go: net: expose context Canceled/DeadlineExceeded error

I believe the point here is to permit using error.Is(err, context.Canceled) on the error returned by net.Dial to see if the Dial failed because the context was canceled. That doesn’t work today because of the mapErr function in net/net.go (https://golang.org/src/net/net.go?#L413) which changes context.Canceled to the net package internal error errCanceled. This was done when DialContext was introduced so that the net package operations would continue returning the errors that they always returned. That is because when DialContext was introduced, the existing DialTimeout function was changed to use a context. Changing the error from the internal errCanceled to context.Canceled would certainly break some tests and possible break some real code as well.

So this issue is basically: drop mapErr.

Do you mean a func net.IsCanceled(error) bool? That could work.

Not quite, I was suggesting to implement interface{ Is(error) bool } on the existing error so that errors.Is(err, context.SomeError) works. It looks like your proposed change will achieve the same effect though. 👍🏻

We know that people do string matching on error strings, and in the past we’ve decided to not change error strings because doing so broke too much existing code. Yes, matching on error strings is typically bad practice. Still, we want to be careful in this area.

This particular change may be perfectly fine. As I said, we don’t what existing code would break if we made the change. Maybe none.

Perhaps the error could remain the same but have an Is(target error) bool function added that returns true for the relevant context error.