encoding/base64: The decoder returned by NewDecoder sometimes drops errors

[akalin]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.8.2

What operating system and processor architecture are you using (go env)?

darwin amd64

What did you do?

When passing an io.Reader to base64.NewDecoder, if that reader emits valid base64 input and an error, I would expect that error to be propagated. Instead, it is dropped.

Example: https://play.golang.org/p/-sJt1NN9G1

In this case, it's because d.err is set to d.readErr only if the (4-byte) buffer isn't filled by the Read call.

This is split off from #20044 .

[akalin]

Similar to #20475, I think this isn't actually a bug, too. Updated test: https://play.golang.org/p/jdYK0CTsjZ