Go: proposal: context: add Context.IsCanceled method

[ilyapashuk]

please add function like that to the context type:
func (c Context) IsCanceled() {
select {
case <- c.Done():
return true
default:
return false
}
}
or something like that to simplify context cancelation checking in channelless situations by something like that:
if ctx.IsCanceled() {
return ctx.Err()
}

[ianlancetaylor]

The proposed function is the same as ctx.Err() != nil. It doesn't seem to add much. In particular the case at the bottom would normally be written

    if err := ctx.Err(); err != nil {
        return err
    }

which seems preferable to me.

[rsc]

As Ian pointed out, ctx.Err is exactly this function (returning an error instead of a boolean).
In #19856, we made sure that this idiom is valid - Err is allowed before the Done channel is closed, and it must return nil.

[carnott-snap]

While you could implement a func that is pure syntax sugar, I agree the justification seems weak:

package context

func IsCanceled(ctx Context) bool { return ctx.Err() == Canceled }

[rsc]

Based on the discussion above, this seems like a likely decline.

[davecheney]

There’s also the issue of a logical race, the context may be canceled after the call to IsCancelled but before the code responding to that condition has run.

[rsc]

No change in consensus, so declined.

[rsc]

Forgot to close in July.