fix(checks): avoid crash when check flag cannot be parsed

Instead make the check failing with a proper description.

Fixes #17816

Author: nijel

docs/changes.rst

@@ -27,6 +27,7 @@ Weblate 5.16
 * Adding plural strings with singular matching existing string is now prohibited for bilingual translations (see :ref:`bimono`).
 * Automatic :ref:`component-repoweb` URL for common code hosting sites.
 * File formats that only differed in encoding (CSV, GWT Properties, Java Properties, iOS Strings) have been merged into single formats.
+* Gracefully handle invalid check flags in :ref:`custom-checks`.
 
 .. rubric:: Compatibility
 

weblate/checks/base.py

@@ -382,9 +382,12 @@ def check_target_unit(
     ) -> bool:
         """Check flag value."""
         if unit.all_flags.has_value(self.enable_string):
-            return self.check_target_params(
-                sources, targets, unit, self.get_value(unit)
-            )
+            try:
+                value = self.get_value(unit)
+            except ValueError:
+                # Value is present, but is syntactically invalid
+                return True
+            return self.check_target_params(sources, targets, unit, value)
         return False
 
     def check_target_params(
@@ -400,6 +403,15 @@ def check_single(self, source: str, target: str, unit: Unit) -> bool:
         """We don't check single phrase here."""
         return False
 
+    def get_description(self, check_obj: Check) -> StrOrPromise:
+        try:
+            self.get_value(check_obj.unit)
+        except ValueError as error:
+            return format_html(
+                gettext("Could not parse {} flag: {}"), self.enable_string, error
+            )
+        return super().get_description(check_obj)
+
 
 class CountingCheck(TargetCheck):
     """Check whether there is same count of given string."""

weblate/checks/tests/test_chars_checks.py

@@ -4,7 +4,7 @@
 
 """Tests for char based quality checks."""
 
-from unittest import TestCase
+from django.test import SimpleTestCase
 
 from weblate.checks.chars import (
     BeginNewlineCheck,
@@ -28,7 +28,10 @@
     PunctuationSpacingCheck,
     ZeroWidthSpaceCheck,
 )
+from weblate.checks.models import Check
 from weblate.checks.tests.test_checks import CheckTestCase, MockUnit
+from weblate.lang.models import Language
+from weblate.trans.models import Component, Project, Translation, Unit
 
 
 class BeginNewlineCheckTest(CheckTestCase):
@@ -316,7 +319,7 @@ def setUp(self) -> None:
         self.test_failure_1 = ("string", "str\u200bing", "")
 
 
-class MaxLengthCheckTest(TestCase):
+class MaxLengthCheckTest(SimpleTestCase):
     def setUp(self) -> None:
         self.check = MaxLengthCheck()
         self.test_good_matching = ("strings", "less than 21", "max-length:12")
@@ -331,6 +334,33 @@ def test_check(self) -> None:
             )
         )
 
+    def test_check_invalid_flag(self) -> None:
+        self.assertTrue(
+            self.check.check_target(
+                [self.test_good_matching[0]],
+                [self.test_good_matching[1]],
+                MockUnit(flags="max-length:*"),
+            )
+        )
+
+    def test_description_invalid_flag(self) -> None:
+        unit = Unit(
+            source=self.test_good_matching[0],
+            target=self.test_good_matching[1],
+            extra_flags="max-length:*",
+            translation=Translation(
+                component=Component(
+                    file_format="po",
+                    source_language=Language(code="en"),
+                    project=Project(),
+                )
+            ),
+        )
+        check = Check(unit=unit)
+        self.assertIn(
+            "Could not parse max-length flag:", str(self.check.get_description(check))
+        )
+
     def test_unicode_check(self) -> None:
         self.assertFalse(
             self.check.check_target(