fix: do not wrap io.EOF in errd.Wrap

[Yanhu007]

Fixes #561

Problem

The errd.Wrap helper wraps all non-nil errors using fmt.Errorf("%w"). When io.EOF is returned (e.g. from Conn.reader), it becomes "failed to get reader: EOF". Per Go convention, io.EOF must never be wrapped:

EOF is the error returned by Read when no more input is available.
(Read must return EOF itself, not an error wrapping EOF,
because callers will test for EOF using ==.)

This breaks callers like io.Copy and bufio.Scanner that test err == io.EOF.

Fix

Skip wrapping in errd.Wrap when the error is exactly io.EOF:

func Wrap(err *error, f string, v ...any) {
    if *err != nil && *err != io.EOF {
        *err = fmt.Errorf(f+": %w", append(v, *err)...)
    }
}

[mafredri]

Hey, thanks for the PR!

I'm not convinced we should do this in Wrap, but rather handle this explicitly in Read methods. I can't think of a reason we should unwrap e.g. writes hitting io.EOF.

[mafredri]

This seems backwards. Go convention does not require callers to do this. The convention asks implementers to return io.EOF due to backwards compatibility and the stdlib implementation being frozen.

Callers should absolutely use errors.Is, it's the modern way.