Golang select priority execution

select key word

go In language select Keywords are used to monitor case The statement corresponds to chan, And execute the corresponding code under it , until case The execution of the next code will end chan The following code . It's kind of like switch.

select {
    
case x := <-ch1:
    fmt.Println(x)
case ch2 <- 1:
    fmt.Println("push 1")
}

empty select

select{
    
  
}

Empty select Statement directly blocks the current goroutine, Make the goroutine Enter a sleep state that cannot be awakened .

only one case

select {
    
case <-ch1:
    fmt.Println("recv")
}

The above code , When ch1 Print operation will be performed when it is readable , Otherwise, it will block .

Yes default sentence

select {
    
case <-ch1:
    fmt.Println("recv")
default:
    time.Sleep(time.Second)
}

The above code , When ch1 Print operation will be performed when it is readable , Otherwise, it will be executed default Code in statement , This is equivalent to doing a non blocking channel Read operation

summary

• select There is no such thing as case： Permanent blocking current goroutine
• select There is only one case： Blocked sending / receive
• select There are multiple case： Randomly select one that meets the conditions case perform
• select There is default, other case When they are not satisfied ： perform default Code in statement

Realization select Priority in

Reference resources k8s Priority in the source code select To deal with

func (tc *NoExecuteTaintManager) worker(worker int, done func(), stopCh <-chan struct{
    }) {
    
    defer done()

    // When processing events we want to prioritize Node updates over Pod updates,
    // as NodeUpdates that interest NoExecuteTaintManager should be handled as soon as possible -
    // we don't want user (or system) to wait until PodUpdate queue is drained before it can
    // start evicting Pods from tainted Nodes.
    for {
    
        select {
    
        case <-stopCh:
            return
        case nodeUpdate := <-tc.nodeUpdateChannels[worker]:
            tc.handleNodeUpdate(nodeUpdate)
            tc.nodeUpdateQueue.Done(nodeUpdate)
        case podUpdate := <-tc.podUpdateChannels[worker]:
            // If we found a Pod update we need to empty Node queue first.
        priority:
            for {
    
                select {
    
                case nodeUpdate := <-tc.nodeUpdateChannels[worker]:
                    tc.handleNodeUpdate(nodeUpdate)
                    tc.nodeUpdateQueue.Done(nodeUpdate)
                default:
                    break priority
                }
            }
            // After Node queue is emptied we process podUpdate.
            tc.handlePodUpdate(podUpdate)
            tc.podUpdateQueue.Done(podUpdate)
        }
    }
}

I need this priority right now select The application scenario is generally , When my program catch When the abnormal signal needs to exit , I'll put... Before I quit select Let's do something else chan Exit after all execution

// Abnormal exit signal 
var kExitSignals = []os.Signal{
    
    syscall.SIGTERM,
    syscall.SIGINT,
    syscall.SIGSEGV,
    syscall.SIGBUS,
    syscall.SIGQUIT,
}

int main() {
    
    exitChan := make(chan os.Signal, 1)
    signal.Notify(exitChan, kExitSignals...)
    
    handleCh := make(chan struct{
    }, 10)
    
    //goroutine with input handleCh 
    
    for {
    
        select {
    
        case <-handleCh:
            //do some handle
        case signl := <-exitChan:
            // Received an exit signal , Also make sure to put chan Exit after processing the data in 
        priority:
            for {
    
                select {
    
                case <-handleCh:
                    // do some handle
                default:
                    break priority
                }
            }
            log.Info("catch signal:", signl)
            // Do some cleanup and resource recycling 
            //os.Exit(1)
             // If it is a server, you can use server.Stop() perhaps server.GraceStop() To quit 
        }
    }
}