Fix KeyError when replacement worker has mismatched collection

When a worker crashes and is replaced, if the replacement worker's
collection doesn't match the original, add_node_collection() returns
early without adding the node to registered_collections. Later, when
schedule() calls _reschedule() on all nodes, _assign_work_unit() crashes
with KeyError accessing registered_collections[node].

Add a guard in _reschedule() to skip nodes that aren't in
registered_collections.

Fixes #1189
Fixes #714

Author: bendrucker

src/xdist/scheduler/loadscope.py

@@ -320,6 +320,11 @@ def _reschedule(self, node: WorkerController) -> None:
         if node.shutting_down:
             return
 
+        # Skip nodes that aren't properly registered (e.g., collection mismatch)
+        # See https://github.com/pytest-dev/pytest-xdist/issues/1189
+        if node not in self.registered_collections:
+            return
+
         # Check that more work is available
         if not self.workqueue:
             node.shutdown()

testing/test_loadscope.py

@@ -0,0 +1,85 @@
+"""Tests for LoadScopeScheduling."""
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import pytest
+
+from xdist.scheduler import LoadScopeScheduling
+
+
+class MockGateway:
+    _count = 0
+
+    def __init__(self) -> None:
+        self.id = str(MockGateway._count)
+        MockGateway._count += 1
+
+
+class MockNode:
+    def __init__(self) -> None:
+        self.sent: list[int] = []
+        self.gateway = MockGateway()
+        self._shutdown = False
+
+    def send_runtest_some(self, indices: Sequence[int]) -> None:
+        self.sent.extend(indices)
+
+    def shutdown(self) -> None:
+        self._shutdown = True
+
+    @property
+    def shutting_down(self) -> bool:
+        return self._shutdown
+
+
+@pytest.fixture(autouse=True)
+def reset_mock_gateway_counter() -> None:
+    MockGateway._count = 0
+
+
+class TestLoadScopeScheduling:
+    def test_replacement_worker_with_mismatched_collection_is_skipped(
+        self, pytester: pytest.Pytester
+    ) -> None:
+        """Regression test for https://github.com/pytest-dev/pytest-xdist/issues/1189"""
+        config = pytester.parseconfig("--tx=2*popen")
+        sched = LoadScopeScheduling(config)
+
+        node1, node2 = MockNode(), MockNode()
+        sched.add_node(node1)
+        sched.add_node(node2)
+
+        collection = [
+            "test_mod.py::test_a",
+            "test_mod.py::test_b",
+            "test_other.py::test_c",
+            "test_other.py::test_d",
+        ]
+        sched.add_node_collection(node1, collection)
+        sched.add_node_collection(node2, collection)
+        sched.schedule()
+
+        # Simulate node1 crashing
+        sched.remove_node(node1)
+
+        # Replacement worker collects different tests (e.g., due to test file changes)
+        replacement_node = MockNode()
+        sched.add_node(replacement_node)
+        different_collection = [
+            "test_mod.py::test_a",
+            "test_mod.py::test_b",
+            "test_mod.py::test_NEW",  # Different
+            "test_other.py::test_d",
+        ]
+        sched.add_node_collection(replacement_node, different_collection)
+
+        # Replacement node should not be in registered_collections due to mismatch
+        assert replacement_node not in sched.registered_collections
+        assert replacement_node in sched.assigned_work
+
+        # schedule() should skip unregistered nodes rather than crashing
+        sched.schedule()
+
+        assert replacement_node.sent == []
+        assert node2 in sched.registered_collections