Fixed #29949 -- Refactored db introspection identifier converters.

Removed DatabaseIntrospection.table_name_converter()/column_name_converter()
and use instead DatabaseIntrospection.identifier_converter().

Removed DatabaseFeatures.uppercases_column_names.

Thanks Tim Graham for the initial patch and review and Simon Charette
for the review.

15 files changed, 64 insertions, 76 deletions

diff --git a/django/core/management/commands/migrate.py b/django/core/management/commands/migrate.py
index 6efae2fc90..c2d0c16816 100644
--- a/django/core/management/commands/migrate.py
+++ b/django/core/management/commands/migrate.py
@@ -308,7 +308,7 @@ class Command(BaseCommand):
 
         def model_installed(model):
             opts = model._meta
-            converter = connection.introspection.table_name_converter
+            converter = connection.introspection.identifier_converter
             return not (
                 (converter(opts.db_table) in tables) or
                 (opts.auto_created and converter(opts.auto_created._meta.db_table) in tables)
diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index 8fec545486..e07adb970d 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -200,8 +200,6 @@ class BaseDatabaseFeatures:
     # If NULL is implied on columns without needing to be explicitly specified
     implied_column_null = False
 
-    uppercases_column_names = False
-
     # Does the backend support "select for update" queries with limit (and offset)?
     supports_select_for_update_with_limit = True
 
diff --git a/django/db/backends/base/introspection.py b/django/db/backends/base/introspection.py
index cefdecea0a..fb362eed1e 100644
--- a/django/db/backends/base/introspection.py
+++ b/django/db/backends/base/introspection.py
@@ -24,22 +24,14 @@ class BaseDatabaseIntrospection:
         """
         return self.data_types_reverse[data_type]
 
-    def table_name_converter(self, name):
+    def identifier_converter(self, name):
         """
-        Apply a conversion to the name for the purposes of comparison.
+        Apply a conversion to the identifier for the purposes of comparison.
 
-        The default table name converter is for case sensitive comparison.
+        The default identifier converter is for case sensitive comparison.
         """
         return name
 
-    def column_name_converter(self, name):
-        """
-        Apply a conversion to the column name for the purposes of comparison.
-
-        Use table_name_converter() by default.
-        """
-        return self.table_name_converter(name)
-
     def table_names(self, cursor=None, include_views=False):
         """
         Return a list of names of all tables that exist in the database.
@@ -83,11 +75,11 @@ class BaseDatabaseIntrospection:
                 )
         tables = list(tables)
         if only_existing:
-            existing_tables = self.table_names(include_views=include_views)
+            existing_tables = set(self.table_names(include_views=include_views))
             tables = [
                 t
                 for t in tables
-                if self.table_name_converter(t) in existing_tables
+                if self.identifier_converter(t) in existing_tables
             ]
         return tables
 
@@ -101,10 +93,10 @@ class BaseDatabaseIntrospection:
         all_models = []
         for app_config in apps.get_app_configs():
             all_models.extend(router.get_migratable_models(app_config, self.connection.alias))
-        tables = list(map(self.table_name_converter, tables))
+        tables = set(map(self.identifier_converter, tables))
         return {
             m for m in all_models
-            if self.table_name_converter(m._meta.db_table) in tables
+            if self.identifier_converter(m._meta.db_table) in tables
         }
 
