buffered channel not blocking for write when reached its capacity-CodePudding

https://play.golang.org/p/5A-dnZVy2aA

func closeChannel(stream chan int) {
    fmt.Println("closing the channel")
    close(stream)
}

func main() {
    chanOwner := func() <-chan int {
        resultStream := make(chan int, 5)
        go func() {
            defer closeChannel(resultStream)
            for i := 0; i <= 5; i   {
                resultStream <- i
            }
        }()
        return resultStream
    }
    resultStream := chanOwner()
    for result := range resultStream { //this blocks until the channel is closed
        fmt.Printf("Received: %d\n", result)
    }
    fmt.Println("done receiving")
}

Output of program

closing the channel
Received: 0
Received: 1
Received: 2
Received: 3
Received: 4
Received: 5
done receiving

why is the closing the channel statement printed before any Received in the above program. since the channel is buffered with a capacity of 5, and we are inserting 6 elements into it, I was expecting it to block at the resultStream <- i before a value was read and cleared up space in the buffer.

CodePudding user response：

The generator goroutine fills the channel to its capacity and blocks. The receiver for loops receives the first item from the channel, which enables the generator goroutine again. The generator goroutine runs to completion before the receiver for-loop can print the Received message, printing the closing the channel message. Then the receiver loop receives all remaining messages.