A joint patch for MDEV-19284 and MDEV-19285 (INSTANT ALTER)

This patch fixes:

- MDEV-19284 INSTANT ALTER with ucs2-to-utf16 conversion produces bad data
- MDEV-19285 INSTANT ALTER from ascii_general_ci to latin1_general_ci produces corrupt data

These regressions were introduced in 10.4.3 by:
- MDEV-15564 Avoid table rebuild in ALTER TABLE on collation or charset changes

Changes:

1. Cleanup: Adding a helper method
   Field_longstr::csinfo_change_allows_instant_alter(),
   to remove some duplicate code in field.cc.

2. Cleanup: removing Type_handler::Charsets_are_compatible() and static
   function charsets_are_compatible() and
   introducing new methods in the recently added class Charset instead:
   - encoding_allows_reinterpret_as()
   - encoding_and_order_allow_reinterpret_as()

3. Bug fix: Removing the code that allowed instant conversion for
   ascii-to->8bit and ucs2-to->utf16.
   This actually fixes MDEV-19284 and MDEV-19285.

4. Bug fix: Adding a helper method Charset::collation_specific_name().
   The old corresponding code in Type_handler::Charsets_are_compatible()
   was not safe against (badly named) user-defined collations whose
   character set name can be longer than collation name.

1 files changed, 8 insertions, 0 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index 39555020696..9f113d8ef6c 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -159,6 +159,14 @@ public:
   {
     swap_variables(CHARSET_INFO*, m_charset, other.m_charset);
   }
+  /*
+    Collation name without the character set name.
+    For example, in case of "latin1_swedish_ci",
+    this method returns "_swedish_ci".
+  */
+  LEX_CSTRING collation_specific_name() const;
+  bool encoding_allows_reinterpret_as(CHARSET_INFO *cs) const;
+  bool encoding_and_order_allow_reinterpret_as(CHARSET_INFO *cs) const;
 };