gh-93575: Use correct way to calculate PyUnicode struct sizes (GH-93602)

* gh-93575: Use correct way to calculate PyUnicode struct sizes

* Add comment to keep test_sys and test_unicode in sync

* Fix case code < 256

1 files changed, 18 insertions, 5 deletions

diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 64abc0c..9765ed9 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -2370,15 +2370,19 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertIs(s.expandtabs(), s)
 
     def test_raiseMemError(self):
-        null_byte = 1
-        ascii_struct_size = sys.getsizeof("a") - len("a") - null_byte
-        compact_struct_size = sys.getsizeof("\xff") - len("\xff") - null_byte
+        asciifields = "nnb"
+        compactfields = asciifields + "nP"
+        ascii_struct_size = support.calcobjsize(asciifields)
+        compact_struct_size = support.calcobjsize(compactfields)
 
         for char in ('a', '\xe9', '\u20ac', '\U0010ffff'):
             code = ord(char)
-            if code < 0x100:
+            if code < 0x80:
                 char_size = 1  # sizeof(Py_UCS1)
                 struct_size = ascii_struct_size
+            elif code < 0x100:
+                char_size = 1  # sizeof(Py_UCS1)
+                struct_size = compact_struct_size
             elif code < 0x10000:
                 char_size = 2  # sizeof(Py_UCS2)
                 struct_size = compact_struct_size
@@ -2390,7 +2394,16 @@ class UnicodeTest(string_tests.CommonTest,
             # be allocatable, given enough memory.
             maxlen = ((sys.maxsize - struct_size) // char_size)
             alloc = lambda: char * maxlen
-            with self.subTest(char=char):
+            with self.subTest(
+                char=char,
+                struct_size=struct_size,
+                char_size=char_size
+            ):
+                # self-check
+                self.assertEqual(
+                    sys.getsizeof(char * 42),
+                    struct_size + (char_size * (42 + 1))
+                )
                 self.assertRaises(MemoryError, alloc)
                 self.assertRaises(MemoryError, alloc)