Fixed bug #20710.

This bug occurs when error message length exceeds allowed limit: my_error()
function outputs "%s" sequences instead of long string arguments.

Formats like %-.64s are very common in errmsg.txt files, however my_error()
function simply ignores precision of those formats.

mysql-test/r/alter_table.result

@@ -803,3 +803,8 @@ ERROR 42000: You have an error in your SQL syntax; check the manual that corresp
 alter table table_24562 order by no_such_col;
 ERROR 42S22: Unknown column 'no_such_col' in 'order clause'
 drop table table_24562;
+CREATE TABLE t1 (c1 CHAR(255));
+INSERT INTO t1 VALUES (REPEAT("x", 255)), (REPEAT("x", 255));
+ALTER TABLE t1 ADD UNIQUE (c1);
+ERROR 23000: Duplicate entry 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx' for key 1
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -583,5 +583,16 @@ alter table table_24562 order by no_such_col;
 
 drop table table_24562;
 
+#
+# Bug #20710: adding unique index of column with duplicated
+# long values to reproduce error message with truncated key value.
+#
+
+CREATE TABLE t1 (c1 CHAR(255));
+INSERT INTO t1 VALUES (REPEAT("x", 255)), (REPEAT("x", 255));
+--error 1062
+ALTER TABLE t1 ADD UNIQUE (c1);
+DROP TABLE t1;
+
 # End of 4.1 tests
 

mysys/my_error.c

@@ -82,6 +82,11 @@ int my_error(int nr,myf MyFlags, ...)
         If "%.*u" or "%.*d" are encountered, the precision number is read
         from the variable argument list but its value is ignored.
       */
+      if (*tpos == '-')
+      {
+        tpos++;
+        olen--;
+      }
       prec_supplied= 0;
       if (*tpos== '.')
       {
@@ -94,6 +99,14 @@ int my_error(int nr,myf MyFlags, ...)
           prec_chars= va_arg(ap, int); /* get length parameter */
           prec_supplied= 1;
         }
+        else
+        {
+          for (prec_chars= 0; my_isdigit(&my_charset_latin1, *tpos); tpos++, olen--)
+          {
+            prec_supplied= 1;
+            prec_chars= prec_chars * 10 + *tpos - '0';
+          }
+        }
       }
 
       if (!prec_supplied)