get-write-flag.ts

// Get the appropriate flag to use for creating files
// We use fmap on Windows platforms for files less than
// 512kb.  This is a fairly low limit, but avoids making
// things slower in some cases.  Since most of what this
// library is used for is extracting tarballs of many
// relatively small files in npm packages and the like,
// it can be a big boost on Windows platforms.

import fs from 'fs'

const platform = process.env.__FAKE_PLATFORM__ || process.platform
const isWindows = platform === 'win32'

/* c8 ignore start */
const { O_CREAT, O_NOFOLLOW, O_TRUNC, O_WRONLY } = fs.constants
const UV_FS_O_FILEMAP =
  Number(process.env.__FAKE_FS_O_FILENAME__) ||
  fs.constants.UV_FS_O_FILEMAP ||
  0
/* c8 ignore stop */

const fMapEnabled = isWindows && !!UV_FS_O_FILEMAP
const fMapLimit = 512 * 1024
const fMapFlag = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY
const noFollowFlag =
  !isWindows && typeof O_NOFOLLOW === 'number' ?
    O_NOFOLLOW | O_TRUNC | O_CREAT | O_WRONLY
  : null
export const getWriteFlag =
  noFollowFlag !== null ? () => noFollowFlag
  : !fMapEnabled ? () => 'w'
  : (size: number) => (size < fMapLimit ? fMapFlag : 'w')