fd

File descriptor manager for Node.js. Available in npm as fd.
fd manages fs.open() and fs.close() calls safely for you where there may be timing issues related to multiple-use of the same descriptor.
fd provides checkin() and checkout() functions so your application can register its intent to use a file descriptor after it's been opened and then register that it has finished with the descriptor so that any pending fs.close() operations may be performed.
fd naturally couples with async-cache to provide a safe pool of file descriptors.

Example

Lets make a static resource web server! This example can be found in the example/ directory of this repository.
We use async-cache to cache both fs.sync() calls and fds, but we hook it up to fd so we can safely manage opens and closes.

const fdman = require('fd')()
    , http  = require('http')
    , fs    = require('fs')
    , path  = require('path')
    , AC    = require('async-cache')
    , mime  = require('mime')

    , ROOT  = path.join(__dirname, 'public')

      // an async cache for fs.stat calls, fresh for 10s
    , statCache = AC({
          max    : 100
        , maxAge : 10000
        , load   : function (path, callback) {
                     fs.stat(path, callback)
                   }
      })

      // an async cache for fds, fresh for 10s
    , fdCache = AC({
          max     : 100
        , maxAge  : 10000
          // use fdman to open & close
        , load    : fdman.open.bind(fdman)
        , dispose : fdman.close.bind(fdman)
      })

    , serveError = function (res) {
        res.statusCode = 404
        res.setHeader('content-type', 'text/plain')
        res.end(http.STATUS_CODES[res.statusCode] + '\n')
      }

http.createServer(function (req, res) {
  var p = path.join(ROOT, req.url)

  // get a fs.stat for this file
  statCache.get(p, function (err, stat) {
    if (err || !stat.isFile())
      return serveError(res)

    // get an fd for this file
    fdCache.get(p, function (err, fd) {
      var mimeType = mime.lookup(path.extname(p))
          // get a safe checkin function from fdman that
          // we could safely all multiple times for this single
          // checkout
        , checkin = fdman.checkinfn(p, fd)

      // check out the fd for use
      fdman.checkout(p, fd)

      res.setHeader(
          'content-type'
          // don't force download, just show it
        , mimeType != 'application/octet-stream' ? mimeType : 'text/plain'
      )

      // stream from the fd to the response
      var st = fs.createReadStream(p, { fd: fd, start: 0, end: stat.size })n
        .on('end', checkin)
        .on('error', checkin)

      // override destroy so we don't close the fd
      st.destroy = function () {}

      st.pipe(res)

    })
  })
}).listen(8080)

API

fd()

Create a new instance of fd. Typically called with var fdman = require('fd')(). You can have multiple, separate instances of fd operating at the same time, hence the need to instantiate.

fdman.open(path, callback)

Equivalent to fs.open(path, callback), you'll get back an err and fd parameters but the descriptor will go into the managed pool.

fdman.close(path, fd)

Will call fs.close(fd) only when the fd is no longer in use. i.e. it will wait till all current uses have been checked in (see below).

fdman.checkout(path, fd)

Called when your application may need to use the fd. This should be called as early as possible, even if your application may not end up using it.
It is important to perform a checkout() as soon as you have a reference to the file descriptor if you may be using it, otherwise an asynchronous call may interrupt and call close() before you use it. You don't have to use the fd to register your intent to use it, as long as you eventually call checkin().

fdman.checkin(path, fd)

Register with fd that you have finished using the descriptor and it can be safely closed if need be.
The descriptor may not need to be closed or there may be other uses of the descriptor currently checked out so a checkin() won't automatically lead to a close().

fdman.checkinfn(path, fd)

Returns a function that, when called, will safely perform a checkin() for you on the given path and descriptor. An important property of the function is that it will only perform a single checkin() regardless of how many times it is called.
This returned function is helpful for calling checkin() from multiple points in your application, such as in case of error, and you don't need to worry about whether it's been previously called for the current checkout().
See the example above how this can be used.

Licence

fd is Copyright (c) 2012 Rod Vagg @rvagg and licenced under the MIT licence. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE file for more details.