What happens when an anonymous function waits on channel forever in golang?

Question

I have a function creating a channel with no buffer. This function goes on to create several other concurrent anonymous functions writing to said channel. The function then goes on to wait for input on the channel and then returns the value.

See example below

package main

import (
    "time"
    "fmt"
    "strconv"
    "math/rand"
)

func main() {
    for{
        text := foo()
        fmt.Println(text)
        time.Sleep(time.Second)
    }

}

func foo() string {    
    ch := make(chan string)
    for i := 0; i < 10; i++ {
        // Create some threads
        go func(i int) {
            time.Sleep(time.Duration(rand.Intn(1000))*time.Millisecond)
            ch <- strconv.Itoa(i)
        }(i)
    }
    return <- ch
}

What happens with the anonymous functions that are still waiting on the channel, even though the entire function (foo in example) is "dead"?

Will they be collected as garbage, or will they forever wander the limbo of my computers memory (or until I kill the main thread) eating away at it in a desperate attempt to send their last message before passing on?

Answer 1

You have a goroutine leak.

package main

import (
    "fmt"
    "math/rand"
    "runtime"
    "strconv"
    "time"
)

func main() {
    for {
        text := foo()
        fmt.Println(text, "NumGoroutine", runtime.NumGoroutine())
        time.Sleep(time.Second)
    }
}

func foo() string {
    ch := make(chan string)
    for i := 0; i < 10; i++ {
        // Create some threads
        go func(i int) {
            time.Sleep(time.Duration(rand.Intn(1000)) * time.Millisecond)
            ch <- strconv.Itoa(i)
        }(i)
    }
    return <-ch
}

Output:

$ go run leak.go
4 NumGoroutine 10
4 NumGoroutine 20
0 NumGoroutine 28
8 NumGoroutine 37
2 NumGoroutine 46
8 NumGoroutine 55
2 NumGoroutine 64
3 NumGoroutine 73
8 NumGoroutine 82
1 NumGoroutine 91
4 NumGoroutine 100
<<--SNIP-->>
4 NumGoroutine 4006
7 NumGoroutine 4015
6 NumGoroutine 4024
9 NumGoroutine 4033
9 NumGoroutine 4042
9 NumGoroutine 4051
1 NumGoroutine 4060
0 NumGoroutine 4069
4 NumGoroutine 4078
0 NumGoroutine 4087
6 NumGoroutine 4096
^Csignal: interrupt
$

Like a memory leak, a goroutine leak is bad.

For some background, see Go: proposal: runtime: garbage collect goroutines blocked forever #19702: Closed.