Bug#15581: COALESCE function truncates mutli-byte TINYTEXT values

field.cc:
  BLOB variations have number-in-bytes limit,
  unlike CHAR/VARCHAR which have number-of-characters limits.
  A tinyblob column can store up to 255 bytes.
  In the case of basic Latin letters (which use 1 byte per character)
  we can store up to 255 characters in a tinyblob column.
  When passing an utf8 tinyblob column as an argument into
  a function (e.g. COALESCE) we need to reserve 3*255 bytes.
  I.e. multiply length in bytes to mbcharlen for the character set.
  Although in reality a tinyblob column can never be 3*255 bytes long,
  we need to set max_length to multiply to make fix_length_and_dec()
  of the function-caller (e.g. COALESCE) calculate the correct max_length
  for the column being created.

ctype_utf8.result, ctype_utf8.test:
  Adding test case.

mysql-test/r/ctype_utf8.result

@@ -1070,3 +1070,11 @@ char(a)
 1
 2
 drop table t1;
+CREATE TABLE t1 (t TINYTEXT CHARACTER SET utf8);
+INSERT INTO t1 VALUES(REPEAT('a', 100));
+CREATE TEMPORARY TABLE t2 SELECT COALESCE(t) AS bug FROM t1;
+SELECT LENGTH(bug) FROM t2;
+LENGTH(bug)
+100
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/ctype_utf8.test

@@ -882,4 +882,14 @@ set names utf8;
 select distinct char(a) from t1;
 drop table t1;
 
+#
+# Bug#15581: COALESCE function truncates mutli-byte TINYTEXT values
+#
+CREATE TABLE t1 (t TINYTEXT CHARACTER SET utf8);
+INSERT INTO t1 VALUES(REPEAT('a', 100));
+CREATE TEMPORARY TABLE t2 SELECT COALESCE(t) AS bug FROM t1;
+SELECT LENGTH(bug) FROM t2;
+DROP TABLE t2;
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/field.cc

@@ -6949,11 +6949,11 @@ uint32 Field_blob::max_length()
   switch (packlength)
   {
   case 1:
-    return 255;
+    return 255 * field_charset->mbmaxlen;
   case 2:
-    return 65535;
+    return 65535 * field_charset->mbmaxlen;
   case 3:
-    return 16777215;
+    return 16777215 * field_charset->mbmaxlen;
   case 4:
     return (uint32) 4294967295U;
   default: