concurrent_map.go

package main

import (
	"fmt"
	"sync"
	"time"
)

// DockerStats contains docker stats
type DockerStats struct {
	previousCPUValues map[string]CPUValues
	age               int
}

// CPUValues struct contains the last cpu-usage values in order to compute properly the current values.
// (see calculateCPUPercent() for more details)
type CPUValues struct {
	totCPU, systemCPU uint64
}

func (d DockerStats) getContainerStats(container string) {
	fmt.Println("Getting container stats")
	oldValue := d.previousCPUValues[container]
	time.Sleep(time.Duration(80) * time.Millisecond)
	cpuvalue := CPUValues{totCPU: 100 + oldValue.totCPU, systemCPU: 222 + oldValue.systemCPU}
	time.Sleep(time.Duration(80) * time.Millisecond)
	d.previousCPUValues[container] = cpuvalue
	time.Sleep(time.Duration(80) * time.Millisecond)
	d.age = 20
}

func main() {
	d := new(DockerStats)
	d.previousCPUValues = map[string]CPUValues{}

	cpuvalue := CPUValues{totCPU: 45, systemCPU: 60}
	d.previousCPUValues["foobar"] = cpuvalue

	d.age = 100
	var wg sync.WaitGroup
	wg.Add(100)
	for i := 0; i < 100; i++ {
		container := fmt.Sprintf("foobar : %d", i)
		d.previousCPUValues[container] = cpuvalue
		go func() {
			defer wg.Done()
			d.getContainerStats(container)
		}()
	}
	wg.Wait()

	fmt.Println("totCPU", d.previousCPUValues["foobar"].totCPU)
	fmt.Println("age", d.age)
}

It crashes with:

Getting container stats
Getting container stats
Getting container stats
Getting container stats
fatal error: concurrent map writes

goroutine 73 [running]:
runtime.throw(0x514040, 0x15)
        /data/go/src/runtime/panic.go:530 +0x90 fp=0xc820102e10 sp=0xc820102df8
runtime.mapassign1(0x4b9d40, 0xc82000e450, 0xc820102f48, 0xc820102f08)
        /data/go/src/runtime/hashmap.go:445 +0xb1 fp=0xc820102eb8 sp=0xc820102e10
main.DockerStats.getContainerStats(0xc82000e450, 0x64, 0x4ff0e0, 0x6)
        /home/hulk/snippets/golang/using_map.go:27 +0x200 fp=0xc820102f78 sp=0xc820102eb8
main.main.func1(0xc82000a480, 0xc82000a430)
        /home/hulk/snippets/golang/using_map.go:47 +0x69 fp=0xc820102fa0 sp=0xc820102f78
runtime.goexit()
        /data/go/src/runtime/asm_amd64.s:1998 +0x1 fp=0xc820102fa8 sp=0xc820102fa0
created by main.main
        /home/hulk/snippets/golang/using_map.go:48 +0x46d