main function not waiting for channel read before exiting?

Issue

Consider the following attempt to implement Dining Philosophers with Go routines and channels.

package main

import "fmt"

func philos(id int, left, right, plate chan bool) {
    fmt.Printf("Philosopher # %d wants to eat\n", id) 
    <-left
    <-right
    plate <- true
    left <- true
    right <- true
    fmt.Printf("Philosopher # %d finished eating\n", id) 
}

func main() {
    const numPhilos = 5 
    var forks [numPhilos]chan bool
    for i := 0; i < numPhilos; i++ {
        forks[i] = make(chan bool, 1)
        forks[i] <- true
    }   
    plates := make(chan bool)
    for i := 0; i < numPhilos; i++ {
        go philos(i, forks[(i-1+numPhilos)%numPhilos], forks[(i+numPhilos)%numPhilos], plates)
    }   
    for i := 0; i < numPhilos; i++ {
        <-plates
    }   
}

Sometimes this works as expected, i.e. all philosophers eat, e.g.:

Philosopher # 4 wants to eat
Philosopher # 3 wants to eat
Philosopher # 2 wants to eat
Philosopher # 1 wants to eat
Philosopher # 4 finished eating
Philosopher # 3 finished eating
Philosopher # 2 finished eating
Philosopher # 1 finished eating
Philosopher # 0 wants to eat
Philosopher # 0 finished eating

However, sometimes, one (or more) philosophers are missed (e.g. Philosopher #0, did not eat in the following case):

Philosopher # 4 wants to eat
Philosopher # 1 wants to eat
Philosopher # 3 wants to eat
Philosopher # 2 wants to eat
Philosopher # 4 finished eating
Philosopher # 0 wants to eat
Philosopher # 2 finished eating
Philosopher # 1 finished eating
Philosopher # 3 finished eating

Question is: why does this happen?

What I already know:

  1. The program will exit if the main go routine finished (even if some other routines are still running).

  2. A go routine will block if it tries to read from a channel, and that channel is empty (i.e. no-one wrote to it previously).

Now, main tries to read 5 times from the channel plates, therefore it should not terminate until the philos routine has run five times. But it seems it somehow still manages to terminate before doing that. Am I missing something? (It seems that plates was read only 4 times.)

EDIT: Ok, after thinking about it a bit more, I came to the conclusion, that maybe the philos routine does always run 5 times, however, it can be interrupted before having the time to print that the philosopher ate. Indeed, if I change the order as follows, it seems to be working always:

func philos(id int, left, right, plate chan bool) {
    fmt.Printf("Philosopher # %d wants to eat\n", id) 
    <-left
    <-right
    left <- true
    right <- true
    fmt.Printf("Philosopher # %d finished eating\n", id) 
    plate <- true
}

Still, it would be great if someone could validate this explanation 🙂

Solution

What you see in stdout isn’t the same as what’s happening. Sometimes, main receives from plates and then returns before the print statement happens. So:

plate <- true
left <- true    // On this line or on
right <- true   // this line, main receives from plate and then returns before
fmt.Printf("Philosopher # %d finished eating\n", id) // this line executes

Because concurrency isn’t deterministic, this doesn’t happen every time. Sometimes the print happens before main returns, sometimes it doesn’t. That doesn’t mean the channel read isn’t happening.

Answered By – Adrian

Answer Checked By – Terry (GoLangFix Volunteer)

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