feat(rollout): remote inference draft

tests/policy_server/test_scheduler.py

@@ -0,0 +1,117 @@
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for the RoundRobinScheduler fairness guarantees.
+
+The scheduler only reads ``client_uuid`` from sessions, so minimal fakes
+stand in for real Session objects.
+"""
+
+from __future__ import annotations
+
+from collections import Counter
+
+from lerobot.policy_server.scheduler import RoundRobinScheduler
+
+
+class FakeSession:
+    """The scheduler touches nothing but client_uuid."""
+
+    def __init__(self, client_uuid: str):
+        self.client_uuid = client_uuid
+
+
+def picks(scheduler: RoundRobinScheduler, ready: list[FakeSession], n: int) -> list[str]:
+    served = []
+    for _ in range(n):
+        chosen = scheduler.select(list(ready))
+        assert len(chosen) == 1
+        served.append(chosen[0].client_uuid)
+    return served
+
+
+def test_empty_ready_returns_empty():
+    scheduler = RoundRobinScheduler()
+    assert scheduler.select([]) == []
+
+
+def test_empty_ready_after_serving_returns_empty():
+    scheduler = RoundRobinScheduler()
+    scheduler.select([FakeSession("a")])
+    assert scheduler.select([]) == []
+
+
+def test_single_session_picked_repeatedly():
+    scheduler = RoundRobinScheduler()
+    only = FakeSession("solo")
+    for _ in range(5):
+        assert scheduler.select([only]) == [only]
+
+
+def test_three_sessions_served_fairly_in_sorted_uuid_order():
+    scheduler = RoundRobinScheduler()
+    a, b, c = FakeSession("a"), FakeSession("b"), FakeSession("c")
+    # Pass ready in non-sorted order: the ring is sorted by uuid internally.
+    served = picks(scheduler, [c, a, b], 9)
+
+    assert served == ["a", "b", "c", "a", "b", "c", "a", "b", "c"]
+    assert Counter(served) == {"a": 3, "b": 3, "c": 3}
+
+
+def test_session_leaving_between_calls_keeps_fairness():
+    scheduler = RoundRobinScheduler()
+    a, b, c = FakeSession("a"), FakeSession("b"), FakeSession("c")
+    assert scheduler.select([a, b, c]) == [a]
+
+    # 'a' leaves; remaining sessions alternate with no crash or starvation.
+    served = picks(scheduler, [b, c], 4)
+    assert served == ["b", "c", "b", "c"]
+
+
+def test_departed_last_served_uuid_resumes_after_it():
+    scheduler = RoundRobinScheduler()
+    a, b, c = FakeSession("a"), FakeSession("b"), FakeSession("c")
+    picks(scheduler, [a, b, c], 2)  # last served is 'b'
+
+    # 'b' leaves; the next pick is the first uuid greater than 'b'.
+    assert scheduler.select([a, c]) == [c]
+    assert scheduler.select([a, c]) == [a]
+
+
+def test_wraparound_from_last_uuid_back_to_first():
+    scheduler = RoundRobinScheduler()
+    a, b, c = FakeSession("a"), FakeSession("b"), FakeSession("c")
+    assert scheduler.select([c]) == [c]  # last served is the highest uuid
+
+    # Everyone is <= last served: wrap back to the first sorted uuid.
+    assert scheduler.select([a, b, c]) == [a]
+
+
+def test_newly_ready_session_joins_ring_fairly():
+    scheduler = RoundRobinScheduler()
+    a, c = FakeSession("a"), FakeSession("c")
+    served = picks(scheduler, [a, c], 2)
+    assert served == ["a", "c"]
+
+    # 'b' becomes ready; wrap-around lands on 'a', then 'b' gets its turn.
+    b = FakeSession("b")
+    served = picks(scheduler, [a, b, c], 3)
+    assert served == ["a", "b", "c"]
+
+
+def test_no_starvation_over_many_cycles():
+    scheduler = RoundRobinScheduler()
+    ready = [FakeSession(f"u{i:02d}") for i in range(5)]
+    served = picks(scheduler, ready, 50)
+    assert Counter(served) == {f"u{i:02d}": 10 for i in range(5)}