MDEV-15926 MEDIUMINT returns wrong I_S attributes

Problem:

The logic in store_column_type() with a switch on field type was
hard to follow. The part for MEDIUMINT (MYSQL_TYPE_INT24) was not correct.
It erroneously calculated the precision of MEDIUMINT UNSIGNED
as 7 instead of 8.

A similar hard-to-follow switch doing some type specific calculations
resided in adjust_max_effective_column_length(). It was also wrong for
MEDIUMINT (reported as a separate issue in MDEV-15946).

Solution:

1. Introducing a new class Information_schema_numeric_attributes
2. Adding a new virtual method Field::information_schema_numeric_attributes()
3. Splitting the logic in store_column_type() into virtual
   implementations of information_schema_numeric_attributes().
4. In order to avoid adding duplicate code for the integer data types,
   adding a new virtual method Field_int::numeric_precision(),
   which returns the number of digits.

Additional changes:

1. Adding the "const" qualifier to Field::max_display_length()

2. Moving the code from adjust_max_effective_column_length()
  directly to Field::max_display_length().
  There was no any sense to have two implementations:
  - a set of wrong virtual implementations for Field_xxx::max_display_length()
  - additional code in adjust_max_effective_column_length() fixing
    bad results of Field_xxx::max_display_length()
  This change is safe:
  - The code using Field::max_display_length()
    in field.cc, sql_show.cc, sql_type.cc is not affected.
  - The code in rpl_utility.cc is also not affected.
    See a new DBUG_ASSSERT and new comments explaining why.

  In the new reduction, Field_xxx::max_display_length() returns
  correct results for all integer types (except MEDIUMINT, see below).

  Putting implementations of numeric_precision() and max_display_length()
  near each other in field.h made the logic much clearer and thus
  helped to reveal bad results for Field_medium::max_display_length(),
  which returns 9 instead of 8 for signed MEDIUMINT fields.
  This problem will be addressed separately (MDEV-15946).

Note, this change is also useful for pluggable data types (see MDEV-4912),
as now a user defined Field_xxx has a way to control what's returned
in INFORMATION_SCHEMA.COLUMNS.NUMERIC_PRECISION and
INFORMATION_SCHEMA.COLUMNS.NUMERIC_SCALE by implementing
a desired behavior in Field_xxx::information_schema_numeric_attributes().

sql/sql_show.cc

@@ -5636,7 +5636,6 @@ static void store_column_type(TABLE *table, Field *field, CHARSET_INFO *cs,
                               uint offset)
 {
   bool is_blob;
-  int decimals, field_length;
   const char *tmp_buff;
   char column_type_buff[MAX_FIELD_WIDTH];
   String column_type(column_type_buff, sizeof(column_type_buff), cs);
@@ -5686,35 +5685,10 @@ static void store_column_type(TABLE *table, Field *field, CHARSET_INFO *cs,
     They are set to -1 if they should not be set (we should return NULL)
   */
 
-  field_length= -1;
-  decimals= field->decimals();
+  Information_schema_numeric_attributes num=
+    field->information_schema_numeric_attributes();
+
   switch (field->type()) {
-  case MYSQL_TYPE_NEWDECIMAL:
-    field_length= ((Field_new_decimal*) field)->precision;
-    break;
-  case MYSQL_TYPE_DECIMAL:
-    field_length= field->field_length - (decimals  ? 2 : 1);
-    break;
-  case MYSQL_TYPE_TINY:
-  case MYSQL_TYPE_SHORT:
-  case MYSQL_TYPE_LONG:
-  case MYSQL_TYPE_INT24:
-    field_length= field->max_display_length() - 1;
-    break;
-  case MYSQL_TYPE_LONGLONG:
-    field_length= field->max_display_length() - 
-      ((field->flags & UNSIGNED_FLAG) ? 0 : 1);
-    break;
-  case MYSQL_TYPE_BIT:
-    field_length= field->max_display_length();
-    decimals= -1;                             // return NULL
-    break;
-  case MYSQL_TYPE_FLOAT:  
-  case MYSQL_TYPE_DOUBLE:
-    field_length= field->field_length;
-    if (decimals == NOT_FIXED_DEC)
-      decimals= -1;                           // return NULL
-    break;
   case MYSQL_TYPE_TIME:
   case MYSQL_TYPE_TIMESTAMP:
   case MYSQL_TYPE_DATETIME:
@@ -5727,15 +5701,15 @@ static void store_column_type(TABLE *table, Field *field, CHARSET_INFO *cs,
   }
 
   /* NUMERIC_PRECISION column */
-  if (field_length >= 0)
+  if (num.has_precision())
   {
-    table->field[offset + 3]->store((longlong) field_length, TRUE);
+    table->field[offset + 3]->store((longlong) num.precision(), true);
     table->field[offset + 3]->set_notnull();
 
     /* NUMERIC_SCALE column */
-    if (decimals >= 0)
+    if (num.has_scale())
     {
-      table->field[offset + 4]->store((longlong) decimals, TRUE);
+      table->field[offset + 4]->store((longlong) num.scale(), true);
       table->field[offset + 4]->set_notnull();
     }
   }