data race example

console

$ go run -race racy.go 
==================
WARNING: DATA RACE
Write by goroutine 5:
  main.func·001()
      /tmp/racy.go:12 +0x158

Previous read by main goroutine:
  main.main()
      /tmp/racy.go:15 +0xdb

Goroutine 5 (running) created at:
  main.main()
      /tmp/racy.go:14 +0xcb
==================
==================
WARNING: DATA RACE
Read by main goroutine:
  errors.(*errorString).Error()
      /usr/lib/golang/src/errors/errors.go:19 +0x48
  fmt.(*pp).handleMethods()
      /usr/lib/golang/src/fmt/print.go:714 +0x534
  fmt.(*pp).printArg()
      /usr/lib/golang/src/fmt/print.go:794 +0x492
  fmt.(*pp).doPrint()
      /usr/lib/golang/src/fmt/print.go:1218 +0x3a7
  fmt.Fprintln()
      /usr/lib/golang/src/fmt/print.go:254 +0x7b
  fmt.Println()
      /usr/lib/golang/src/fmt/print.go:264 +0xab
  main.main()
      /tmp/racy.go:18 +0x233

Previous write by goroutine 5:
  fmt.Errorf()
      /usr/lib/golang/src/fmt/print.go:212 +0xcb
  main.func·001()
      /tmp/racy.go:12 +0x135

Goroutine 5 (running) created at:
  main.main()
      /tmp/racy.go:14 +0xcb
==================
error: watch error #4
Found 2 data race(s)
exit status 66

racy.go

package main

import (
	"fmt"
	"time"
)

func main() {
	var watchError error
	go func() {
		for i := 0; ; i = (i + 1) % 10 {
			watchError = fmt.Errorf("watch error #%v", i)
		}
	}()
	for watchError == nil {
		time.Sleep(200 * time.Millisecond)
	}
	fmt.Println("error:", watchError)
}