The task "MDEV-25829 Change default Unicode collation to uca1400_ai_ci"
previously changed collation derivation for string user variables
from DERIVATION_EXPLICIT to DERIVATION_COERCIBLE, to resolve illegal
collation mix conflicts between table columns and user variables
when they have different collations.
However, DERIVATION_COERCIBLE was a wrong choice because it caused
conflicts between string literals and user variables when they have
different collations.
Adding a new collation derivation level DERIVATION_USERVAR.
This makes the collation of a user variable:
- weaker than a table column (like it was intended by MDEV-25829)
- but stronger than a literal (like it was in pre-MDEV-25829)
Cleanup in sql_type.h:
Removing the line "- BINARY(expr)" from the before-DERIVATION_CAST
comment, as it was on a wrong place. It's also listed on the correct
place before DERIVATION_IMPLICIT.
Step#1 - Changing collation derivation for string user variables
from IMPLICIT to COERCIBLE.
Retionale:
Without this preparatory change, switching the default collation for
Unicode character sets from xxx_general_ci to uca1400_ai_ci would cause
"Illegal mix of collations" errors in scenarios comparing a column with
a non-default collation to a string user variable
This is especially important for queries to INFORMATION_SCHEMA tables,
whose columns use utf8mb3_general_ci.
See the description of MDEV-25829 for more details and SQL script examples.
This patch changes the main name of 3 byte character set from utf8 to
utf8mb3. New old_mode UTF8_IS_UTF8MB3 is added and set TRUE by default,
so that utf8 would mean utf8mb3. If not set, utf8 would mean utf8mb4.
* It uses stack a lot, limit the recursion depth like
we used to do for pcre.
* But don't do that for newer pcre2 that uses stack very little
(and doesn't support limiting recursion depth anyway).
* Fix tests to verify only that deep recursion doesn't crash, but
ignore results and warnings (which depend on pcre2 version)
* Also different pcre2 versions report different offset in error messages,
take that into account.
