BUG#27788685: NO WARNING WHEN TRUNCATING A STRING WITH DATA

              LOSS

ANALYSIS:
=========
When converting from a BLOB/TEXT type to a smaller
BLOB/TEXT type, no warning/error is reported to the user
informing about the truncation/data loss.

FIX:
====
We are now reporting a warning in non-strict mode and an
appropriate error in strict mode.

mysql-test/r/alter_table.result

@@ -1505,3 +1505,53 @@ SELECT * FROM t1;
 
 10
 DROP TABLE t1;
+#
+# BUG#27788685: NO WARNING WHEN TRUNCATING A STRING WITH DATA LOSS
+#
+SET GLOBAL max_allowed_packet=17825792;
+CREATE TABLE t1 (t1_fld1 TEXT) ENGINE=InnoDB;
+CREATE TABLE t2 (t2_fld1 MEDIUMTEXT) ENGINE=InnoDB;
+CREATE TABLE t3 (t3_fld1 LONGTEXT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (REPEAT('a',300));
+INSERT INTO t2 VALUES (REPEAT('b',65680));
+INSERT INTO t3 VALUES (REPEAT('c',16777300));
+SELECT LENGTH(t1_fld1) FROM t1;
+LENGTH(t1_fld1)
+300
+SELECT LENGTH(t2_fld1) FROM t2;
+LENGTH(t2_fld1)
+65680
+SELECT LENGTH(t3_fld1) FROM t3;
+LENGTH(t3_fld1)
+16777300
+# With strict mode
+SET SQL_MODE='STRICT_ALL_TABLES';
+ALTER TABLE t1 CHANGE `t1_fld1` `my_t1_fld1` TINYTEXT;
+ERROR 22001: Data too long for column 'my_t1_fld1' at row 1
+ALTER TABLE t2 CHANGE `t2_fld1` `my_t2_fld1` TEXT;
+ERROR 22001: Data too long for column 'my_t2_fld1' at row 1
+ALTER TABLE t3 CHANGE `t3_fld1` `my_t3_fld1` MEDIUMTEXT;
+ERROR 22001: Data too long for column 'my_t3_fld1' at row 1
+# With non-strict mode
+SET SQL_MODE='';
+ALTER TABLE t1 CHANGE `t1_fld1` `my_t1_fld1` TINYTEXT;
+Warnings:
+Warning	1265	Data truncated for column 'my_t1_fld1' at row 1
+ALTER TABLE t2 CHANGE `t2_fld1` `my_t2_fld1` TEXT;
+Warnings:
+Warning	1265	Data truncated for column 'my_t2_fld1' at row 1
+ALTER TABLE t3 CHANGE `t3_fld1` `my_t3_fld1` MEDIUMTEXT;
+Warnings:
+Warning	1265	Data truncated for column 'my_t3_fld1' at row 1
+SELECT LENGTH(my_t1_fld1) FROM t1;
+LENGTH(my_t1_fld1)
+44
+SELECT LENGTH(my_t2_fld1) FROM t2;
+LENGTH(my_t2_fld1)
+144
+SELECT LENGTH(my_t3_fld1) FROM t3;
+LENGTH(my_t3_fld1)
+84
+DROP TABLE t1, t2, t3;
+SET SQL_MODE=default;
+SET GLOBAL max_allowed_packet=default;

mysql-test/r/myisam-blob.result

@@ -29,6 +29,8 @@ select length(data) from t1;
 length(data)
 18874368
 alter table t1 modify data blob;
+Warnings:
+Warning	1265	Data truncated for column 'data' at row 1
 select length(data) from t1;
 length(data)
 0

mysql-test/t/alter_table.test

@@ -1280,3 +1280,52 @@ ALTER TABLE t1 ADD INDEX (`
 SELECT * FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # BUG#27788685: NO WARNING WHEN TRUNCATING A STRING WITH DATA LOSS
+--echo #
+
+SET GLOBAL max_allowed_packet=17825792;
+
+--connect(con1, localhost, root,,)
+CREATE TABLE t1 (t1_fld1 TEXT) ENGINE=InnoDB;
+CREATE TABLE t2 (t2_fld1 MEDIUMTEXT) ENGINE=InnoDB;
+CREATE TABLE t3 (t3_fld1 LONGTEXT) ENGINE=InnoDB;
+
+INSERT INTO t1 VALUES (REPEAT('a',300));
+INSERT INTO t2 VALUES (REPEAT('b',65680));
+INSERT INTO t3 VALUES (REPEAT('c',16777300));
+
+SELECT LENGTH(t1_fld1) FROM t1;
+SELECT LENGTH(t2_fld1) FROM t2;
+SELECT LENGTH(t3_fld1) FROM t3;
+
+--echo # With strict mode
+SET SQL_MODE='STRICT_ALL_TABLES';
+
+--error ER_DATA_TOO_LONG
+ALTER TABLE t1 CHANGE `t1_fld1` `my_t1_fld1` TINYTEXT;
+--error ER_DATA_TOO_LONG
+ALTER TABLE t2 CHANGE `t2_fld1` `my_t2_fld1` TEXT;
+--error ER_DATA_TOO_LONG
+ALTER TABLE t3 CHANGE `t3_fld1` `my_t3_fld1` MEDIUMTEXT;
+
+--echo # With non-strict mode
+SET SQL_MODE='';
+
+ALTER TABLE t1 CHANGE `t1_fld1` `my_t1_fld1` TINYTEXT;
+ALTER TABLE t2 CHANGE `t2_fld1` `my_t2_fld1` TEXT;
+ALTER TABLE t3 CHANGE `t3_fld1` `my_t3_fld1` MEDIUMTEXT;
+
+SELECT LENGTH(my_t1_fld1) FROM t1;
+SELECT LENGTH(my_t2_fld1) FROM t2;
+SELECT LENGTH(my_t3_fld1) FROM t3;
+
+# Cleanup
+--disconnect con1
+--source include/wait_until_disconnected.inc
+
+--connection default
+DROP TABLE t1, t2, t3;
+
+SET SQL_MODE=default;
+SET GLOBAL max_allowed_packet=default;

sql/field_conv.cc

-/* Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -287,9 +287,23 @@ static void do_copy_next_number(Copy_field *copy)
 
 static void do_copy_blob(Copy_field *copy)
 {
-  ulong length=((Field_blob*) copy->from_field)->get_length();
-  ((Field_blob*) copy->to_field)->store_length(length);
+  ulong from_length=((Field_blob*) copy->from_field)->get_length();
+  ((Field_blob*) copy->to_field)->store_length(from_length);
   memcpy(copy->to_ptr, copy->from_ptr, sizeof(char*));
+  ulong to_length=((Field_blob*) copy->to_field)->get_length();
+  if (to_length < from_length)
+  {
+    if (copy->to_field->table->in_use->abort_on_warning)
+    {
+      copy->to_field->set_warning(MYSQL_ERROR::WARN_LEVEL_WARN,
+                                  ER_DATA_TOO_LONG, 1);
+    }
+    else
+    {
+      copy->to_field->set_warning(MYSQL_ERROR::WARN_LEVEL_WARN,
+                                  WARN_DATA_TRUNCATED, 1);
+    }
+  }
 }
 
 static void do_conv_blob(Copy_field *copy)