Reject invalid Copier updates

copier_update/tests/test_updater.py

@@ -54,16 +54,19 @@ def update_repository(self, repository: Repository, answers_file: str, token: st
 
 
 class CommandRecordingUpdater(Updater):
-    def __init__(self, client: FakeClient, *, status: str) -> None:
+    def __init__(self, client: FakeClient, *, status: str, invalid_diff: str = "") -> None:
         super().__init__(client, now=lambda: datetime(2026, 8, 1, 12, 34, 56, tzinfo=UTC))
         self.commands: list[tuple[list[str], str | None]] = []
         self.status = status
+        self.invalid_diff = invalid_diff
 
     def _run(self, command, *, cwd=None, token=None, capture_output=False):
         command = list(command)
         self.commands.append((command, token))
         if command[:2] == ["git", "clone"]:
             Path(command[-1]).mkdir()
+        if command == ["git", "diff", "--check"] and self.invalid_diff:
+            raise subprocess.CalledProcessError(2, command, output=self.invalid_diff)
         stdout = self.status if command[:3] == ["git", "status", "--porcelain"] else ""
         return subprocess.CompletedProcess(command, 0, stdout=stdout, stderr="")
 
@@ -195,3 +198,19 @@ def test_repository_update_stops_when_copier_changes_only_ignored_files():
     assert not updater.update_repository(repository, ".copier-answers.yaml", "repository-token")
     assert not any(command[:2] == ["git", "commit"] for command, _ in updater.commands)
     assert client.pull_requests == []
+
+
+def test_repository_update_stops_when_copier_leaves_conflicts():
+    repository = Repository(id=20, full_name="owner/repository", default_branch="main")
+    client = FakeClient([repository])
+    updater = CommandRecordingUpdater(
+        client,
+        status=" M README.md\n",
+        invalid_diff="README.md:1: leftover conflict marker\n",
+    )
+
+    with pytest.raises(RuntimeError, match="Copier produced invalid changes"):
+        updater.update_repository(repository, ".copier-answers.yaml", "repository-token")
+
+    assert not any(command[:2] == ["git", "push"] for command, _ in updater.commands)
+    assert client.pull_requests == []

copier_update/updater.py

@@ -130,6 +130,11 @@ def update_repository(self, repository: Repository, answers_file: str, token: st
                 token=token,
             )
             self._run(["copier", "update", "-A", "-f", "-a", answers_file], cwd=repository_path, token=token)
+            try:
+                self._run(["git", "diff", "--check"], cwd=repository_path, capture_output=True)
+            except subprocess.CalledProcessError as error:
+                details = (error.stdout or error.stderr or "").strip()
+                raise RuntimeError(f"Copier produced invalid changes for {repository.full_name}:\n{details}") from error
 
             if not self._has_meaningful_changes(repository_path):
                 LOGGER.info("No update available for %s", repository.full_name)