Report overflowing durations as invalid instead of crashing

CHANGELOG.rst

@@ -2,6 +2,7 @@ v4.26.0
 =======
 
 * Decrease import time by delaying importing of ``urllib.request`` (#1416).
+* Report durations whose amounts overflow ``Decimal`` (e.g. ``P1E1000000D``) as invalid instead of raising an uncaught ``decimal.Overflow`` from the ``duration`` format checker (#1511).
 
 v4.25.1
 =======

jsonschema/_format.py

@@ -518,7 +518,7 @@ def is_uri_template(instance: object) -> bool:
     @_checks_drafts(
         draft201909="duration",
         draft202012="duration",
-        raises=isoduration.DurationParsingException,
+        raises=(isoduration.DurationParsingException, ArithmeticError),
     )
     def is_duration(instance: object) -> bool:
         if not isinstance(instance, str):

jsonschema/tests/test_format.py

@@ -2,7 +2,8 @@
 Tests for the parts of jsonschema related to the :kw:`format` keyword.
 """
 
-from unittest import TestCase
+from importlib.util import find_spec
+from unittest import TestCase, skipIf
 
 from jsonschema import FormatChecker, ValidationError
 from jsonschema.exceptions import FormatError
@@ -80,6 +81,23 @@ def test_format_checkers_come_with_defaults(self):
         with self.assertRaises(FormatError):
             checker.check(instance="not-an-ipv4", format="ipv4")
 
+    @skipIf(
+        find_spec("isoduration") is None,
+        "The isoduration dependency is not installed",
+    )
+    def test_it_rejects_durations_that_overflow_decimal(self):
+        # Decimal parsing in isoduration raises decimal.Overflow (an
+        # ArithmeticError, not a DurationParsingException) for amounts
+        # past the context's Emax, e.g. an exponent of 1E1000000 or a
+        # plain digit run longer than 999999. Those must be reported as
+        # invalid durations rather than escaping the checker.
+        checker = FormatChecker()
+        for instance in ("P1E1000000D", "P" + "9" * 1000000 + "D"):
+            with self.assertRaises(FormatError):
+                checker.check(instance=instance, format="duration")
+        # A well-formed duration is still accepted.
+        self.assertTrue(checker.conforms("P1Y2M3DT4H5M6S", "duration"))
+
     def test_repr(self):
         checker = FormatChecker(formats=())
         checker.checks("foo")(lambda thing: True)  # pragma: no cover