用于println和fmt.Println的竞争检测器中的输出不同

Question

I have a different output for println and fmt.Println in race detector which I couldn't explain. I expected both to be race, or at least both to be no race.

package main

var a int

func f() {
    a = 1
}

func main() {
    go f()
    println(a)
}

And, it finds race condition as expected.

0
==================
WARNING: DATA RACE
Write by goroutine 5:
  main.f()
/home/felmas/test.go:6 +0x30

Previous read by main goroutine:
  main.main()
/home/felmas/test.go:11 +0x4d

Goroutine 5 (running) created at:
  main.main()
/home/felmas/test.go:10 +0x38
==================
Found 1 data race(s)

However, this one runs without any detected race.

package main

import "fmt"

var a int

func f() {
    a = 1
}

func main() {
    go f()
    fmt.Println(a)
}

To my knowledge, no race is detected doesn't mean there is no race so is this one of these deficiencies or is there a deeper explanation since println is builtin and quite special?

Answer 1

The race detector is a dynamic testing tool and no static analysis. In order to get reliable results from the race detector, you should strife for a high test coverage of your program, preferable by writing lots of benchmarks using multiple processes (by setting GOMAXPROCS > 1, GOMAXPROCS=NumCPU is the default for Go 1.5) and use a continuous integration tool that executes those tests regularly.

The race detector does not report any false positives so you should take every output serious. On the other hand it might not detect every race on every run, depending on the order goroutines and processes are scheduled.

In your example, wrapping everything in a tight loop and re-executing the tests reports the race correctly in both cases.