[BugFix] Fix handling of stop strings and stop token ids (#3672)

vllm/sampling_params.py

@@ -166,6 +166,13 @@ class SamplingParams:
         self.logits_processors = logits_processors
         self.include_stop_str_in_output = include_stop_str_in_output
         self.truncate_prompt_tokens = truncate_prompt_tokens
+        # Number of characters to hold back for stop string evaluation
+        # until sequence is finished.
+        if self.stop and not include_stop_str_in_output:
+            self.output_text_buffer_length = max(len(s) for s in self.stop) - 1
+        else:
+            self.output_text_buffer_length = 0
+
         self._verify_args()
         if self.use_beam_search:
             self._verify_beam_search()
@@ -226,6 +233,8 @@ class SamplingParams:
                 and self.truncate_prompt_tokens < 1):
             raise ValueError(f"truncate_prompt_tokens must be >= 1, "
                              f"got {self.truncate_prompt_tokens}")
+        if any(not stop_str for stop_str in self.stop):
+            raise ValueError("stop cannot contain an empty string.")
         if self.stop and not self.detokenize:
             raise ValueError(
                 "stop strings are only supported when detokenize is True. "