Fixed #5725 -- Fixed varchar column size introspection for MySQL

Thanks ferdonline for the initial patch and Karen Tracey for the
related post on django-users.

django/db/backends/mysql/introspection.py

@@ -36,9 +36,20 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         return [row[0] for row in cursor.fetchall()]
 
     def get_table_description(self, cursor, table_name):
-        "Returns a description of the table, with the DB-API cursor.description interface."
+        """
+        Returns a description of the table, with the DB-API cursor.description interface."
+        """
+        # varchar length returned by cursor.description is an internal length,
+        # not visible length (#5725), use information_schema database to fix this
+        cursor.execute("""
+            SELECT column_name, character_maximum_length FROM information_schema.columns
+            WHERE table_name = %s AND table_schema = DATABASE()
+                AND character_maximum_length IS NOT NULL""", [table_name])
+        length_map = dict(cursor.fetchall())
+
         cursor.execute("SELECT * FROM %s LIMIT 1" % self.connection.ops.quote_name(table_name))
-        return cursor.description
+        return [line[:3] + (length_map.get(line[0], line[3]),) + line[4:]
+            for line in cursor.description]
 
     def _name_to_index(self, cursor, table_name):
         """

tests/regressiontests/introspection/tests.py

@@ -89,6 +89,11 @@ class IntrospectionTests(six.with_metaclass(IgnoreNotimplementedError, TestCase)
             [datatype(r[1], r) for r in desc],
             ['IntegerField', 'CharField', 'CharField', 'CharField', 'BigIntegerField']
         )
+        # Check also length of CharFields
+        self.assertEqual(
+            [r[3] for r in desc if datatype(r[1], r) == 'CharField'],
+            [30, 30, 75]
+        )
 
     # Oracle forces null=True under the hood in some cases (see
     # https://docs.djangoproject.com/en/dev/ref/databases/#null-and-empty-strings)