Introduce backend rollout-completions interface and decouple OpenEnv helper from vLLM internals

tests/experimental/test_openenv_utils.py

@@ -0,0 +1,83 @@
+# Copyright 2020-2026 The HuggingFace 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.
+
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+import pytest
+
+from trl.experimental.openenv.utils import generate_rollout_completions
+from trl.generation.backend import RolloutCompletion
+
+
+class TestOpenEnvUtils:
+    def test_generate_rollout_completions_dispatch_and_schema(self):
+        backend = MagicMock()
+        backend.generate_rollout_completions.return_value = [
+            RolloutCompletion(prompt_ids=[1, 2], completion_ids=[10, 11], logprobs=[-0.1, -0.2], text="hello")
+        ]
+        trainer = SimpleNamespace(generation_backend=backend, processing_class=MagicMock())
+
+        result = generate_rollout_completions(
+            trainer,
+            prompts=["hi"],
+            generation_overrides={"temperature": 0.7},
+            as_chat=False,
+        )
+
+        assert result == [
+            {
+                "prompt_ids": [1, 2],
+                "completion_ids": [10, 11],
+                "logprobs": [-0.1, -0.2],
+                "text": "hello",
+            }
+        ]
+        backend.generate_rollout_completions.assert_called_once_with(
+            prompts=["hi"],
+            processing_class=trainer.processing_class,
+            generation_overrides={"temperature": 0.7},
+            as_chat=False,
+        )
+
+    def test_generate_rollout_completions_forwards_as_chat_none(self):
+        backend = MagicMock()
+        backend.generate_rollout_completions.return_value = []
+        trainer = SimpleNamespace(generation_backend=backend, processing_class=MagicMock())
+
+        generate_rollout_completions(trainer, prompts=[{"role": "user", "content": "hi"}], as_chat=None)
+
+        backend.generate_rollout_completions.assert_called_once_with(
+            prompts=[{"role": "user", "content": "hi"}],
+            processing_class=trainer.processing_class,
+            generation_overrides=None,
+            as_chat=None,
+        )
+
+    def test_generate_rollout_completions_empty_prompts(self):
+        backend = MagicMock()
+        trainer = SimpleNamespace(generation_backend=backend, processing_class=MagicMock())
+
+        result = generate_rollout_completions(trainer, prompts=[])
+
+        assert result == []
+        backend.generate_rollout_completions.assert_not_called()
+
+    def test_generate_rollout_completions_surfaces_capability_error(self):
+        backend = MagicMock()
+        backend.generate_rollout_completions.side_effect = RuntimeError("does not support rollout completions")
+        trainer = SimpleNamespace(generation_backend=backend, processing_class=MagicMock())
+
+        with pytest.raises(RuntimeError, match="does not support rollout completions"):
+            generate_rollout_completions(trainer, prompts=["x"])