Send on Closed Channel

Why This Happens

Closing a channel signals that no more values will be sent. If you send on a closed channel, Go panics. This usually happens when multiple goroutines send to the same channel and one closes it while others are still sending, or when you close a channel too early.

The Problem

func main() {
    ch := make(chan int)
    close(ch)
    ch <- 42 // panic: send on closed channel
}

The Fix

func main() {
    ch := make(chan int)
    go func() {
        ch <- 42
        close(ch) // close after done sending
    }()
    for v := range ch {
        fmt.Println(v)
    }
}

Step-by-Step Fix

1. 1

   Identify who closes the channel

   Find the close(ch) call and determine which goroutine calls it.

2. 2

   Ensure single closer

   Only one goroutine should close the channel, typically the sender.

3. 3

   Coordinate with sync

   Use sync.WaitGroup or sync.Once to ensure the channel is closed only after all senders are done.