     def sequence_list(self):
diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index c5a5ee1d04..85caec688d 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -1048,7 +1048,7 @@ class BaseDatabaseSchemaEditor:
         """Return all constraint names matching the columns and conditions."""
         if column_names is not None:
             column_names = [
-                self.connection.introspection.column_name_converter(name)
+                self.connection.introspection.identifier_converter(name)
                 for name in column_names
             ]
         with self.connection.cursor() as cursor:
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index ef1189df27..7e288e9840 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -28,7 +28,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     requires_literal_defaults = True
     closed_cursor_error_class = InterfaceError
     bare_select_suffix = " FROM DUAL"
-    uppercases_column_names = True
     # select for update with limit can be achieved on Oracle, but not with the current backend.
     supports_select_for_update_with_limit = False
     supports_temporal_subtraction = True
diff --git a/django/db/backends/oracle/introspection.py b/django/db/backends/oracle/introspection.py
index e5597fd56c..51566365bc 100644
--- a/django/db/backends/oracle/introspection.py
+++ b/django/db/backends/oracle/introspection.py
@@ -50,7 +50,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         """Return a list of table and view names in the current database."""
         cursor.execute("SELECT TABLE_NAME, 't' FROM USER_TABLES UNION ALL "
                        "SELECT VIEW_NAME, 'v' FROM USER_VIEWS")
-        return [TableInfo(row[0].lower(), row[1]) for row in cursor.fetchall()]
+        return [TableInfo(self.identifier_converter(row[0]), row[1]) for row in cursor.fetchall()]
 
     def get_table_description(self, cursor, table_name):
         """
@@ -86,13 +86,13 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             internal_size, default, is_autofield = field_map[name]
             name = name % {}  # cx_Oracle, for some reason, doubles percent signs.
             description.append(FieldInfo(
-                name.lower(), *desc[1:3], internal_size, desc[4] or 0,
+                self.identifier_converter(name), *desc[1:3], internal_size, desc[4] or 0,
                 desc[5] or 0, *desc[6:], default, is_autofield,
             ))
         return description
 
-    def table_name_converter(self, name):
-        """Table name comparison is case insensitive under Oracle."""
+    def identifier_converter(self, name):
+        """Identifier comparison is case insensitive under Oracle."""
         return name.lower()
 
     def get_sequences(self, cursor, table_name, table_fields=()):
@@ -114,7 +114,11 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         # Oracle allows only one identity column per table.
         row = cursor.fetchone()
         if row:
-            return [{'name': row[0].lower(), 'table': table_name, 'column': row[1].lower()}]
+            return [{
+                'name': self.identifier_converter(row[0]),
+                'table': self.identifier_converter(table_name),
+                'column': self.identifier_converter(row[1]),
+            }]
         # To keep backward compatibility for AutoFields that aren't Oracle
         # identity columns.
         for f in table_fields:
@@ -136,10 +140,12 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
            user_constraints.r_constraint_name = cb.constraint_name AND
            ca.position = cb.position""", [table_name])
 
-        relations = {}
-        for row in cursor.fetchall():
-            relations[row[0].lower()] = (row[2].lower(), row[1].lower())
-        return relations
+        return {
+            self.identifier_converter(field_name): (
+                self.identifier_converter(rel_field_name),
+                self.identifier_converter(rel_table_name),
+            ) for field_name, rel_table_name, rel_field_name in cursor.fetchall()
+        }
 
     def get_key_columns(self, cursor, table_name):
         cursor.execute("""
@@ -150,8 +156,10 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             JOIN user_cons_columns rcol
               ON rcol.constraint_name = c.r_constraint_name
             WHERE c.table_name = %s AND c.constraint_type = 'R'""", [table_name.upper()])
-        return [tuple(cell.lower() for cell in row)
-                for row in cursor.fetchall()]
+        return [
+            tuple(self.identifier_converter(cell) for cell in row)
+            for row in cursor.fetchall()
+        ]
 
     def get_constraints(self, cursor, table_name):
         """
@@ -186,6 +194,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             GROUP BY user_constraints.constraint_name, user_constraints.constraint_type
         """, [table_name])
         for constraint, columns, pk, unique, check in cursor.fetchall():
+            constraint = self.identifier_converter(constraint)
             constraints[constraint] = {
                 'columns': columns.split(','),
                 'primary_key': pk,
@@ -213,6 +222,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             GROUP BY cons.constraint_name, rcols.table_name, rcols.column_name
         """, [table_name])
         for constraint, columns, other_table, other_column in cursor.fetchall():
+            constraint = self.identifier_converter(constraint)
             constraints[constraint] = {
                 'primary_key': False,
                 'unique': False,
@@ -240,6 +250,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             GROUP BY ind.index_name, ind.index_type
         """, [table_name])
         for constraint, type_, columns, orders in cursor.fetchall():
+            constraint = self.identifier_converter(constraint)
             constraints[constraint] = {
                 'primary_key': False,
                 'unique': False,
diff --git a/django/db/models/query.py b/django/db/models/query.py
index 85c695d298..1479bea47f 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -1347,7 +1347,7 @@ class RawQuerySet:
 
