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"""Async per-line stream prefixer for ``ts serve``."""

from __future__ import annotations

import asyncio
from typing import Protocol

ENGINE_TAG = "ts"
GATEWAY_TAG = "smg"


class _Sink(Protocol):
    def write(self, data: str) -> int: ...
    def flush(self) -> None: ...


async def tag_stream(reader: asyncio.StreamReader, tag: str, sink: _Sink) -> None:
    """Read lines from ``reader`` until EOF, write ``[tag] <line>`` to ``sink``.

    A trailing partial line (without a newline) is still emitted with a
    synthesized newline so the last line of a crash message is not lost.
    """
    prefix = f"[{tag}] "
    while True:
        line = await reader.readline()
        if not line:
            return
        text = line.decode("utf-8", errors="replace")
        if text.endswith("\n"):
            sink.write(prefix + text)
        else:
            sink.write(prefix + text + "\n")