Question

这是我的程序产生死锁，我该如何避免它以及处理这种情况的推荐模式。

问题是在超时后如何检测到我的频道上没有阅读器？

var wg sync.WaitGroup

func main() {   
    wg.Add(1)
    c := make(chan int)
    go readFromChannel(c, time.After(time.Duration(2)*time.Second))
    time.Sleep(time.Duration(5) * time.Second)
    c <- 10
    wg.Wait()
}

func readFromChannel(c chan int, ti <-chan time.Time) {
    select {
    case x := <-c:
        fmt.Println("Read", x)
    case <-ti:
        fmt.Println("TIMED OUT")
    }
    wg.Done()
}

Answer 1

所以，让我们来看看你的资源中真正发生了什么。您有两个 goroutines（其中有两个以上，但我们将重点关注明确的那些），main和readFromChannel。

让我们看一下readFromChannel的作用：

if channel `c` is not empty before `ti` has expired, print its contents and return, after signalling its completion to wait group.
if `ti` has expired before `c` is not empty, print "TIMED OUT" and return, after signalling its completion to wait group.

现在主要：

adds to waitgroup 
make a channel `c`
start a goroutine `readFromChannel`
sleep for 5 seconds
send 10 to channel `c`
call wait for waitgroup

现在，让我们同时执行代码的执行流程（您的代码可能/可能不会每次按此顺序执行，请记住这一点）

1) wg.Add(1)
2) c := make(chan int)
3) go readFromChannel(c, time.After(time.Duration(2)*time.Second))
#timer ti starts#
4) time.Sleep(time.Duration(5) * time.Second)
#MAIN Goroutine begins sleep
#timer ti expires#
5) case <-ti:
6) fmt.Println("TIMED OUT")
7) wg.Done()
# readFromChannel Goroutine returns #
#MAIN Goroutine exits sleep#
8) c<-10
9) ......#DEADLOCK#

现在你可以猜到为什么会遇到僵局。在go中，非缓冲频道将阻止，直到频道另一端发生某些事情，无论您是否正在发送或接收。所以c <- 10将会阻止，直到某些内容从c的另一端读取，但是你已经为此做出的goroutine已经在2秒前退出了画面。因此，c会永久阻止，并且由于main是最后一个goroutine，因此会出现死锁。

如何预防？使用频道时，请确保每个receive频道的另一端始终为send。func main() { wg.Add(1) c := make(chan int) go readFromChannel(c, time.After(time.Duration(2)*time.Second)) time.Sleep(time.Duration(5) * time.Second) c <- 10 wg.Wait() } func readFromChannel(c chan int, ti <-chan time.Time) { // the forloop will run forever loop: // ** for { select { case x := <-c: fmt.Println("Read", x) break loop // breaks out of the for loop and the select ** case <-ti: fmt.Println("TIMED OUT") } } wg.Done() }。您也可以使用缓冲频道，但在上面的代码中，它不会是＆＃34;右边＆＃34;解。

