"""Locked, atomically replaced JSON files for the small per-user registries."""
from __future__ import annotations

import fcntl
import json
import os
from pathlib import Path
import tempfile
from typing import Callable, Generic, TypeVar


Value = TypeVar("Value")
Outcome = TypeVar("Outcome")


class JsonRegistry(Generic[Value]):
    """A private JSON file updated under an exclusive lock.

    Subclasses provide the user-facing error type and the mapping between the
    stored payload and a Python value. Every update reads, modifies and writes
    under the lock so concurrent Hub processes cannot lose each other's
    changes, the file is replaced atomically with mode 0600, and a corrupt
    file is never overwritten.
    """

    error_type: type[RuntimeError] = RuntimeError

    def __init__(self, path: Path) -> None:
        self.path = path

    @property
    def lock_path(self) -> Path:
        return self.path.with_name(f".{self.path.name}.lock")

    def _empty(self) -> Value:
        raise NotImplementedError

    def _parse(self, payload: object) -> Value:
        raise NotImplementedError

    def _serialize(self, value: Value) -> object:
        raise NotImplementedError

    def load(self) -> Value:
        if not self.path.exists():
            return self._empty()
        try:
            payload = json.loads(self.path.read_text(encoding="utf-8"))
        except (OSError, json.JSONDecodeError) as error:
            raise self.error_type(f"cannot read {self.path}: {error}") from error
        return self._parse(payload)

    def _update(self, callback: Callable[[Value], tuple[Value, Outcome]]) -> Outcome:
        try:
            self.path.parent.mkdir(mode=0o700, parents=True, exist_ok=True)
            with self.lock_path.open("a+", encoding="utf-8") as lock:
                fcntl.flock(lock.fileno(), fcntl.LOCK_EX)
                current = self.load()
                updated, outcome = callback(current)
                if updated != current:
                    self._write(updated)
                return outcome
        except self.error_type:
            raise
        except OSError as error:
            raise self.error_type(f"cannot update {self.path}: {error}") from error

    def _write(self, value: Value) -> None:
        payload = self._serialize(value)
        temporary_name: str | None = None
        try:
            with tempfile.NamedTemporaryFile(
                mode="w",
                encoding="utf-8",
                dir=self.path.parent,
                prefix=f".{self.path.name}.",
                delete=False,
            ) as temporary:
                temporary_name = temporary.name
                json.dump(payload, temporary, ensure_ascii=False, indent=2)
                temporary.write("\n")
                temporary.flush()
                os.fsync(temporary.fileno())
            os.chmod(temporary_name, 0o600)
            os.replace(temporary_name, self.path)
            directory_fd = os.open(self.path.parent, os.O_RDONLY | os.O_DIRECTORY)
            try:
                os.fsync(directory_fd)
            finally:
                os.close(directory_fd)
        finally:
            if temporary_name is not None:
                try:
                    Path(temporary_name).unlink(missing_ok=True)
                except OSError:
                    pass