# plugins/prometheus/scraper.py
"""Fetching one /metrics endpoint. Every failure comes back as one sentence
a page can show, never a stack trace."""
from __future__ import annotations

import httpx

from steward.core import prometheus

TIMEOUT_SECONDS = 10.0
# A /metrics page past this is not one to store a series of; reading stops.
MAX_BYTES = 20 * 1024 * 1024

ACCEPT = "text/plain;version=0.0.4;q=1,*/*;q=0.1"


class ScrapeError(Exception):
    """Why a scrape got nothing, said to the person."""


async def scrape(url: str, token: str = "", verify_tls: bool = True,
                 transport: httpx.AsyncBaseTransport | None = None) -> dict[str, prometheus.Family]:
    """The endpoint's families. `transport` stands in for the network in tests."""
    headers = {"Accept": ACCEPT}
    if token:
        headers["Authorization"] = f"Bearer {token}"
    try:
        async with httpx.AsyncClient(timeout=TIMEOUT_SECONDS, verify=verify_tls,
                                     transport=transport, follow_redirects=False) as client:
            async with client.stream("GET", url, headers=headers) as response:
                if response.status_code in (401, 403):
                    raise ScrapeError(f"Answered {response.status_code}: the token was refused. "
                                      "Check the token, or make a new one on the service.")
                if response.status_code != 200:
                    raise ScrapeError(f"Answered {response.status_code} {response.reason_phrase}, not metrics.")
                body = bytearray()
                async for chunk in response.aiter_bytes():
                    body.extend(chunk)
                    if len(body) > MAX_BYTES:
                        raise ScrapeError(f"The page is over {MAX_BYTES // (1024 * 1024)} MB; "
                                          "is this a /metrics address?")
    except ScrapeError:
        raise
    except httpx.TimeoutException:
        raise ScrapeError(f"No answer within {TIMEOUT_SECONDS:.0f} s.") from None
    except httpx.ConnectError as exc:
        raise ScrapeError(f"Couldn't connect: {_reason(exc)}.") from None
    except httpx.HTTPError as exc:
        raise ScrapeError(f"The request failed: {_reason(exc)}.") from None
    families = prometheus.parse(body.decode("utf-8", errors="replace"))
    if not families:
        raise ScrapeError("It answered, but with no metrics in Prometheus format.")
    return families


def _reason(exc: Exception) -> str:
    text = str(exc).strip() or type(exc).__name__
    return text.rstrip(".")