Fix switching CommandTag back to string

copy_from.go

@@ -287,11 +287,11 @@ func (c *Conn) CopyFrom(tableName Identifier, columnNames []string, rowSrc CopyF
 // CopyFromReader uses the PostgreSQL textual format of the copy protocol
 func (c *Conn) CopyFromReader(r io.Reader, sql string) (pgconn.CommandTag, error) {
 	if err := c.sendSimpleQuery(sql); err != nil {
-		return nil, err
+		return "", err
 	}
 
 	if err := c.readUntilCopyInResponse(); err != nil {
-		return nil, err
+		return "", err
 	}
 	buf := c.wbuf
 
@@ -306,7 +306,7 @@ func (c *Conn) CopyFromReader(r io.Reader, sql string) (pgconn.CommandTag, error
 		pgio.SetInt32(buf[sp:], int32(n+4))
 
 		if _, err := c.pgConn.Conn().Write(buf); err != nil {
-			return nil, err
+			return "", err
 		}
 	}
 
@@ -315,25 +315,25 @@ func (c *Conn) CopyFromReader(r io.Reader, sql string) (pgconn.CommandTag, error
 	buf = pgio.AppendInt32(buf, 4)
 
 	if _, err := c.pgConn.Conn().Write(buf); err != nil {
-		return nil, err
+		return "", err
 	}
 
 	for {
 		msg, err := c.rxMsg()
 		if err != nil {
-			return nil, err
+			return "", err
 		}
 
 		switch msg := msg.(type) {
 		case *pgproto3.ReadyForQuery:
 			c.rxReadyForQuery(msg)
-			return nil, err
+			return "", err
 		case *pgproto3.CommandComplete:
 			return pgconn.CommandTag(msg.CommandTag), nil
 		case *pgproto3.ErrorResponse:
-			return nil, c.rxErrorResponse(msg)
+			return "", c.rxErrorResponse(msg)
 		default:
-			return nil, c.processContextFreeMsg(msg)
+			return "", c.processContextFreeMsg(msg)
 		}
 	}
 }