ISSUE #627: convert typed-nils to untyped-nils on error

client.go

@@ -892,6 +892,19 @@ func (cl *Client) hasExtension(ext *sshfx.ExtensionPair) bool {
 	return cl.exts[ext.Name] == ext.Data
 }
 
+// StatVFS retrieves VFS statistics from a remote host.
+//
+// It implements the [email protected] SSH_FXP_EXTENDED feature from
+// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL
+func (cl *Client) StatVFS(path string) (*openssh.StatVFSExtendedReplyPacket, error) {
+	resp, err := getPacket[*openssh.StatVFSExtendedReplyPacket](context.Background(), nil, cl,
+		&openssh.StatVFSExtendedPacket{
+			Path: path,
+		},
+	)
+	return valOrPathError("statvfs", path, resp, err)
+}
+
 // Link creates newname as a hard link to oldname file.
 //
 // If the server did not announce support for the "[email protected]" extension,

localfs/localfs_integration_test.go

@@ -751,6 +751,32 @@ func TestReadFrom(t *testing.T) {
 	}
 }
 
+func TestStatVFS(t *testing.T) {
+	if !*testServerImpl {
+		t.Skip("not testing against localfs server implementation")
+	}
+
+	if _, ok := any(handler).(sftp.StatVFSServerHandler); !ok {
+		t.Skip("handler does not implement statvfs")
+	}
+
+	dir := t.TempDir()
+
+	targetNotExist := filepath.Join(dir, "statvfs-does-not-exist")
+
+	_, err := cl.StatVFS(toRemotePath(targetNotExist))
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("unexpected error, got %v, should be fs.ErrNotFound", err)
+	}
+
+	resp, err := cl.StatVFS(toRemotePath(dir))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("%+v", resp)
+}
+
 var benchBuf []byte
 
 func benchHelperWriteTo(b *testing.B, length int) {

server.go

@@ -344,7 +344,12 @@ func Hijack[REQ sshfx.Packet](srv *Server, fn func(context.Context, REQ) error)
 // This is really only useful for supporting newer versions of the SFTP standard.
 func HijackWithResponse[REQ, RESP sshfx.Packet](srv *Server, fn func(context.Context, REQ) (RESP, error)) error {
 	wrap := wrapHandler(func(ctx context.Context, req sshfx.Packet) (sshfx.Packet, error) {
-		return fn(ctx, req.(REQ))
+		resp, err := fn(ctx, req.(REQ))
+		if err != nil {
+			// We have to convert maybe typed-zero to untyped-nil.
+			return nil, err
+		}
+		return resp, nil
 	})
 
 	var pkt REQ
@@ -515,6 +520,7 @@ func (srv *Server) handle(req sshfx.Packet, hint []byte, maxDataLen uint32) (ssh
 
 	if len(srv.hijacks) > 0 {
 		if fn := srv.hijacks[req.Type()]; fn != nil {
+			// Hijack takes care of wrapping the getter into an untyped-nil on error.
 			return get(srv, req, fn)
 		}
 	}
@@ -595,7 +601,13 @@ func (srv *Server) handle(req sshfx.Packet, hint []byte, maxDataLen uint32) (ssh
 
 		case *openssh.StatVFSExtendedPacket:
 			if statvfser, ok := srv.Handler.(StatVFSServerHandler); ok {
-				return get(srv, req, statvfser.StatVFS)
+				resp, err := get(srv, req, statvfser.StatVFS)
+				if err != nil {
+					// We have to convert typed-nil to untyped-nil.
+					return nil, err
+				}
+
+				return resp, nil
 			}
 
 		case interface{ GetHandle() string }:
@@ -610,15 +622,27 @@ func (srv *Server) handle(req sshfx.Packet, hint []byte, maxDataLen uint32) (ssh
 
 			case *openssh.FStatVFSExtendedPacket:
 				if statvfser, ok := file.(StatVFSFileHandler); ok {
-					return statvfser.StatVFS()
+					resp, err := statvfser.StatVFS()
+					if err != nil {
+						// We have to convert typed-nil to untyped-nil.
+						return nil, err
+					}
+
+					return resp, nil
 				}
 
 				if statvfser, ok := srv.Handler.(StatVFSServerHandler); ok {
 					req := &openssh.StatVFSExtendedPacket{
 						Path: file.Name(),
 					}
 
-					return get(srv, req, statvfser.StatVFS)
+					resp, err := get(srv, req, statvfser.StatVFS)
+					if err != nil {
+						// We have to convert typed-nil to untyped-nil.
+						return nil, err
+					}
+
+					return resp, nil
 				}
 			}
 		}
@@ -701,7 +725,7 @@ func (srv *Server) handle(req sshfx.Packet, hint []byte, maxDataLen uint32) (ssh
 			}
 
 			hint = slices.Grow(hint[:0], int(req.Length))[:req.Length]
-			 
+
 			n, err := file.ReadAt(hint, int64(req.Offset))
 			if err != nil {
 				// We cannot return results AND a status like SSH_FX_EOF,
@@ -729,7 +753,8 @@ func (srv *Server) handle(req sshfx.Packet, hint []byte, maxDataLen uint32) (ssh
 				return nil, io.ErrShortWrite
 			}
 
-			return nil, nil
+			// explicitly return statusOK here, rather than both nil.
+			return statusOK, nil
 
 		case *sshfx.FStatPacket:
 			attrs, err := file.Stat()