     def resolve_model_init_order(self):
         """Resolve the init field names and value positions."""
-        converter = connections[self.db].introspection.column_name_converter
+        converter = connections[self.db].introspection.identifier_converter
         model_init_fields = [f for f in self.model._meta.fields if converter(f.column) in self.columns]
         annotation_fields = [(column, pos) for pos, column in enumerate(self.columns)
                              if column not in self.model_fields]
@@ -1469,7 +1469,7 @@ class RawQuerySet:
     @cached_property
     def model_fields(self):
         """A dict mapping column names to model field names."""
-        converter = connections[self.db].introspection.table_name_converter
+        converter = connections[self.db].introspection.identifier_converter
         model_fields = {}
         for field in self.model._meta.fields:
             name, column = field.get_attname_column()
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index d924638b64..7d991b6b84 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -92,7 +92,7 @@ class RawQuery:
     def get_columns(self):
         if self.cursor is None:
             self._execute_query()
-        converter = connections[self.using].introspection.column_name_converter
+        converter = connections[self.using].introspection.identifier_converter
         return [converter(column_meta[0])
                 for column_meta in self.cursor.description]
 
diff --git a/docs/releases/2.2.txt b/docs/releases/2.2.txt
index c00cb9cd41..8a15262ff9 100644
--- a/docs/releases/2.2.txt
+++ b/docs/releases/2.2.txt
@@ -312,6 +312,13 @@ backends.
   * ``sql_create_fk`` is replaced with ``sql_foreign_key_constraint``,
     ``sql_constraint``, and ``sql_create_constraint``.
 
+* ``DatabaseIntrospection.table_name_converter()`` and
+  ``column_name_converter()`` are removed. Third party database backends may
+  need to instead implement ``DatabaseIntrospection.identifier_converter()``.
+  In that case, the constraint names that
+  ``DatabaseIntrospection.get_constraints()`` returns must be normalized by
+  ``identifier_converter()``.
+
 Admin actions are no longer collected from base ``ModelAdmin`` classes
 ----------------------------------------------------------------------
 
diff --git a/tests/backends/tests.py b/tests/backends/tests.py
index 6e6868edfb..19b46e916e 100644
--- a/tests/backends/tests.py
+++ b/tests/backends/tests.py
@@ -80,7 +80,7 @@ class ParameterHandlingTest(TestCase):
         "An executemany call with too many/not enough parameters will raise an exception (Refs #12612)"
         with connection.cursor() as cursor:
             query = ('INSERT INTO %s (%s, %s) VALUES (%%s, %%s)' % (
-                connection.introspection.table_name_converter('backends_square'),
+                connection.introspection.identifier_converter('backends_square'),
                 connection.ops.quote_name('root'),
                 connection.ops.quote_name('square')
             ))
@@ -217,7 +217,7 @@ class BackendTestCase(TransactionTestCase):
 
     def create_squares(self, args, paramstyle, multiple):
         opts = Square._meta
-        tbl = connection.introspection.table_name_converter(opts.db_table)
+        tbl = connection.introspection.identifier_converter(opts.db_table)
         f1 = connection.ops.quote_name(opts.get_field('root').column)
         f2 = connection.ops.quote_name(opts.get_field('square').column)
         if paramstyle == 'format':
@@ -303,7 +303,7 @@ class BackendTestCase(TransactionTestCase):
                 'SELECT %s, %s FROM %s ORDER BY %s' % (
                     qn(f3.column),
                     qn(f4.column),
-                    connection.introspection.table_name_converter(opts2.db_table),
+                    connection.introspection.identifier_converter(opts2.db_table),
                     qn(f3.column),
                 )
             )
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index ddcaf7cfe0..d53892c3a3 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -48,8 +48,6 @@ class CheckConstraintTests(TestCase):
     def test_name(self):
         constraints = get_constraints(Product._meta.db_table)
         expected_name = 'price_gt_discounted_price'
-        if connection.features.uppercases_column_names:
-            expected_name = expected_name.upper()
         self.assertIn(expected_name, constraints)
 
