index.js

const fs = require('fs');
const util = require('util');
const stream = require('stream');
const Readable = stream.Readable;
const Writable = stream.Writable;
const PassThrough = stream.PassThrough;
const Pend = require('pend');
const EventEmitter = require('events').EventEmitter;

exports.createFromBuffer = createFromBuffer;
exports.createFromFd = createFromFd;
exports.BufferSlicer = BufferSlicer;
exports.FdSlicer = FdSlicer;

util.inherits(FdSlicer, EventEmitter);
function FdSlicer(fd, options) {
  options = options || {};
  EventEmitter.call(this);

  this.fd = fd;
  this.pend = new Pend();
  this.pend.max = 1;
  this.refCount = 0;
  this.autoClose = !!options.autoClose;
}

FdSlicer.prototype.read = function(buffer, offset, length, position, callback) {
  const self = this;
  self.pend.go(function(cb) {
    fs.read(self.fd, buffer, offset, length, position, function(err, bytesRead, buffer) {
      cb();
      callback(err, bytesRead, buffer);
    });
  });
};

FdSlicer.prototype.write = function(buffer, offset, length, position, callback) {
  const self = this;
  self.pend.go(function(cb) {
    fs.write(self.fd, buffer, offset, length, position, function(err, written, buffer) {
      cb();
      callback(err, written, buffer);
    });
  });
};

FdSlicer.prototype.createReadStream = function(options) {
  return new ReadStream(this, options);
};

FdSlicer.prototype.createWriteStream = function(options) {
  return new WriteStream(this, options);
};

FdSlicer.prototype.ref = function() {
  this.refCount += 1;
};

FdSlicer.prototype.unref = function() {
  const self = this;
  self.refCount -= 1;

  if (self.refCount > 0) return;
  if (self.refCount < 0) throw new Error("invalid unref");

  if (self.autoClose) {
    fs.close(self.fd, onCloseDone);
  }

  function onCloseDone(err) {
    if (err) {
      self.emit('error', err);
    } else {
      self.emit('close');
    }
  }
};

util.inherits(ReadStream, Readable);
function ReadStream(context, options) {
  options = options || {};
  Readable.call(this, options);

  this.context = context;
  this.context.ref();

  this.start = options.start || 0;
  this.endOffset = options.end;
  this.pos = this.start;
  this.destroyed = false;
}

ReadStream.prototype._read = function(n) {
  const self = this;
  if (self.destroyed) return;

  let toRead = Math.min(self._readableState.highWaterMark, n);
  if (self.endOffset != null) {
    toRead = Math.min(toRead, self.endOffset - self.pos);
  }
  if (toRead <= 0) {
    self.destroyed = true;
    self.push(null);
    self.context.unref();
    return;
  }
  self.context.pend.go(function(cb) {
    if (self.destroyed) return cb();
    const buffer = Buffer.alloc(toRead);
    fs.read(self.context.fd, buffer, 0, toRead, self.pos, function(err, bytesRead) {
      if (err) {
        self.destroy(err);
      } else if (bytesRead === 0) {
        self.destroyed = true;
        self.push(null);
        self.context.unref();
      } else {
        self.pos += bytesRead;
        self.push(buffer.slice(0, bytesRead));
      }
      cb();
    });
  });
};

ReadStream.prototype.destroy = function(err) {
  if (this.destroyed) return;

  err = err || new Error("stream destroyed");

  this.destroyed = true;
  this.emit('error', err);
  this.context.unref();
};

util.inherits(WriteStream, Writable);
function WriteStream(context, options) {
  options = options || {};
  Writable.call(this, options);

  this.context = context;
  this.context.ref();

  this.start = options.start || 0;
  this.endOffset = (options.end == null) ? Infinity : +options.end;
  this.bytesWritten = 0;
  this.pos = this.start;
  this.destroyed = false;

  this.on('finish', this.destroy.bind(this));
}

WriteStream.prototype._write = function(buffer, encoding, callback) {
  const self = this;
  if (self.destroyed) return;

  if (self.pos + buffer.length > self.endOffset) {
    const err = new Error("maximum file length exceeded");
    err.code = 'ETOOBIG';
    self.destroy();
    callback(err);

    return;
  }
  self.context.pend.go(function(cb) {
    if (self.destroyed) return cb();
    fs.write(self.context.fd, buffer, 0, buffer.length, self.pos, function(err, bytes) {
      if (err) {
        self.destroy();
        cb();
        callback(err);
      } else {
        self.bytesWritten += bytes;
        self.pos += bytes;
        self.emit('progress');
        cb();
        callback();
      }
    });
  });
};

