package main

import (
	"fmt"
	"time"

	"github.com/rosedblabs/rosedb/v2"
	"github.com/rosedblabs/rosedb/v2/utils"
)

// this file shows how to use the Watch feature of rosedb.

func main() {
	// specify the options
	options := rosedb.DefaultOptions
	options.DirPath = "/tmp/rosedb_watch"
	options.WatchQueueSize = 1000

	// open a database
	db, err := rosedb.Open(options)
	if err != nil {
		panic(err)
	}
	// defer func() { // <- comment it
	// 	_ = db.Close()
	// }()

	// run a new goroutine to handle db event.
	go func() {
		eventCh, err := db.Watch()
		if err != nil {
			return
		}
		for {
			event := <-eventCh
			// when db closed, the event will receive nil.
			if event == nil {
				fmt.Println("The db is closed, so the watch channel is closed.")
				return
			}
			// events can be captured here for processing
			fmt.Printf("Get a new event: key%s \n", event.Key)
		}
	}()

	// write some data
	for i := 0; i < 10; i++ {
		_ = db.Put(utils.GetTestKey(i), utils.RandomValue(64))
	}
	// delete some data
	for i := 0; i < 10/2; i++ {
		_ = db.Delete(utils.GetTestKey(i))
	}

	_ = db.Close() // <- close db

	// wait for watch goroutine to finish.
	time.Sleep(1 * time.Second)
}

[Liang Zheng]

The PR https://github.com/rosedblabs/rosedb/pull/252/files mainly refactors from the following three aspects.

1. fix potential panic: send on closed channel
2. using wait group to wait for goroutine in example
3. refactor event watch

Add Watcher interface, the interface mainly refers to etcd WatchStream and watchStream, it contains two methods, Watch and Close.

type Watcher interface {
	Watch() WatchChan
	Close() error
}

The watcher struct implements Watcher interface like follows.

type watcher struct {
	queue   eventQueue
	mu      sync.RWMutex
	watchCh chan *Event // new added, which is event channels. Watch() will return it.

	cond     *sync.Cond
	capacity uint64 // new added, which is used to record how many events currently, and does not exceed the capacity of the event queue

	closeMu    sync.Mutex
	notifyCh   chan struct{} // it is a notification channel, which means watch channel has been closed.
	cancelFunc func() // Close() will invoke it
}

@roseduan