"""Local configuration snapshots and bounded, process-safe updates."""
import copy
import errno
import json
import os
import tempfile
import time
from contextlib import contextmanager


class ConfigSnapshot(dict):
    def __init__(self, values):
        super().__init__(values)
        self.original = copy.deepcopy(values)


@contextmanager
def file_lock(path, timeout=45):
    # An OS lock is released even if the owning process crashes. Do not delete
    # the lock file: replacing its inode would break coordination on POSIX.
    with open(path, "a+b") as handle:
        handle.seek(0, os.SEEK_END)
        if handle.tell() == 0:
            handle.write(b"\0")
            handle.flush()
        deadline = time.monotonic() + timeout
        while True:
            try:
                if os.name == "nt":
                    import msvcrt
                    handle.seek(0)
                    msvcrt.locking(handle.fileno(), msvcrt.LK_NBLCK, 1)
                else:
                    import fcntl
                    fcntl.flock(handle.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
                break
            except OSError as exc:
                if exc.errno not in (errno.EACCES, errno.EAGAIN, errno.EDEADLK):
                    raise
                if time.monotonic() >= deadline:
                    raise TimeoutError("等待本机登录/配置锁超过 %s 秒" % timeout)
                time.sleep(0.05)
        try:
            yield
        finally:
            if os.name == "nt":
                handle.seek(0)
                msvcrt.locking(handle.fileno(), msvcrt.LK_UNLCK, 1)
            else:
                fcntl.flock(handle.fileno(), fcntl.LOCK_UN)


def read_config(path):
    with open(path, "r", encoding="utf-8-sig") as handle:
        return ConfigSnapshot(json.load(handle))


def update_config(path, values, ignored=(), initialize=False):
    """Merge only changed fields into the latest file, then atomically replace it."""
    original = getattr(values, "original", {})
    changes = {key: value for key, value in values.items()
               if key not in ignored and (key not in original or value != original[key])}
    removed = {key for key in original if key not in values and key not in ignored}
    with file_lock(path + ".lock"):
        try:
            latest = read_config(path)
            if initialize:
                return latest
        except FileNotFoundError:
            latest = {}
        if not changes and not removed and os.path.exists(path):
            return ConfigSnapshot(latest)
        latest.update(changes)
        for key in removed:
            latest.pop(key, None)
        fd, temporary = tempfile.mkstemp(prefix="config.json.", suffix=".tmp", dir=os.path.dirname(path))
        try:
            with os.fdopen(fd, "w", encoding="utf-8") as handle:
                json.dump(latest, handle, ensure_ascii=False, indent=2)
                handle.flush()
                os.fsync(handle.fileno())
            os.replace(temporary, path)
        finally:
            if os.path.exists(temporary):
                os.unlink(temporary)
    if isinstance(values, ConfigSnapshot):
        values.original = copy.deepcopy(dict(values))
    return ConfigSnapshot(latest)