Question

I’ve a long running server written in Go. Main fires off several goroutines where the logic of the program executes. After that main does nothing useful. Once main exits, the program will quit. The method I am using right now to keep the program running is just a simple call to fmt.Scanln(). I’d like to know how others keep main from exiting. Below is a basic example. What ideas or best practices could be used here?

I considered creating a channel and delaying exit of main by receiving on said channel, but I think that could be problematic if all my goroutines become inactive at some point.

Side note: In my server (not the example), the program isn’t actually running connected to a shell, so it doesn’t really make sense to interact with the console anyway. For now it works, but I’m looking for the “correct” way, assuming there is one.

package main

import (
    "fmt"
    "time"
)

func main() {
    go forever()
    //Keep this goroutine from exiting
    //so that the program doesn't end.
    //This is the focus of my question.
    fmt.Scanln()
}

func forever() {
    for ; ; {
    //An example goroutine that might run
    //indefinitely. In actual implementation
    //it might block on a chanel receive instead
    //of time.Sleep for example.
        fmt.Printf("%v+\n", time.Now())
        time.Sleep(time.Second)
    }
}

Answer 1

The current design of Go’s runtime assumes that the programmer is responsible for detecting when to terminate a goroutine and when to terminate the program. The programmer needs to compute the termination condition for goroutines and also for the entire program. A program can be terminated in a normal way by calling os.Exit or by returning from the main() function.

Creating a channel and delaying exit of main() by immediately receiving on said channel is a valid approach of preventing main from exiting. But it does not solve the problem of detecting when to terminate the program.

If the number of goroutines cannot be computed before the main() function enters the wait-for-all-goroutines-to-terminate loop, you need to be sending deltas so that main function can keep track of how many goroutines are in flight:

// Receives the change in the number of goroutines
var goroutineDelta = make(chan int)

func main() {
    go forever()

    numGoroutines := 0
    for diff := range goroutineDelta {
        numGoroutines += diff
        if numGoroutines == 0 { os.Exit(0) }
    }
}

// Conceptual code
func forever() {
    for {
        if needToCreateANewGoroutine {
            // Make sure to do this before "go f()", not within f()
            goroutineDelta <- +1

            go f()
        }
    }
}

func f() {
    // When the termination condition for this goroutine is detected, do:
    goroutineDelta <- -1
}

An alternative approach is to replace the channel with sync.WaitGroup. A drawback of this approach is that wg.Add(int) needs to be called before calling wg.Wait(), so it is necessary to create at least 1 goroutine in main() while subsequent goroutines can be created in any part of the program:

var wg sync.WaitGroup

func main() {
    // Create at least 1 goroutine
    wg.Add(1)
    go f()

    go forever()
    wg.Wait()
}

// Conceptual code
func forever() {
    for {
        if needToCreateANewGoroutine {
            wg.Add(1)
            go f()
        }
    }
}

func f() {
    // When the termination condition for this goroutine is detected, do:
    wg.Done()
}