How to Avoid Data Races When Launching Goroutines in Go?

Data Race in Concurrent Go Routine

This code demonstrates a data race issue in a concurrent Go routine:

package main

import (
    "fmt"
    "time"
)

type field struct {
    name string
}

func (p *field) print() {
    fmt.Println(p.name)
}

func main() {
    data := []field{{"one"}, {"two"}, {"three"}}
    for _, v := range data {
        go v.print()
    }
    <-time.After(1 * time.Second)
}

Issue:

The code prints "three" three times instead of printing "one", "two", and "three" in any order. This is because there is a data race.

Explanation:

Implicitly, the code takes the address of the variable v when evaluating arguments to the goroutine function. The goroutine function v.print() is equivalent to (&v).print(). The loop changes the value of v, and when the goroutines execute, they happen to have the last value of the loop ("three").

Fixes:

There are several ways to fix this data race:

• Create a new variable within the loop:

for _, v := range data {
    v := v        // short variable declaration of new variable `v`.
    go v.print()
}

• Use a slice of pointers:

data := []*field{{"one"}, {"two"}, {"three"}} // note '*'
for _, v := range data {
    go v.print()
}

• Use the address of the slice element:

data := []field{{"one"}, {"two"}, {"three"}} // note '*'
for i := range data {
    v := &data[i]
    go v.print()
}

• Pass the range variable as an argument to an anonymous function:

for _, v := range data {
  go func(v field) {
    v.print() // take address of argument v, not range variable v.
  }(v)
}

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