set identifier length for MySQL constraints to 64

The rule to limit index names to 64 also applies to all
DDL names, such as those coming from naming conventions.
Add another limiting variable for constraint names and
create test cases against all constraint types.

Additionally, codified in the test suite MySQL's lack of
support for naming of a FOREIGN KEY constraint after
the name was given, which apparently assigns the name to an
associated KEY but not the constraint itself, until MySQL 8
and MariaDB 10.5 which appear to have resolved the
behavior.  However it's not clear how Alembic hasn't had
issues reported with this so far.

Fixed long-lived bug in MySQL dialect where the maximum identifier length
of 255 was too long for names of all types of constraints, not just
indexes, all of which have a size limit of 64. As metadata naming
conventions can create too-long names in this area, apply the limit to the
identifier generator within the DDL compiler.

Fixes: #5898
Change-Id: I79549474845dc29922275cf13321c07598dcea08

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index e5dda7d3e..f5286c85d 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -460,6 +460,18 @@ class SuiteRequirements(Requirements):
         return exclusions.closed()
 
     @property
+    def foreign_key_constraint_name_reflection(self):
+        """Target supports refleciton of FOREIGN KEY constraints and
+        will return the name of the constraint that was used in the
+        "CONSTRANT <name> FOREIGN KEY" DDL.
+
+        MySQL prior to version 8 and MariaDB prior to version 10.5
+        don't support this.
+
+        """
+        return exclusions.closed()
+
+    @property
     def implicit_default_schema(self):
         """target system has a strong concept of 'default' schema that can
         be referred to implicitly.