[bugfix] improved authenticate post inbox error handling (#2803)

* improved PostInboxScheme() error handling / logging in case of failed auth

* dumbass kim. returning err instead of errWithCode...

* add checks for the slightly changed error handling in tests, add notes in codebase about the odd way of working

internal/federation/federatingactor.go

@@ -80,8 +80,23 @@ func (f *federatingActor) PostInboxScheme(ctx context.Context, w http.ResponseWr
 	}
 
 	// Authenticate request by checking http signature.
+	//
+	// NOTE: the behaviour here is a little strange as we have
+	// the competing code styles of the go-fed interface expecting
+	// that any 'err' is fatal, but 'authenticated' bool is intended to
+	// be the main passer of whether failed auth occurred, but we in
+	// the gts codebase use errors to pass-back non-200 status codes,
+	// so we specifically have to check for already wrapped with code.
+	//
 	ctx, authenticated, err := f.sideEffectActor.AuthenticatePostInbox(ctx, w, r)
-	if err != nil {
+	if errors.As(err, new(gtserror.WithCode)) {
+		// If it was already wrapped with an
+		// HTTP code then don't bother rewrapping
+		// it, just return it as-is for caller to
+		// handle. AuthenticatePostInbox already
+		// calls WriteHeader() in some situations.
+		return false, err
+	} else if err != nil {
 		err := gtserror.Newf("error authenticating post inbox: %w", err)
 		return false, gtserror.NewErrorInternalError(err)
 	}