"""Backing store for traces: plain files under ``TRACE_DIR``.

On a Space, ``TRACE_DIR`` points inside a **private HF bucket mounted as a
read-write volume**. The platform mounts it and handles authentication, so the
app holds no credentials at all — there is no HF token in the environment for a
bug or a dependency to leak. Locally, ``TRACE_DIR`` is just a directory.

The mount is a network filesystem, so writes go through a temp file and an
atomic rename: a reader must never observe a half-written trace.
"""

from __future__ import annotations

import json
import logging
import os
import secrets
from pathlib import Path
from typing import Any

log = logging.getLogger("tracebin.storage")


class NotFound(Exception):
    pass


class FileStore:
    def __init__(self, root: str) -> None:
        self.root = Path(root)
        self.root.mkdir(parents=True, exist_ok=True)

    def _path(self, trace_id: str) -> Path:
        # Shard two chars deep so no single directory accumulates every trace;
        # large directories are especially slow to stat over a network mount.
        return self.root / trace_id[:2] / f"{trace_id}.json"

    def put(self, trace_id: str, record: dict[str, Any]) -> None:
        path = self._path(trace_id)
        path.parent.mkdir(parents=True, exist_ok=True)
        tmp = path.parent / f".{trace_id}.{secrets.token_hex(4)}.tmp"
        try:
            tmp.write_text(json.dumps(record), encoding="utf-8")
            os.replace(tmp, path)
        except OSError:
            tmp.unlink(missing_ok=True)
            raise

    def get(self, trace_id: str) -> dict[str, Any]:
        try:
            return json.loads(self._path(trace_id).read_text(encoding="utf-8"))
        except FileNotFoundError as exc:
            raise NotFound(trace_id) from exc


def build_store() -> FileStore:
    root = os.environ.get("TRACE_DIR", "./data").strip() or "./data"
    store = FileStore(root)
    if not os.access(root, os.W_OK):
        # Better to fail loudly at boot than to 500 on the first upload.
        raise RuntimeError(f"TRACE_DIR {root!r} is not writable")
    log.info("storage: %s", root)
    return store