Testing Concurrent Programs

In this lesson, explore testing concurrent programs through one small Go program, and connect its output to the language rules behind it.

Concurrent tests should verify outcomes and shutdown behavior rather than assume workers run in a fixed order. Use synchronization, cancellation and time bounds to prevent tests from hanging indefinitely.

Run go test -race as an additional signal, and avoid time.Sleep as a substitute for waiting on an event.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

concurrent_test.go

package main
import (
    "reflect"
    "testing"
)
func TestConcurrentSquares(t *testing.T) {
    got := concurrentSquares([]int{2,3,4})
    if !reflect.DeepEqual(got, []int{4,9,16}) { t.Fatalf("got %v",got) }
}

Run go test -race ./....

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"sync"
)

func concurrentSquares(nums []int) []int {
	result := make([]int, len(nums))
	var wg sync.WaitGroup
	for i, n := range nums {
		wg.Add(1)
		go func(index, value int) { defer wg.Done(); result[index] = value * value }(i, n)
	}
	wg.Wait()
	return result
}
func main() { fmt.Println(concurrentSquares([]int{2, 3, 4})) }

Output

[4 9 16]

Several goroutines write different result indexes. Wait ensures a completed result before returning, so [4 9 16] is deterministic.

• A race detector reports observed races, not logical deadlocks that never run.
• Tests should not assume worker scheduling or completion order.

Add a TestConcurrentSquares that compares the returned slice and run it with -race.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Testing Concurrent Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.