import * as fs from 'node:fs';
import { CsoError } from './contracts';

/** Read one caller-supplied control file without following or blocking on a raced special file. */
export function readBoundedStable(path: string, max: number, label: string): Buffer {
  let named: fs.Stats, fd: number | undefined;
  try {
    named = fs.lstatSync(path);
  } catch {
    throw new CsoError('MISSING_INPUT', `${label} does not exist`);
  }
  if (named.isSymbolicLink() || !named.isFile() || named.nlink !== 1 || named.size > max)
    throw new CsoError('MISSING_INPUT', `${label} must be one bounded regular file`);
  try {
    fd = fs.openSync(
      path,
      fs.constants.O_RDONLY | (fs.constants.O_NOFOLLOW ?? 0) | (fs.constants.O_NONBLOCK ?? 0),
    );
    const opened = fs.fstatSync(fd);
    if (
      !opened.isFile() ||
      opened.nlink !== 1 ||
      opened.dev !== named.dev ||
      opened.ino !== named.ino ||
      opened.mode !== named.mode ||
      opened.size !== named.size
    )
      throw new CsoError('SNAPSHOT_RACE', `${label} changed before it could be read`);
    const data = Buffer.alloc(max + 1);
    let bytes = 0,
      count = 0;
    while (bytes < data.length && (count = fs.readSync(fd, data, bytes, data.length - bytes, null)) > 0)
      bytes += count;
    const after = fs.fstatSync(fd),
      current = fs.lstatSync(path);
    if (bytes > max) throw new CsoError('MISSING_INPUT', `${label} exceeds the ${max}-byte limit`);
    if (
      !current.isFile() ||
      current.isSymbolicLink() ||
      current.nlink !== 1 ||
      current.dev !== opened.dev ||
      current.ino !== opened.ino ||
      current.mode !== opened.mode ||
      after.size !== opened.size ||
      after.mtimeMs !== opened.mtimeMs ||
      after.ctimeMs !== opened.ctimeMs
    )
      throw new CsoError('SNAPSHOT_RACE', `${label} changed while it was read`);
    return data.subarray(0, bytes);
  } catch (error) {
    if (error instanceof CsoError) throw error;
    const code = (error as NodeJS.ErrnoException).code;
    if (['ELOOP', 'ENOENT', 'ENOTDIR', 'ENXIO'].includes(code ?? ''))
      throw new CsoError('SNAPSHOT_RACE', `${label} changed before it could be opened`);
    throw new CsoError('MISSING_INPUT', `${label} is missing or unreadable`);
  } finally {
    if (fd !== undefined) fs.closeSync(fd);
  }
}