gh-125318: Fix validation of zoneinfo false friends

Reject fold=2 before it can reach the transition array, and use the
correct format specifier for the long fold value. Match the datetime
year error message and use MAXYEAR for the upper bound.

Add regression tests for invalid folds and years, conversion errors,
and valid boundary values.

Author: pganssle

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -13,6 +13,7 @@
 import shutil
 import struct
 import tempfile
+import types
 import unittest
 from datetime import date, datetime, time, timedelta, timezone
 from functools import cached_property
@@ -446,6 +447,52 @@ def __eq__(self, other):
 class CZoneInfoTest(ZoneInfoTest):
     module = c_zoneinfo
 
+    def test_duck_typed_invalid_values(self):
+        long_max = (1 << (struct.calcsize("l") * 8 - 1)) - 1
+        cases = [
+            ("fold", value, ValueError, f"fold must be 0 or 1, got {value}")
+            for value in (-1, 2, 3, 256, long_max, -long_max - 1)
+        ]
+        cases.extend(
+            ("year", value, ValueError, f"year must be in 1..9999, not {value}")
+            for value in (0, -1, 10000)
+        )
+        cases.extend(
+            (attr, value, exc, message)
+            for attr in ("fold", "year")
+            for value, exc, message in (
+                (1 << 100, OverflowError, ""),
+                (-(1 << 100), OverflowError, ""),
+                (None, TypeError, ""),
+            )
+        )
+        # Use a date beyond the last transition to exercise year validation.
+        dt = datetime(2040, 1, 1)
+        zi = self.zone_from_key("Europe/London")
+        for attr, value, exc, message in cases:
+            false_friend = types.SimpleNamespace(
+                year=dt.year, hour=0, minute=0, second=0, fold=0,
+                toordinal=dt.toordinal,
+            )
+            setattr(false_friend, attr, value)
+            for method in (zi.utcoffset, zi.dst, zi.tzname):
+                with self.subTest(attr=attr, value=value, method=method):
+                    with self.assertRaisesRegex(exc, re.escape(message)):
+                        method(false_friend)
+
+    def test_duck_typed_valid_boundaries(self):
+        zi = self.zone_from_key("Europe/London")
+        for year in (1, 9999):
+            for fold in (0, 1):
+                dt = datetime(year, 1, 1, fold=fold)
+                false_friend = types.SimpleNamespace(
+                    year=year, hour=0, minute=0, second=0, fold=fold,
+                    toordinal=dt.toordinal,
+                )
+                for method in (zi.utcoffset, zi.dst, zi.tzname):
+                    with self.subTest(year=year, fold=fold, method=method):
+                        self.assertEqual(method(false_friend), method(dt))
+
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")
     def test_signatures(self):

Modules/_zoneinfo.c

@@ -2253,9 +2253,9 @@ find_ttinfo(zoneinfo_state *state, PyZoneInfo_ZoneInfo *self, PyObject *dt)
         if (PyErr_Occurred()) {
             return NULL;
         }
-        if (fold_int < 0 || fold_int > 2) {
+        if (fold_int < 0 || fold_int > 1) {
             PyErr_Format(PyExc_ValueError,
-                         "fold must be 0 or 1, got %d", fold_int);
+                         "fold must be 0 or 1, got %ld", fold_int);
             return NULL;
         }
         fold = (unsigned char)fold_int;
@@ -2285,13 +2285,10 @@ find_ttinfo(zoneinfo_state *state, PyZoneInfo_ZoneInfo *self, PyObject *dt)
                 return NULL;
             }
 
-            if (year < MINYEAR || year > 9999) {
+            if (year < MINYEAR || year > MAXYEAR) {
                 PyErr_Format(PyExc_ValueError,
-                        "year out of range, should be in (%d, %d) but got %d",
-                        MINYEAR,
-                        MAXYEAR,
-                        year
-                );
+                             "year must be in %d..%d, not %d",
+                             MINYEAR, MAXYEAR, year);
                 return NULL;
             }
         }