Description
This is an offshoot of #74327.
The following program contains a data race as reported by the race detector:
func Test(t *testing.T) {
synctest.Test(t, func(t *testing.T) {
var v int
go func() {
time.Sleep(1)
v = 0 // data race: write
}()
_ = v // data race: read
time.Sleep(2)
})
}It should be obvious that the access to v is racy when executed outside of a synctest bubble: The time.Sleep(1) call does not establish a happens-before relationship between the read and the write. Within the fake time environment of a synctest bubble, the time.Sleep(1) will not return until after the read has occurred. However, we deliberately do not tell the race detector to establish a happens-before relationship here, to allow it to detect racing operations in a system under test that might be obscured by the use of fake time.
The synctest.Wait function establishes a happens-before relationship between all activity in a bubble and the Wait call returning. Therefore, this program does not contain a data race:
func Test(t *testing.T) {
synctest.Test(t, func(t *testing.T) {
var v int
go func() {
time.Sleep(1)
v = 0
}()
time.Sleep(2)
synctest.Wait() // v = 0 happens-before Wait returns
_ = v
})
}However, perhaps surprisingly, the race detector does report a race in this program:
func Test(t *testing.T) {
synctest.Test(t, func(t *testing.T) {
var v int
go func() {
time.Sleep(1)
v = 0
}()
synctest.Wait()
_ = v
synctest.Wait()
time.Sleep(2)
})
}The user's intent in this test seems clear, but nothing in this program establishes a happens-before relationship between the two accesses to v.
Perhaps there's some tweak we can make to the happens-before relationships established by Wait that would allow us to avoid reporting a data race in this case. I'm not sure exactly what that would be.
We do want to take care not to make a change that masks data races in goroutines which do not call Wait, such as in this example:
func Test(t *testing.T) {
synctest.Test(t, func(t *testing.T) {
var v int
go func() {
time.Sleep(1)
v = 0
}()
go func() {
time.Sleep(2)
_ = v
}()
// None of these Waits should establish a happens-before relationship
// between the two goroutines above.
synctest.Wait()
time.Sleep(1)
synctest.Wait()
time.Sleep(1)
})
}