net: expose context Canceled/DeadlineExceeded error

[pascaldekloe]

Issue #28529 could be solved with the new error wrapping now.

What did you do?

Check for context.Canceled with errors.Is seems reasonable.

https://play.golang.org/p/-JTEZXGyfQ6

$ go version
go version go1.13.5 darwin/amd64

[dmitshur]

Thanks for making this issue.

Can you elaborate on what problem it would help solve to expose this information? What is it that cannot be done (or can be done, but inconveniently) before this is resolved, and how will it be done after?

/cc @mikioh @bradfitz @ianlancetaylor

Also /cc @bcmills who commented on the relevant issue #28529.

[pascaldekloe]

Generally speaking you want to continue the abort (by returning context.Canceled) if the context expired. Dial errors on the other hand must be reported [logged]. Maybe a retry or two?

[bcmills]

@pascaldekloe, that doesn't really answer @dmitshur's questions.

[pascaldekloe]

Yes it does @bcmills. You always block my issues with the WaitingForInfo label for some reason. Not helping at all.

[ianlancetaylor]

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.