Fixed #31395 -- Made setUpTestData enforce in-memory data isolation.

Since it's introduction in Django 1.8 setUpTestData has been suffering
from a documented but confusing caveat due to its sharing of attributes
assigned during its execution with all test instances.

By keeping track of class attributes assigned during the setUpTestData
phase its possible to ensure only deep copies are provided to test
instances on attribute retreival and prevent manual setUp gymnastic to
work around the previous lack of in-memory data isolation.

Thanks Adam Johnson for the extensive review.

django/test/testcases.py

@@ -5,9 +5,10 @@ import posixpath
 import sys
 import threading
 import unittest
+import warnings
 from collections import Counter
 from contextlib import contextmanager
-from copy import copy
+from copy import copy, deepcopy
 from difflib import get_close_matches
 from functools import wraps
 from unittest.suite import _DebugResult
@@ -40,6 +41,7 @@ from django.test.utils import (
     CaptureQueriesContext, ContextList, compare_xml, modify_settings,
     override_settings,
 )
+from django.utils.deprecation import RemovedInDjango41Warning
 from django.utils.functional import classproperty
 from django.views.static import serve
 
@@ -1071,6 +1073,59 @@ def connections_support_transactions(aliases=None):
     return all(conn.features.supports_transactions for conn in conns)
 
 
+class TestData:
+    """
+    Descriptor to provide TestCase instance isolation for attributes assigned
+    during the setUpTestData() phase.
+
+    Allow safe alteration of objects assigned in setUpTestData() by test
+    methods by exposing deep copies instead of the original objects.
+
+    Objects are deep copied using a memo kept on the test case instance in
+    order to maintain their original relationships.
+    """
+    memo_attr = '_testdata_memo'
+
+    def __init__(self, name, data):
+        self.name = name
+        self.data = data
+
+    def get_memo(self, testcase):
+        try:
+            memo = getattr(testcase, self.memo_attr)
+        except AttributeError:
+            memo = {}
+            setattr(testcase, self.memo_attr, memo)
+        return memo
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self.data
+        memo = self.get_memo(instance)
+        try:
+            data = deepcopy(self.data, memo)
+        except TypeError:
+            # RemovedInDjango41Warning.
+            msg = (
+                "Assigning objects which don't support copy.deepcopy() during "
+                "setUpTestData() is deprecated. Either assign the %s "
+                "attribute during setUpClass() or setUp(), or add support for "
+                "deepcopy() to %s.%s.%s."
+            ) % (
+                self.name,
+                owner.__module__,
+                owner.__qualname__,
+                self.name,
+            )
+            warnings.warn(msg, category=RemovedInDjango41Warning, stacklevel=2)
+            data = self.data
+        setattr(instance, self.name, data)
+        return data
+
+    def __repr__(self):
+        return '<TestData: name=%r, data=%r>' % (self.name, self.data)
+
+
 class TestCase(TransactionTestCase):
     """
     Similar to TransactionTestCase, but use `transaction.atomic()` to achieve
@@ -1119,12 +1174,16 @@ class TestCase(TransactionTestCase):
                     cls._rollback_atomics(cls.cls_atomics)
                     cls._remove_databases_failures()
                     raise
+        pre_attrs = cls.__dict__.copy()
         try:
             cls.setUpTestData()
         except Exception:
             cls._rollback_atomics(cls.cls_atomics)
             cls._remove_databases_failures()
             raise
+        for name, value in cls.__dict__.items():
+            if value is not pre_attrs.get(name):
+                setattr(cls, name, TestData(name, value))
 
     @classmethod
     def tearDownClass(cls):