Yes, it makes sense that it would be impossible to really check at that
level. What surprised me was that this does not trigger vet:
for i := range slice {
go f(i)
_ = 1
}
Yet this does trigger vet:
for i := range slice {
_ = 1
go f(i)
}
Is there something special about the last statement that makes it easier to
check?
For what it's worth, this exact bug caused real user-impacting issues at my
company. The team responsible for the code was very surprised that vet did
not alert them to the issue before it made it into production.
On Thursday, August 10, 2017 at 4:27:15 PM UTC-4, Steven Blenkinsop wrote:
>
> If you have a loop like:
>
> for i := range slice {
> f(&i)
> }
>
> There's no way to know whether f launches a goroutine that captures the
> loop variable without also evaluating the body of f and potentially every
> function directly or indirectly called by f. If i escapes into a global
> variable, you also have to evaluate the entire program to see if a
> goroutine is ever launched which could access i by way of that
> variable. This is what they mean by whole program analysis.
>
> However, I feel like only catching a go or defer statement which is the
> last statement in the loop body is very far from a best effort. I also
> wonder if the analysis could take advantage of the existing escape analysis
> in the compiler to flag likely misuses of loop variables. There are
> certainly valid use cases for loop variables escaping (such as when this
> coincides with returning from the function or otherwise breaking the loop),
> so I'm not sure how easy it would be to discriminate, even if it's feasible
> to begin with.
>
> On Thu, Aug 10, 2017 at 1:18 PM <spe...@justin.tv <javascript:>> wrote:
>
>> I've noticed that cmd/vet's rangeloop check, which warns you about
>> binding to range loop variables in function literals which are launched
>> with 'go' or 'defer', is more limited than I thought:
>>
>>
>> https://github.com/golang/go/blob/d5ad7793d610bddfb3e7e09b8dafa0b0837f0cb2/src/cmd/vet/rangeloop.go#L6-L9
>>
>>
>> <https://github.com/golang/go/blob/d5ad7793d610bddfb3e7e09b8dafa0b0837f0cb2/src/cmd/vet/rangeloop.go#L6-L9>
>>
>>> This file contains the code to check range loop variables bound inside
>>> function literals that are deferred or launched in new goroutines. We only
>>> check instances where the defer or go statement is the last statement in
>>> the loop body, as otherwise we would need whole program analysis.
>>>
>>
>> I don't understand why whole program analysis is necessary here. Why
>> can't cmd/vet check each function literal declared within the loop's braces?
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "golang-nuts" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to golang-nuts...@googlegroups.com <javascript:>.
>> For more options, visit https://groups.google.com/d/optout.
>>
>
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
