common/errors: implement Unwrap on *Error for errors.Is/As support

[bindreams]

Add Unwrap() to *Error so errors.Is/As/Unwrap traverse the inner chain. Until now *Error exposed its wrapped error only through Inner()/Cause(), so standard-library unwrapping stopped at the first *Error.

A low-level error wrapped in *Error layers can now be recovered directly:

var errno syscall.Errno
errors.As(listenErr, &errno) // now found through the *Error chain

Strictly additive: Unwrap returns the existing inner field (nil when unset), so it can only surface matches that were previously unreachable, never change one.

[bindreams]

Let me take this moment to thank you for the good work on v2ray! Internet freedom, one commit at a time :)

[xiaokangwang]

I think it is okay to add this Unwrap(), but could you please explain what user observable change it makes?

[bindreams]

Hi @xiaokangwang sorry for the late reply.

The only observable change is for the developers inspecting v2ray errors. Unwrap() is a stdlib contract (see https://pkg.go.dev/errors) and it allows me as a consumer of v2ray to inspect the error using standard Go methods errors.Is/errors.As:

conn, err := core.Dial(ctx, instance, dest)
    if err != nil {
        // An error — abort, or retry? Let me cast to Errno and check:
        var errno syscall.Errno
        if errors.As(err, &errno) && (errno == syscall.ECONNREFUSED || errno == syscall.ETIMEDOUT) {
            // back off and retry
        }
        return err // abort
  }

See also the last test from the ones I've added.