MDEV-11672 mysql_list_field() returns wrong default values for VIEW

The problem happened because Item_ident_for_show::field_type() always
returned MYSQL_TYPE_DOUBLE and ignored the actual data type of the
referenced Field. As a result, the execution always used
Item_ident_for_show::val_real() to send the default value of the field,
so most default values for non-numeric types were displayed as '0'.

This patch:
1. Cleanup:
   a. Removes Send_field::charsetnr, as it's been unused since
      introduction of Item::charset_for_protocol() in MySQL-5.5.
   b. Adds the "const" qualifier to Field::char_length().
      This is needed for (5.a), see below.

2. Introduces a new virtual method Type_handler::charset_for_protocol(),
   returning item->collation.collation for string data types, or
   &my_charset_bin for non-string data types.

3. Changes Item::charset_for_protocol() from virtual to non-virtual.
   It now calls type_handler()->charset_for_protocol().
   As a good side effect, duplicate code in Item::charset_for_protocol() and
   Item_temporal_hybrid_func::charset_for_protocol() is now gone.

4. Fixes Item_ident_for_show::field_type() to correctly return
   its data type according to the data type of the referenced field.
   This actually fixes the problem reported in MDEV-11672.
   Now the default value is sent using a correct method, e.g.
   val_str() for VARCHAR/TEXT, or val_int() for INT/BIGINT.
   This required additional changes:
   a. in DBUG_ASSERT in Protocol::store(const char *,size_t,CHARSET_INFO),
      This method is now used by mysqld_list_fields(), which
      (unlike normal SELECT queries) does not set
      field_types/field_pos/field_count.
   b. Item_ident_for_show::Item_ident_for_show() now set standard attributes
      (collation, decimals, max_length, unsigned_flag) according to the
      referenced field, to make charset_for_protocol() return the correct
      value and to make mysqld_list_fields() correctly send default
      values.

5. In order to share the code between Item_field::set_field() and
   Item_ident_for_show::Item_ident_for_show():
   a. Introduces a new method Type_std_attributes::set(const Field*)
   b. To make (a) possible, moves Item::fix_char_length() from Item
      to Type_std_attributes, also moves char_to_byte_length_safe()
      from item.h to sql_type.h
   c. Additionally, moves Item::fix_length_and_charset() and
      Item::max_char_length() from Item to Type_std_attributes.
      This is not directly needed for the fix and is done just for symmetry
      with fix_char_length(), as these three methods are directly related
      to each other.

1 files changed, 71 insertions, 0 deletions

diff --git a/tests/mysql_client_test.c b/tests/mysql_client_test.c
index 58e3dda2b0d..a1c28f0d750 100644
--- a/tests/mysql_client_test.c
+++ b/tests/mysql_client_test.c
@@ -8398,6 +8398,76 @@ static void test_list_fields()
 }
 
 
+static void test_list_fields_default()
+{
+  int rc, i;
+  myheader("test_list_fields_default");
+
+  rc= mysql_query(mysql, "DROP TABLE IF EXISTS t1");
+  myquery(rc);
+
+  rc= mysql_query(mysql,
+                  "CREATE TABLE t1 ("
+                  " i1 INT NOT NULL DEFAULT 0,"
+                  " i3 BIGINT UNSIGNED NOT NULL DEFAULT 0xFFFFFFFFFFFFFFFF,"
+                  " s1 VARCHAR(10) CHARACTER SET latin1 NOT NULL DEFAULT 's1def',"
+                  " d1 DECIMAL(31,1) NOT NULL DEFAULT 111111111122222222223333333333.9,"
+                  " t1 DATETIME(6) NOT NULL DEFAULT '2001-01-01 10:20:30.123456',"
+                  " e1 ENUM('a','b') NOT NULL DEFAULT 'a'"
+                  ")");
+  myquery(rc);
+
+  rc= mysql_query(mysql, "DROP VIEW IF EXISTS v1");
+  myquery(rc);
+
+  rc= mysql_query(mysql, "CREATE VIEW v1 AS SELECT * FROM t1");
+  myquery(rc);
+
+  /*
+    Checking that mysql_list_fields() returns the same result
+    for a TABLE and a VIEW on the same table.
+  */
+  for (i= 0; i < 2; i++)
+  {
+    const char *table_name= i == 0 ? "t1" : "v1";
+    MYSQL_RES *result= mysql_list_fields(mysql, table_name, NULL);
+    mytest(result);
+
+    rc= my_process_result_set(result);
+    DIE_UNLESS(rc == 0);
+
+    verify_prepare_field(result, 0, "i1", "i1", MYSQL_TYPE_LONG,
+                         table_name, table_name, current_db,
+                         11, "0");
+
+    verify_prepare_field(result, 1, "i3", "i3", MYSQL_TYPE_LONGLONG,
+                         table_name, table_name, current_db,
+                         20, "18446744073709551615");
+
+    verify_prepare_field(result, 2, "s1", "s1", MYSQL_TYPE_VAR_STRING,
+                         table_name, table_name, current_db,
+                         10, "s1def");
+
+    verify_prepare_field(result, 3, "d1", "d1", MYSQL_TYPE_NEWDECIMAL,
+                         table_name, table_name, current_db,
+                         33, "111111111122222222223333333333.9");
+
+    verify_prepare_field(result, 4, "t1", "t1", MYSQL_TYPE_DATETIME,
+                         table_name, table_name, current_db,
+                         26, "2001-01-01 10:20:30.123456");
+
+    verify_prepare_field(result, 5, "e1", "e1", MYSQL_TYPE_STRING,
+                         table_name, table_name, current_db,
+                         1, "a");
+
+    mysql_free_result(result);
+  }
+
+  myquery(mysql_query(mysql, "DROP VIEW v1"));
+  myquery(mysql_query(mysql, "DROP TABLE t1"));
+}
+
+
 static void test_bug19671()
 {
   MYSQL_RES *result;
@@ -19558,6 +19628,7 @@ static struct my_tests_st my_tests[]= {
   { "test_fetch_column", test_fetch_column },
   { "test_mem_overun", test_mem_overun },
   { "test_list_fields", test_list_fields },
+  { "test_list_fields_default", test_list_fields_default },
   { "test_free_result", test_free_result },
   { "test_free_store_result", test_free_store_result },
   { "test_sqlmode", test_sqlmode },