 
@@ -87,6 +85,4 @@ class UniqueConstraintTests(TestCase):
     def test_name(self):
         constraints = get_constraints(Product._meta.db_table)
         expected_name = 'unique_name'
-        if connection.features.uppercases_column_names:
-            expected_name = expected_name.upper()
         self.assertIn(expected_name, constraints)
diff --git a/tests/gis_tests/gis_migrations/test_operations.py b/tests/gis_tests/gis_migrations/test_operations.py
index 3694a7eb67..c5794eebc2 100644
--- a/tests/gis_tests/gis_migrations/test_operations.py
+++ b/tests/gis_tests/gis_migrations/test_operations.py
@@ -63,12 +63,9 @@ class OperationTestCase(TransactionTestCase):
         self.current_state = self.apply_operations('gis', ProjectState(), operations)
 
     def assertGeometryColumnsCount(self, expected_count):
-        table_name = 'gis_neighborhood'
-        if connection.features.uppercases_column_names:
-            table_name = table_name.upper()
         self.assertEqual(
             GeometryColumns.objects.filter(**{
-                GeometryColumns.table_name_col(): table_name,
+                '%s__iexact' % GeometryColumns.table_name_col(): 'gis_neighborhood',
             }).count(),
             expected_count
         )
diff --git a/tests/inspectdb/tests.py b/tests/inspectdb/tests.py
index b578dbf3df..6452002c3e 100644
--- a/tests/inspectdb/tests.py
+++ b/tests/inspectdb/tests.py
@@ -184,7 +184,7 @@ class InspectDBTestCase(TestCase):
         out = StringIO()
         call_command('inspectdb', table_name_filter=special_table_only, stdout=out)
         output = out.getvalue()
-        base_name = 'field' if connection.features.uppercases_column_names else 'Field'
+        base_name = connection.introspection.identifier_converter('Field')
         self.assertIn("field = models.IntegerField()", output)
         self.assertIn("field_field = models.IntegerField(db_column='%s_')" % base_name, output)
         self.assertIn("field_field_0 = models.IntegerField(db_column='%s__')" % base_name, output)
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 6f2b6df765..fc39b77f87 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -85,7 +85,7 @@ class SchemaTests(TransactionTestCase):
 
     def delete_tables(self):
         "Deletes all model tables for our models for a clean test environment"
-        converter = connection.introspection.table_name_converter
+        converter = connection.introspection.identifier_converter
         with connection.schema_editor() as editor:
             connection.disable_constraint_checking()
             table_names = connection.introspection.table_names()
@@ -1868,9 +1868,6 @@ class SchemaTests(TransactionTestCase):
                 table_name=AuthorWithIndexedName._meta.db_table,
                 column_names=('name',),
             )
-        if connection.features.uppercases_column_names:
-            author_index_name = author_index_name.upper()
-            db_index_name = db_index_name.upper()
         try:
             AuthorWithIndexedName._meta.indexes = [index]
             with connection.schema_editor() as editor:
@@ -1908,8 +1905,6 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.add_index(Author, index)
         if connection.features.supports_index_column_ordering:
-            if connection.features.uppercases_column_names:
-                index_name = index_name.upper()
             self.assertIndexOrder(Author._meta.db_table, index_name, ['ASC', 'DESC'])
         # Drop the index
         with connection.schema_editor() as editor:
@@ -2122,12 +2117,14 @@ class SchemaTests(TransactionTestCase):
         field = get_field()
         table = model._meta.db_table
         column = field.column
+        identifier_converter = connection.introspection.identifier_converter
 
         with connection.schema_editor() as editor:
             editor.create_model(model)
             editor.add_field(model, field)
 
