go/neo/neonet: Dedicate an error type to indicate "protocol version mismatch"...

go/neo/neonet: Dedicate an error type to indicate "protocol version mismatch" as handshake failure cause

We will soon need to detect if a handshake failure was due to mismatch
of protocol encodings and that would require introduction of dedicated
error type for that cause. As a preparatory step first refactor "version
mismatch cause" to follow the same style for symmetry.

go/neo/neonet/newlink.go

-// Copyright (C) 2016-2021  Nexedi SA and Contributors.
+// Copyright (C) 2016-2023  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -60,6 +60,13 @@ type _HandshakeError struct {
 	Err        error
 }
 
+// _VersionMismatchError is reported as cause when handshake detects that peer
+// uses protocol version different than ours.
+type _VersionMismatchError struct {
+	LocalVer  uint32
+	RemoteVer uint32
+}
+
 func (e *_HandshakeError) Error() string {
 	role := ""
 	switch e.LocalRole {
@@ -73,6 +80,10 @@ func (e *_HandshakeError) Error() string {
 func (e *_HandshakeError) Cause()  error { return e.Err }
 func (e *_HandshakeError) Unwrap() error { return e.Err }
 
+func (e *_VersionMismatchError) Error() string {
+	return fmt.Sprintf("protocol version mismatch: peer = %08x  ; our side = %08x", e.RemoteVer, e.LocalVer)
+}
+
 // handshakeClient implements client-side NEO protocol handshake just after raw
 // connection between 2 nodes was established.
 //
@@ -130,7 +141,7 @@ func _handshakeClient(ctx context.Context, conn net.Conn, version uint32, encPre
 
 		// verify version
 		if peerVer != version {
-			return fmt.Errorf("protocol version mismatch: peer = %08x  ; our side = %08x", peerVer, version)
+			return &_VersionMismatchError{version, peerVer}
 		}
 
 		return nil
@@ -174,7 +185,7 @@ func _handshakeServer(ctx context.Context, conn net.Conn, version uint32) (enc p
 
 		// verify version
 		if peerVer != version {
-			return fmt.Errorf("protocol version mismatch: peer = %08x  ; our side = %08x", peerVer, version)
+			return &_VersionMismatchError{version, peerVer}
 		}
 
 		return nil

go/neo/neonet/newlink_test.go

-// Copyright (C) 2016-2021  Nexedi SA and Contributors.
+// Copyright (C) 2016-2023  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -24,6 +24,7 @@ import (
 	"errors"
 	"io"
 	"net"
+	"reflect"
 	"testing"
 
 	"lab.nexedi.com/kirr/go123/exc"
@@ -81,14 +82,32 @@ func _TestHandshake(t *T) {
 	xclose(p1)
 	xclose(p2)
 
-	err1Want := "pipe - pipe: handshake (client): protocol version mismatch: peer = 00000002  ; our side = 00000001"
-	err2Want := "pipe - pipe: handshake (server): protocol version mismatch: peer = 00000001  ; our side = 00000002"
+	e1ok := &_HandshakeError{
+		LocalRole:   _LinkClient,
+		LocalAddr:   p1.LocalAddr(),
+		RemoteAddr:  p1.RemoteAddr(),
+		Err:         &_VersionMismatchError{
+			LocalVer:  1,
+			RemoteVer: 2,
+		},
+	}
+	e2ok := &_HandshakeError{
+		LocalRole:   _LinkServer,
+		LocalAddr:   p2.LocalAddr(),
+		RemoteAddr:  p2.RemoteAddr(),
+		Err:         &_VersionMismatchError{
+			LocalVer:  2,
+			RemoteVer: 1,
+		},
+	}
 
-	if !(err1 != nil && err1.Error() == err1Want) {
-		t.Errorf("handshake ver mismatch: p1: unexpected error:\nhave: %v\nwant: %v", err1, err1Want)
+	if !reflect.DeepEqual(err1, e1ok) {
+		t.Errorf("handshake ver mismatch: p1: unexpected error:\nhave: %#v\n     %q\nwant: %#v\n     %q",
+			 err1, estr(err1), e1ok, e1ok.Error())
 	}
-	if !(err2 != nil && err2.Error() == err2Want) {
-		t.Errorf("handshake ver mismatch: p2: unexpected error:\nhave: %v\nwant: %v", err2, err2Want)
+	if !reflect.DeepEqual(err2, e2ok) {
+		t.Errorf("handshake ver mismatch: p2: unexpected error:\nhave: %#v\n     %q\nwant: %#v\n     %q",
+			 err2, estr(err2), e2ok, e2ok.Error())
 	}
 
 	// tx & rx problem (client)
@@ -162,3 +181,12 @@ func _TestHandshake(t *T) {
 		t.Errorf("handshake (server): cancel: unexpected error: %#v", err)
 	}
 }
+
+// estr returns err.Error() or "<nil>".
+func estr(err error) string {
+	if err == nil {
+		return "<nil>"
+	} else {
+		return err.Error()
+	}
+}