Add a Scheduling Protocol

- Clearly delineate the required interface of schedulers
- Useful for typing

src/xdist/scheduler/protocol.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+from typing import Protocol
+from typing import Sequence
+
+from xdist.workermanage import WorkerController
+
+
+class Scheduling(Protocol):
+    @property
+    def nodes(self) -> list[WorkerController]: ...
+
+    @property
+    def collection_is_completed(self) -> bool: ...
+
+    @property
+    def tests_finished(self) -> bool: ...
+
+    @property
+    def has_pending(self) -> bool: ...
+
+    def add_node(self, node: WorkerController) -> None: ...
+
+    def add_node_collection(
+        self,
+        node: WorkerController,
+        collection: Sequence[str],
+    ) -> None: ...
+
+    def mark_test_complete(
+        self,
+        node: WorkerController,
+        item_index: int,
+        duration: float = 0,
+    ) -> None: ...
+
+    def mark_test_pending(self, item: str) -> None: ...
+
+    def remove_pending_tests_from_node(
+        self,
+        node: WorkerController,
+        indices: Sequence[int],
+    ) -> None: ...
+
+    def remove_node(self, node: WorkerController) -> str | None: ...
+
+    def schedule(self) -> None: ...