-            constraint_name = "CamelCaseIndex"
+            constraint_name = 'CamelCaseIndex'
+            expected_constraint_name = identifier_converter(constraint_name)
             editor.execute(
                 editor.sql_create_index % {
                     "table": editor.quote_name(table),
@@ -2138,22 +2135,20 @@ class SchemaTests(TransactionTestCase):
                     "condition": "",
                 }
             )
-            if connection.features.uppercases_column_names:
-                constraint_name = constraint_name.upper()
-            self.assertIn(constraint_name, self.get_constraints(model._meta.db_table))
+            self.assertIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
             editor.alter_field(model, get_field(db_index=True), field, strict=True)
-            self.assertNotIn(constraint_name, self.get_constraints(model._meta.db_table))
+            self.assertNotIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
 
-            constraint_name = "CamelCaseUniqConstraint"
+            constraint_name = 'CamelCaseUniqConstraint'
+            expected_constraint_name = identifier_converter(constraint_name)
             editor.execute(editor._create_unique_sql(model, [field.column], constraint_name))
-            if connection.features.uppercases_column_names:
-                constraint_name = constraint_name.upper()
-            self.assertIn(constraint_name, self.get_constraints(model._meta.db_table))
+            self.assertIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
             editor.alter_field(model, get_field(unique=True), field, strict=True)
-            self.assertNotIn(constraint_name, self.get_constraints(model._meta.db_table))
+            self.assertNotIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
 
             if editor.sql_foreign_key_constraint:
-                constraint_name = "CamelCaseFKConstraint"
+                constraint_name = 'CamelCaseFKConstraint'
+                expected_constraint_name = identifier_converter(constraint_name)
                 fk_sql = editor.sql_foreign_key_constraint % {
                     "column": editor.quote_name(column),
                     "to_table": editor.quote_name(table),
@@ -2170,11 +2165,9 @@ class SchemaTests(TransactionTestCase):
                         "constraint": constraint_sql,
                     }
                 )
-                if connection.features.uppercases_column_names:
-                    constraint_name = constraint_name.upper()
-                self.assertIn(constraint_name, self.get_constraints(model._meta.db_table))
+                self.assertIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
                 editor.alter_field(model, get_field(Author, CASCADE, field_class=ForeignKey), field, strict=True)
-                self.assertNotIn(constraint_name, self.get_constraints(model._meta.db_table))
+                self.assertNotIn(expected_constraint_name, self.get_constraints(model._meta.db_table))
 
     def test_add_field_use_effective_default(self):
         """
@@ -2491,11 +2484,7 @@ class SchemaTests(TransactionTestCase):
         new_field.set_attributes_from_name('weight')
         with connection.schema_editor() as editor:
             editor.alter_field(Author, old_field, new_field, strict=True)
-
-        expected = 'schema_author_weight_587740f9'
-        if connection.features.uppercases_column_names:
-            expected = expected.upper()
-        self.assertEqual(self.get_constraints_for_column(Author, 'weight'), [expected])
+        self.assertEqual(self.get_constraints_for_column(Author, 'weight'), ['schema_author_weight_587740f9'])
 
         # Remove db_index=True to drop index.
         with connection.schema_editor() as editor:
diff --git a/tests/select_for_update/tests.py b/tests/select_for_update/tests.py
index a750dc61db..f359dc2650 100644
--- a/tests/select_for_update/tests.py
+++ b/tests/select_for_update/tests.py
@@ -113,11 +113,10 @@ class SelectForUpdateTests(TransactionTestCase):
             ))
         features = connections['default'].features
         if features.select_for_update_of_column:
-            expected = ['"select_for_update_person"."id"', '"select_for_update_country"."id"']
+            expected = ['select_for_update_person"."id', 'select_for_update_country"."id']
         else:
-            expected = ['"select_for_update_person"', '"select_for_update_country"']
-        if features.uppercases_column_names:
-            expected = [value.upper() for value in expected]
+            expected = ['select_for_update_person', 'select_for_update_country']
+        expected = [connection.ops.quote_name(value) for value in expected]
         self.assertTrue(self.has_for_update_sql(ctx.captured_queries, of=expected))
 
     @skipUnlessDBFeature('has_select_for_update_of')