How to stop reading from channel on error?

php editor Zimo will introduce you how to stop reading from the channel when an error occurs. When performing a channel read operation, you sometimes encounter an error condition, such as the channel being closed or the read timeout. In order to ensure the stability and reliability of the program, we need to stop the read operation in time when these errors occur. This can be achieved by setting up an error handler or using an exception handling mechanism. After catching the error, the read operation will be terminated immediately and corresponding error handling will be performed. This can effectively avoid abnormal situations caused by errors in the program and improve the robustness and maintainability of the code.

Question content

I want to stop reading from the channel once an error is encountered. I think I need to use errroup.WithContext() but I can't quite understand how to integrate it.

This is the output:

Thread 2: STARTED
Thread 1: STARTED
Thread 0: STARTED
Thread 2: GOT=0
Thread 1: GOT=1
Thread 0: GOT=2
Thread 0: FAILED
Thread 1: GOT=3
Thread 2: GOT=4
Thread 2: GOT=5
Thread 1: GOT=6
Thread 1: GOT=7
Thread 2: GOT=8
panic: Thread 0: FAILED

package main

import (
    "fmt"
    "time"

    "golang.org/x/sync/errgroup"
)

func main() {
    const threads = 3

    ch := make(chan int, threads)

    eg := errgroup.Group{}

    for i := 0; i < threads; i++ {
        i := i
        eg.Go(func() error {
            fmt.Printf("Thread %d: STARTED\n", i)
            for n := range ch {
                fmt.Printf("Thread %d: GOT=%d\n", i, n)
                time.Sleep(time.Duration(1) * time.Second)
                // Simulate failed thread
                if n == 2 {
                    fmt.Printf("Thread %d: FAILED\n", i)
                    return fmt.Errorf("Thread %d: FAILED", i)
                }
            }
            return nil
        })
    }

    for i := 0; i < 9; i++ {
        ch <- i
    }

    close(ch)

    if err := eg.Wait(); err != nil {
        panic(err)
    }
}

................................................................. .............................................................

Solution

errroup.WithContext() is probably the best way (as you guessed). This answer is worth reading; it provides a comprehensive explanation (which I won't try to copy!).

However, since this may be difficult to understand, here is a solution using your code (playground):

package main

import (
    "context"
    "fmt"
    "time"

    "golang.org/x/sync/errgroup"
)

func main() {
    const threads = 3

    ch := make(chan int, threads)

    eg, ctx := errgroup.WithContext(context.Background())
    for i := 0; i < threads; i++ {
        i := i
        eg.Go(func() error {
            fmt.Printf("Thread %d: STARTED\n", i)
            for n := range ch {
                fmt.Printf("Thread %d: GOT=%d\n", i, n)
                time.Sleep(time.Duration(1) * time.Second)
                // Simulate failed thread
                if n == 2 {
                    fmt.Printf("Thread %d: FAILED\n", i)
                    return fmt.Errorf("Thread %d: FAILED", i)
                }
                // For the purpose of this example we will just check the context after
                // doing the work.
                if ctx.Err() != nil {
                    fmt.Printf("Thread %d: Quiting due to context\n", i)
                    return fmt.Errorf("Thread %d: Context cancelled: ", i)
                }
            }
            return nil
        })
    }

    // We want to stop sending new work if there is a failure (otherwise `ch <- i` will block permanently);
sendLoop:
    for i := 0; i < 9; i++ {
        select {
        case ch <- i:
        case <-ctx.Done():
            fmt.Printf("stopped\n")
            break sendLoop // Just exit the loop if something goes wrong (you may want to do something else here)
        }
    }
    close(ch) // Note: Important that this happens even if there is an error (otherwise `for n := range ch` may block, leaking goroutines)
    fmt.Printf("all sent\n")
    if err := eg.Wait(); err != nil {
        panic(err)
    }
}

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