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speech_to_speech/pipeline/control.py

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+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+
+class ControlKind(str, Enum):
+    """Strongly-typed kinds for :class:`PipelineControlMessage`."""
+
+    SESSION_END = "session_end"
+
+
+@dataclass(frozen=True)
+class PipelineControlMessage:
+    kind: ControlKind
+    # Session that enqueued the message, when known. Lets the pooled realtime
+    # send loop ignore a SESSION_END from a force-released session so it can't
+    # satisfy the drain wait of the session that claimed the unit afterwards.
+    session_id: str | None = None
+
+
+SESSION_END = PipelineControlMessage(ControlKind.SESSION_END)
+
+
+def is_control_message(message: object, kind: ControlKind | None = None) -> bool:
+    return isinstance(message, PipelineControlMessage) and (kind is None or message.kind == kind)