Goroutine lifecycle

Learn the running, waiting and dead states of a goroutine.

Goroutine lifecycle

A goroutine goes through these states: runnable (in a queue, waiting to run) → running (running on an M+P) → waiting (waiting on a channel, lock, timer or network) → runnable again → finally dead.

ch := make(chan int)
go func() { ch <- 1 }()  // runnable → running
v := <-ch                 // main waiting, then runnable

A waiting goroutine releases the P; the P immediately runs another G.

Leaked goroutines

A goroutine waiting for an event that will never come stays waiting forever and never frees its memory; this is a goroutine leak. A typical cause is writing to a channel whose result nobody reads:

func ilk(urls []string) string {
    ch := make(chan string)        // unbuffered!
    for _, u := range urls {
        go func() { ch <- getir(u) }()
    }
    return <-ch                     // the others wait forever
}

Fix: make(chan string, len(urls)) or cancellation with a context.

Exercise First result without leaks

Write ilk(n int) int: start n goroutines, each sending its own index to a channel, and return the first one that arrives. Prevent the leak by creating the channel with capacity n. Print ilk(3) >= 0.

Show example solution

package main

import "fmt"

func ilk(n int) int {
    ch := make(chan int, n)
    for i := 0; i < n; i++ {
        go func(i int) { ch <- i }(i)
    }
    return <-ch
}

func main() {
    fmt.Println(ilk(3) >= 0)
}

Test yourself

A goroutine sending <code>ch <- 1</code> with no reader is in which state?
- waiting (on the channel)
- running
- dead
- runnable
Show answer

waiting (on the channel) — On an unbuffered channel the send puts the goroutine into waiting until a receiver arrives, releasing the P.

Prevent the leak

ch := make(chan string, ____)

Show answer

ch := make(chan string, len(urls))

State and cause

runnable

Waiting its turn in a queue
running

Running on a P
waiting

Waiting on channel/lock/network
dead

The function returned

Next lesson: Work stealing

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