WriteStream.prototype.destroy = function() {
  if (this.destroyed) return;
  this.destroyed = true;
  this.context.unref();
};

util.inherits(BufferSlicer, EventEmitter);
function BufferSlicer(buffer, options) {
  EventEmitter.call(this);

  options = options || {};
  this.refCount = 0;
  this.buffer = buffer;
  this.maxChunkSize = options.maxChunkSize || Number.MAX_SAFE_INTEGER;
}

BufferSlicer.prototype.read = function(buffer, offset, length, position, callback) {
  const end = position + length;
  const delta = end - this.buffer.length;
  const written = (delta > 0) ? delta : length;
  this.buffer.copy(buffer, offset, position, end);
  setImmediate(function() {
    callback(null, written);
  });
};

BufferSlicer.prototype.write = function(buffer, offset, length, position, callback) {
  buffer.copy(this.buffer, position, offset, offset + length);
  setImmediate(function() {
    callback(null, length, buffer);
  });
};

BufferSlicer.prototype.createReadStream = function(options) {
  options = options || {};
  const readStream = new PassThrough(options);

  readStream.destroyed = false;
  readStream.start = options.start || 0;
  readStream.endOffset = options.end;
  // by the time this function returns, we'll be done.
  readStream.pos = readStream.endOffset || this.buffer.length;

  // respect the maxChunkSize option to slice up the chunk into smaller pieces.
  const entireSlice = this.buffer.slice(readStream.start, readStream.pos);
  let offset = 0;

  while (true) {
    let nextOffset = offset + this.maxChunkSize;
    if (nextOffset >= entireSlice.length) {
      // last chunk
      if (offset < entireSlice.length) {
        readStream.write(entireSlice.slice(offset, entireSlice.length));
      }
      break;
    }
    readStream.write(entireSlice.slice(offset, nextOffset));
    offset = nextOffset;
  }

  readStream.end();
  readStream.destroy = function() {
    readStream.destroyed = true;
  };
  return readStream;
};

BufferSlicer.prototype.createWriteStream = function(options) {
  const bufferSlicer = this;
  options = options || {};
  const writeStream = new Writable(options);
  writeStream.start = options.start || 0;
  writeStream.endOffset = (options.end == null) ? this.buffer.length : +options.end;
  writeStream.bytesWritten = 0;
  writeStream.pos = writeStream.start;
  writeStream.destroyed = false;
  writeStream._write = function(buffer, encoding, callback) {
    if (writeStream.destroyed) return;

    const end = writeStream.pos + buffer.length;
    if (end > writeStream.endOffset) {
      const err = new Error("maximum file length exceeded");

      err.code = 'ETOOBIG';
      writeStream.destroyed = true;
      callback(err);

      return;
    }
    buffer.copy(bufferSlicer.buffer, writeStream.pos, 0, buffer.length);

    writeStream.bytesWritten += buffer.length;
    writeStream.pos = end;
    writeStream.emit('progress');
    callback();
  };
  writeStream.destroy = function() {
    writeStream.destroyed = true;
  };
  return writeStream;
};

BufferSlicer.prototype.ref = function() {
  this.refCount += 1;
};

BufferSlicer.prototype.unref = function() {
  this.refCount -= 1;

  if (this.refCount < 0) {
    throw new Error("invalid unref");
  }
};

function createFromBuffer(buffer, options) {
  return new BufferSlicer(buffer, options);
}

function createFromFd(fd, options) {
  return new FdSlicer(fd, options);
}