From 7c140644c4baca468c42c2f4987d24ab97ca1d27 Mon Sep 17 00:00:00 2001
From: Larry Gao <75778943+Larry-Labs@users.noreply.github.com>
Date: Sat, 10 Oct 2026 03:27:14 +0800
Subject: [PATCH] Preserve PathSpec state when backend reconstruction fails

---
 pathspec/pathspec.py      |  6 ++++--
 tests/test_05_pathspec.py | 40 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/pathspec/pathspec.py b/pathspec/pathspec.py
index 6db7fff..fa11546 100644
--- a/pathspec/pathspec.py
+++ b/pathspec/pathspec.py
@@ -141,8 +141,10 @@ def __iadd__(self: Self, other: PathSpec) -> Self:  # type: ignore[misc]
 		(:class:`PathSpec`) to this instance.
 		"""
 		if isinstance(other, PathSpec):
-			self.patterns = [*self.patterns, *other.patterns]
-			self._backend = self._make_backend(self._backend_name, self.patterns)
+			patterns = [*self.patterns, *other.patterns]
+			backend = self._make_backend(self._backend_name, patterns)
+			self.patterns = patterns
+			self._backend = backend
 			return self
 		else:
 			return NotImplemented
diff --git a/tests/test_05_pathspec.py b/tests/test_05_pathspec.py
index c55ac66..97771a0 100644
--- a/tests/test_05_pathspec.py
+++ b/tests/test_05_pathspec.py
@@ -28,8 +28,11 @@
 from unittest import (
 	SkipTest,
 	TestCase)
+from unittest.mock import (
+	patch)
 
 from pathspec import (
+	GitIgnoreSpec,
 	PathSpec,
 	RegexPattern)
 from pathspec.backend import (
@@ -784,6 +787,43 @@ def test_03_iadd_2_mixed(self):
 			'test.txt',
 		}, debug)
 
+	def test_03_iadd_backend_failure(self):
+		"""
+		A failed backend rebuild must leave the original spec usable and unchanged.
+		"""
+		for spec_class in (PathSpec, GitIgnoreSpec):
+			with self.subTest(spec_class=spec_class):
+				spec = spec_class.from_lines('gitignore', ['*.txt'], backend='simple')
+				original_patterns = spec.patterns
+				other = spec_class.from_lines('gitignore', ['!keep.txt'], backend='simple')
+				with patch.object(spec_class, '_make_backend', side_effect=ValueError('Cannot compile patterns')):
+					with self.assertRaisesRegex(ValueError, 'Cannot compile patterns'):
+						spec += other
+
+				self.assertIs(spec.patterns, original_patterns)
+				self.assertEqual(len(spec), 1)
+				self.assertTrue(spec.match_file('keep.txt'))
+				spec += other
+				self.assertEqual(len(spec), 2)
+				self.assertFalse(spec.match_file('keep.txt'))
+				self.assertTrue(spec.match_file('other.txt'))
+
+	def test_03_iadd_incompatible_re2_pattern(self):
+		"""An unsupported regex must not become part of the RE2 spec."""
+		require_backend('re2')
+		import re2
+		spec = PathSpec([RegexPattern(re.compile(r'.*\.txt$'), include=True)], backend='re2')
+		other = PathSpec([RegexPattern(re.compile(r'(?=keep).*'), include=False)], backend='simple')
+		original_patterns = spec.patterns
+		with self.assertRaises(re2.error):
+			spec += other
+		self.assertIs(spec.patterns, original_patterns)
+		self.assertTrue(spec.match_file('keep.txt'))
+		spec += PathSpec.from_lines('gitignore', ['!keep.txt'], backend='simple')
+		self.assertEqual(len(spec), 2)
+		self.assertFalse(spec.match_file('keep.txt'))
+		self.assertTrue(spec.match_file('other.txt'))
+
 	def test_04_len(self):
 		"""
 		Test spec length.