这是我对僵局的解决方法：

        [root@idas-new centos]# tail -500f nohup.out
Warning: No configuration directory set! Use --conf <dir> to override.
Warning: JAVA_HOME is not set!
+ exec /usr/bin/java -Xmx20m -Dflume.root.logger=DEBUG,console -cp '/usr/cygnus/lib/*:/usr/cygnus/plugins.d/cygnus/lib/*:/usr/cygnus/agent_test.conf -n cygnusagent
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/usr/cygnus/lib/slf4j-log4j12-1.6.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/usr/cygnus/plugins.d/cygnus/lib/cygnus-0.12.0_SNAPSHOT-jar-with-dependencies.jar!/org/slf4j/impl/
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
16/05/18 09:26:46 INFO node.PollingPropertiesFileConfigurationProvider: Configuration provider starting
16/05/18 09:26:46 INFO node.PollingPropertiesFileConfigurationProvider: Reloading configuration file:/usr/cygnus/conf/agent_test.con
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Added sinks: hdfs-sink Agent: cygnusagent
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Processing:hdfs-sink
16/05/18 09:26:46 INFO conf.FlumeConfiguration: Post-validation flume configuration contains configuration for agents: [cygnusagent]
16/05/18 09:26:46 INFO node.AbstractConfigurationProvider: Creating channels
16/05/18 09:26:46 INFO channel.DefaultChannelFactory: Creating instance of channel hdfs-channel type memory
16/05/18 09:26:46 INFO node.AbstractConfigurationProvider: Created channel hdfs-channel
16/05/18 09:26:46 INFO source.DefaultSourceFactory: Creating instance of source http-source, type org.apache.flume.source.http.HTTPS
16/05/18 09:26:46 INFO handlers.OrionRestHandler: Cygnus version (0.12.0_SNAPSHOT.d6ee0e4548ca6bf9116c1dee4c4cbcc7da8b1752)
16/05/18 09:26:46 INFO handlers.OrionRestHandler: Startup completed
16/05/18 09:26:46 INFO sink.DefaultSinkFactory: Creating instance of sink: hdfs-sink, type: com.telefonica.iot.cygnus.sinks.OrionHDF
16/05/18 09:26:46 ERROR node.AbstractConfigurationProvider: Sink hdfs-sink has been removed due to an error during configuration
java.lang.NullPointerException
        at com.telefonica.iot.cygnus.sinks.OrionHDFSSink.configure(OrionHDFSSink.java:339)
        at org.apache.flume.conf.Configurables.configure(Configurables.java:41)
        at org.apache.flume.node.AbstractConfigurationProvider.loadSinks(AbstractConfigurationProvider.java:418)
        at org.apache.flume.node.AbstractConfigurationProvider.getConfiguration(AbstractConfigurationProvider.java:103)
        at org.apache.flume.node.PollingPropertiesFileConfigurationProvider$FileWatcherRunnable.run(PollingPropertiesFileConfigurati
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask.runAndReset(Unknown Source)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(Unknown Source)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
16/05/18 09:26:46 INFO node.AbstractConfigurationProvider: Channel hdfs-channel connected to [http-source]
16/05/18 09:26:46 INFO node.Application: Starting new configuration:{ sourceRunners:{http-source=EventDrivenSourceRunner: { source:oyChannel{name: hdfs-channel}} }
16/05/18 09:26:46 INFO node.Application: Starting Channel hdfs-channel
16/05/18 09:26:46 INFO instrumentation.MonitoredCounterGroup: Monitoried counter group for type: CHANNEL, name: hdfs-channel, regist
16/05/18 09:26:46 INFO instrumentation.MonitoredCounterGroup: Component type: CHANNEL, name: hdfs-channel started
16/05/18 09:26:46 INFO node.Application: Starting Source http-source
16/05/18 09:26:46 INFO interceptors.GroupingRules: No grouping rules have been read
Exception in thread "Thread-1" java.lang.NullPointerException
        at java.io.File.<init>(Unknown Source)
        at com.telefonica.iot.cygnus.interceptors.GroupingInterceptor$ConfigurationReader.run(GroupingInterceptor.java:244)
16/05/18 09:26:46 INFO mortbay.log: Logging to org.slf4j.impl.Log4jLoggerAdapter(org.mortbay.log) via org.mortbay.log.Slf4jLog
16/05/18 09:26:46 INFO mortbay.log: jetty-6.1.26
16/05/18 09:26:47 INFO mortbay.log: Started SocketConnector@0.0.0.0:5050

** see this answer for details

Answer 2

您有一个无缓冲的频道。根据{{3}}：

如果频道未缓冲，则发送方将阻塞，直到接收方具有收到了价值。如果通道有缓冲区，则发送方会阻塞只有将值复制到缓冲区

通过将频道更改为缓冲，我们可以避免死锁。

c := make(chan int, 10) // holds 10 ints

我还建议阅读docs，这里有一些与频道有关的好东西。

Answer 3

您的问题是您使用的是select声明，但您没有在goroutine中使用。

go func() {
    for {
        select {
        case x := <-c:
            fmt.Println("Read", x)
        case <-ti:
            fmt.Println("TIMED OUT")
        }
    }
}()

可以使用select关键字从不同的并发执行goroutine中获取值，该关键字非常类似于switch控制语句，有时也称为通信开关。

在具有默认大小写的select语句中使用send操作可确保发送将是非阻塞的！如果没有任何情况，则select块永远执行。

https://play.golang.org/p/Ai1ggveb4s

Answer 4

这是一个较老的问题，但我自己深入学习频道，并在此处找到了。

我认为您需要在完成发送后关闭频道吗？

代码：

func main() {
    wg.Add(1)
    c := make(chan int)
    go readFromChannel(c, time.After(time.Duration(2)*time.Second))
    time.Sleep(time.Duration(5) * time.Second)
    c <- 10
    close(c) // <- CLOSE IT HERE
    wg.Wait()
}

Answer 5

func main() {

    wg.Add(1)
    c := make(chan int)
    go func() {
        c <- 10

    }()
    go readFromChannel(c, time.After(time.Duration(2)*time.Second))
    time.Sleep(time.Duration(5) * time.Second)

    wg.Wait()

}//This will read without deadlock

func main() {
    wg.Add(1)
    c := make(chan int)

    go readFromChannel(c, time.After(time.Duration(2)*time.Second))
    time.Sleep(time.Duration(5) * time.Second)

    go func() {
        c <- 10

    }()
    wg.Wait()

}//Time out without deadlock

如何在golang程序中避免死锁？

5 个答案: