Bug#22278524: ALTER TABLE SOMETIMES CONVERTS TEXT TO JSON WITHOUT

              SYNTAX CHECKING

If ALTER TABLE changes the type of a JSON column to TEXT with
character set utf8mb4 and collation utf8mb4_bin (the same character
set and collation as the JSON type has), it does not convert the
existing column values from JSON binary format to JSON text format.
Instead, the resulting column values will contain the raw binary
representation of the original JSON values.

If the type is changed in the opposite direction, the same thing
happens, with the result that the JSON column contains text data
instead of binary data. Subsequent attempts to access the column
values will raise an error because the JSON binary parser doesn't
understand JSON text format.

This patch makes Copy_field::get_copy_func() pick do_conv_blob()
instead of do_copy_blob() when copying between JSON and non-JSON
types. This ensures that the values are properly converted between the
different formats, and not just copied byte by byte.

Author: kahatlen

mysql-test/r/json.result

@@ -13410,3 +13410,31 @@ Warning	1235	This version of MySQL doesn't yet support 'comparison of JSON in th
 Warning	1235	This version of MySQL doesn't yet support 'comparison of JSON in the LEAST and GREATEST operators'
 Warning	1235	This version of MySQL doesn't yet support 'comparison of JSON in the LEAST and GREATEST operators'
 Warning	1235	This version of MySQL doesn't yet support 'comparison of JSON in the LEAST and GREATEST operators'
+#
+#  Bug#22278524: ALTER TABLE SOMETIMES CONVERTS TEXT TO JSON WITHOUT
+#                SYNTAX CHECKING
+#
+CREATE TABLE t1(txt TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin);
+INSERT INTO t1 VALUES ('not JSON');
+ALTER TABLE t1 MODIFY COLUMN txt JSON;
+ERROR 22032: Invalid JSON text: "Invalid value." at position 1 in value (or column) 'not JSON'.
+SELECT * FROM t1;
+txt
+not JSON
+CREATE TABLE t2(j JSON);
+INSERT INTO t2 VALUES (JSON_OBJECT('a', 'b'));
+ALTER TABLE t2 MODIFY COLUMN j TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin;
+SELECT * FROM t2;
+j
+{"a": "b"}
+CREATE TABLE t3 (j JSON);
+INSERT INTO t3 VALUES (JSON_OBJECT('a', 'b'));
+CREATE TABLE t4 AS SELECT UPPER(j) AS jj FROM t3;
+INSERT INTO t4 VALUES ('not JSON');
+ALTER TABLE t4 MODIFY COLUMN jj JSON;
+ERROR 22032: Invalid JSON text: "Invalid value." at position 1 in value (or column) 'not JSON'.
+SELECT * FROM t4;
+jj
+{"A": "B"}
+not JSON
+DROP TABLE t1, t2, t3, t4;

mysql-test/t/json.test

@@ -6144,6 +6144,28 @@ SELECT JSON_ARRAY(LEAST(NULL, NULL), GREATEST(NULL, NULL), LEAST(j1, NULL),
                   GREATEST(NULL, j2), LEAST(j1, j2), GREATEST(j1, j2)) AS j
 FROM (SELECT CAST('1' AS JSON) AS j1, CAST('2' AS JSON) AS j2) t;
 
+--echo #
+--echo #  Bug#22278524: ALTER TABLE SOMETIMES CONVERTS TEXT TO JSON WITHOUT
+--echo #                SYNTAX CHECKING
+--echo #
+CREATE TABLE t1(txt TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin);
+INSERT INTO t1 VALUES ('not JSON');
+--error ER_INVALID_JSON_TEXT
+ALTER TABLE t1 MODIFY COLUMN txt JSON;
+SELECT * FROM t1;
+CREATE TABLE t2(j JSON);
+INSERT INTO t2 VALUES (JSON_OBJECT('a', 'b'));
+ALTER TABLE t2 MODIFY COLUMN j TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin;
+SELECT * FROM t2;
+CREATE TABLE t3 (j JSON);
+INSERT INTO t3 VALUES (JSON_OBJECT('a', 'b'));
+CREATE TABLE t4 AS SELECT UPPER(j) AS jj FROM t3;
+INSERT INTO t4 VALUES ('not JSON');
+--error ER_INVALID_JSON_TEXT
+ALTER TABLE t4 MODIFY COLUMN jj JSON;
+SELECT * FROM t4;
+DROP TABLE t1, t2, t3, t4;
+
 
 # Local Variables:
 # mode: sql

sql/field_conv.cc

@@ -749,7 +749,13 @@ Copy_field::get_copy_func(Field *to,Field *from)
   }
   else if (to->flags & BLOB_FLAG)
   {
-    if (!(from->flags & BLOB_FLAG) || from->charset() != to->charset())
+    /*
+      We need to do conversion if we are copying from BLOB to
+      non-BLOB, or if we are copying between BLOBs with different
+      character sets, or if we are copying between JSON and non-JSON.
+    */
+    if (!(from->flags & BLOB_FLAG) || from->charset() != to->charset() ||
+        ((to->type() == MYSQL_TYPE_JSON) != (from->type() == MYSQL_TYPE_JSON)))
       return do_conv_blob;
     if (m_from_length != m_to_length || !compatible_db_low_byte_first)
     {