问题描述
我在使用 -race
标志进行测试时发现了数据竞争。更新结构体并从结构体方法读取值时发生数据竞争。
后来我发现将方法从非指针接收器改为指针接收器可以解决数据竞争。但我不明白原因。谁能解释一下原因?
package main
import (
"fmt"
"testing"
)
type TestStruct struct {
display bool
OtherValue int
}
func (t TestStruct) display() bool {
return t.display
}
func (t *TestStruct) displayP() bool {
return t.display
}
func TestNonPointerRecevier(t *testing.T) {
v := &TestStruct{
display: true,}
go func() {
v.OtherValue = 1
}()
go func() {
fmt.Println(v.display())
}()
}
func TestPointerRecevier(t *testing.T) {
v := &TestStruct{
display: true,}
go func() {
v.OtherValue = 1
}()
go func() {
fmt.Println(v.displayP())
}()
}
go test -race -run ^TestPointerRecevier$
true
PASS
ok _/Users/xxxxx/projects/golang/datarace 0.254s
go test -race -run ^TestNonPointerRecevier$
==================
WARNING: DATA RACE
Read at 0x00c00001c2c8 by goroutine 9:
_/Users/xxxxx/projects/golang/datarace.TestNonPointerRecevier.func2()
/Users/xxxxx/projects/golang/datarace/main_test.go:30 +0x47
PrevIoUs write at 0x00c00001c2c8 by goroutine 8:
_/Users/xxxxx/projects/golang/datarace.TestNonPointerRecevier.func1()
/Users/xxxxx/projects/golang/datarace/main_test.go:27 +0x3e
Goroutine 9 (running) created at:
_/Users/xxxxx/projects/golang/datarace.TestNonPointerRecevier()
/Users/xxxxx/projects/golang/datarace/main_test.go:29 +0xba
testing.tRunner()
/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1123 +0x202
Goroutine 8 (finished) created at:
_/Users/xxxxx/projects/golang/datarace.TestNonPointerRecevier()
/Users/xxxxx/projects/golang/datarace/main_test.go:26 +0x98
testing.tRunner()
/usr/local/Cellar/go/1.15.6/libexec/src/testing/testing.go:1123 +0x202
==================
true
FAIL
exit status 1
FAIL _/Users/xxxxx/projects/golang/datarace 0.103s
解决方法
当方法的接收者值是一个结构体(而不是指向结构体的指针)时,完整的结构体被复制以按值传递给该方法。
因此调用 v.Display()
会隐式读取字段 OtherValue
(在制作结构副本时),因此存在竞争条件。
另一方面,当使用指针时,只会复制指针,并且同时访问 v.display
和 v.OtherValue
不会触发竞争条件。
import (
"fmt"
"testing"
)
type TestStruct struct {
display bool
OtherValue int
}
func Display(t TestStruct) bool { // equal func (t TestStruct) Display() bool
return t.display
}
func DisplayP(t *TestStruct) bool { // equal func (t *TestStruct) DisplayP() bool
return t.display
}
func TestNonPointerRecevier(t *testing.T) {
v := &TestStruct{
display: true,}
go func() {
v.OtherValue = 1 // write
}()
go func() {
fmt.Println(Display(*v)) // *v read value
}()
}
func TestPointerRecevier(t *testing.T) {
v := &TestStruct{
display: true,}
go func() {
v.OtherValue = 1 // write
}()
go func() {
fmt.Println(DisplayP(v)) // un read,just pass parameter
}()
}
关注数据争用错误信息,调用read
方法时发生了Display
,go会将v(type *TestStruct
)转换为(type TestStruct
)。