[Bugfix] Fix llguidance backend, rollback when EOS was encountered (#25905)

Signed-off-by: Rémi Delacourt <remi@mistral.ai>
Signed-off-by: remi <remi@mistral.ai>
Co-authored-by: Russell Bryant <rbryant@redhat.com>

tests/v1/structured_output/test_backend_guidance.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from transformers import AutoTokenizer
+
+from vllm.config import StructuredOutputsConfig, VllmConfig
+from vllm.config.model import ModelConfig
+from vllm.config.speculative import SpeculativeConfig
+from vllm.sampling_params import SamplingParams, StructuredOutputsParams
+from vllm.v1.request import Request
+from vllm.v1.structured_output import StructuredOutputManager
+from vllm.v1.structured_output.backend_guidance import GuidanceBackend
+from vllm.v1.structured_output.backend_types import StructuredOutputOptions
+
+TOKENIZER = "gpt2"
+
+
+def test_backend_guidance_rollback_terminated():
+    # Test that the backend guidance successfully rollbacks from a
+    # terminated state. This can happen with speculative decoding,
+    # where the draft model proposes EOS and it is verified by the
+    # guidance backend. In that case we are in a stopped state, but
+    # it should be reverted in case EOS is not accepted by the target
+    # model.
+    vllm_config = VllmConfig(
+        decoding_config=StructuredOutputsConfig(
+            backend="guidance",
+        )
+    )
+    tokenizer = AutoTokenizer.from_pretrained(TOKENIZER)
+
+    backend = GuidanceBackend(
+        vllm_config,
+        tokenizer=tokenizer,
+        vocab_size=50257,
+    )
+
+    grammar = backend.compile_grammar(
+        StructuredOutputOptions.JSON, '{"type": "object"}'
+    )
+
+    prompt = tokenizer.encode('{"a": "b"}')
+    assert len(prompt) > 1
+    dummy_wrong = tokenizer.encode('{"a"}')
+    for token in prompt:
+        assert grammar.accept_tokens("", [token])
+    assert not grammar.is_terminated()
+    assert grammar.accept_tokens("", [tokenizer.eos_token_id])
+    assert grammar.is_terminated()
+    # Giving any other token should also be accepted
+    assert grammar.accept_tokens("", dummy_wrong)
+    # Rollback is done from where state was terminated, so from '}' not EOS
+    grammar.rollback(len(prompt) - 1)
+    assert not grammar.is_terminated()
+    assert grammar.validate_tokens([tokenizer.eos_token_id]) == []
+    assert grammar.validate_tokens(dummy_wrong) != dummy_wrong
+    assert grammar.accept_tokens("", prompt[1:])
+    assert not grammar.is_terminated()
+    assert grammar.accept_tokens("", [tokenizer.eos_token_id])
+    assert grammar.is_terminated()
+    # Rollback of <= 0 should not change the terminated state
+    grammar.rollback(0)
+    assert grammar.is_terminated()
+    grammar.rollback(-1)
+    assert grammar.is_terminated()
+
+
+def test_grammar_bitmask_with_specdec():
+    tokenizer = AutoTokenizer.from_pretrained(TOKENIZER)
+    prompt = tokenizer.encode('{"a": "b"}')
+    vllm_config = VllmConfig(
+        model_config=ModelConfig(tokenizer=TOKENIZER),
+        structured_outputs_config=StructuredOutputsConfig(backend="guidance"),
+        speculative_config=SpeculativeConfig(model="[ngram]", num_speculative_tokens=3),
+    )
+    structured_output_manager = StructuredOutputManager(vllm_config)
+
+    for i in range(1, 2):
+        sampling_params = SamplingParams(
+            structured_outputs=StructuredOutputsParams(
+                json='{"type": "object"}',
+            ),
+        )
+        sampling_params.structured_outputs._backend = "guidance"
+
+        my_req_id = f"my_req_id_{i}"
+        request = Request(
+            my_req_id,
+            prompt_token_ids=prompt[:i],
+            sampling_params=sampling_params,
+            pooling_params=None,
+            eos_token_id=tokenizer.eos_token_id,
+        )
+
+        structured_output_manager.grammar_init(request)
+
+        def grammar_bitmask(req: Request, tokens: list[int]) -> None:
+            structured_output_manager.grammar_bitmask(
+                requests={req.request_id: req},
+                structured_output_request_ids={req.request_id: 0},
+                scheduled_spec_decode_tokens={req.request_id: tokens},
+            )
+            # At this point, we rolled-back, so should not be terminated
+            assert not req.structured_output_request.grammar.is_terminated()
+
+        # The grammar might not yet be compiled, so we wait for it
+        while not request.structured_output_request._check_grammar_completion():
+            continue
+
+        assert request.structured_output_request.grammar.accept_tokens(
+            request.request_id, prompt[:i]
+        )
+
+        grammar_bitmask(request, prompt[i:] + [tokenizer.eos_token_id])
+        grammar_bitmask(
+            request, prompt[i:] + [tokenizer.eos_token_id] + prompt
+        )  # EOS not the final token
+        grammar_bitmask(request, prompt[i:])  # EOS not present
+        grammar_bitmask(request, prompt[i:] + [tokenizer.eos_token_id])

vllm/v1/structured_output/backend_guidance.py

@@ -111,6 +111,7 @@ class GuidanceGrammar(StructuredOutputGrammar):
     vocab_size: int
     printed_error: bool = False
     terminated: bool = False
+    rollback_lag: int = 0
 
     def check_error(self):
         if not self.printed_error:
@@ -127,6 +128,8 @@ class GuidanceGrammar(StructuredOutputGrammar):
         """
 
         if self.ll_tokenizer.eos_token in tokens:
+            if self.ll_matcher.is_stopped() and not self.terminated:
+                self.rollback_lag = 1
             self.terminated = True
 
         if self.ll_matcher.is_stopped():
@@ -163,8 +166,11 @@ class GuidanceGrammar(StructuredOutputGrammar):
         return tokens[:num_tokens]
 
     def rollback(self, num_tokens: int) -> None:
-        self.ll_matcher.rollback(num_tokens)
-        self.check_error()
+        if num_tokens > 0:
+            self.ll_matcher.rollback(num_tokens - self.rollback_lag)
+            self.terminated = False
+            self.rollback_lag = 0
+            self.check_error()
 
     def fill_bitmask(self, bitmask: torch.Tensor, idx: int) -> None:
         # this will automatically return [EOS] mask if the matcher is stopped