Why Does a Go `select` Statement Need `fmt.Print()` to Terminate in a Specific Goroutine?

goroute with select Does Not Terminate Without fmt.Print()

In the Go Tour exercise #71, a goroute with a select statement executes continuously unless a fmt.Print() statement is added in the default case. This behavior arises due to select's altered operation when a default statement is present.

Without a default, select blocks until a message is received on any of the channels being monitored. By adding a default, select executes the default branch whenever none of the channels have messages.

In the original code, the default statement creates an infinite loop:

for {
    select {
        case todo := <-toDoList:
            ...
        case <-doneCrawling:
            ...
        default:
            if os.Args[1] == "ok" {
                fmt.Print("")
            }
            if crawling == 0 {
                goto END
            }
        }
    }

Without the fmt.Print() within the default, the scheduler cannot schedule other goroutines, resulting in a deadlock. Adding fmt.Print() allows the scheduler to continue its normal function, enabling the completion of the goroutine.

Modifying the code to use a non-blocking select resolves the issue:

for {
    select {
        case todo := <-toDoList:
            ...
        case <-doneCrawling:
            ...
    }
    if crawling == 0 {
        break
    }
}

Alternatively, the original code can be made to work by setting GOMAXPROCS=2, indicating an issue with an overloaded scheduler.

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