.gitignore

node_modules
*.zip

.jshintrc

{
  "node": true,
  "browser": true,
  "esnext": true,
  "bitwise": true,
  "camelcase": true,
  "curly": true,
  "eqeqeq": true,
  "immed": true,
  "indent": 4,
  "latedef": true,
  "newcap": true,
  "noarg": true,
  "quotmark": "single",
  "regexp": true,
  "undef": true,
  "unused": true,
  "strict": true,
  "trailing": true,
  "smarttabs": true,
  "globals": {
    "angular": false
  }
}

README.md

pipe-to-file fail

Forgetting to call stream.pause and releasing the tick lets data spill out between your request callback and the next function.

To try it out:

npm install
rm v3.1.1.zip
node .
unzip -t v3.1.1.zip

Follow the sorry saga in the commits.

index.js

'use strict';

var assert = require('assert'),
    path = require('path'),
    http = require('http'),
    https = require('https'),
    fs = require('fs'),
    url = require('url');

function fetch(uri, callback) {
    'fetch a file';

    console.error('requesting', uri);

    var modules = {
            'http:': http,
            'https:': https
        },
        module = modules[url.parse(uri).protocol];

    module.get(uri, function (res) {
        console.error('headers:', res.headers);
        switch (res.statusCode) {
        case 200:
            return callback(null, res);
        case 301:
        case 302:
            res.socket.end();
            return fetch(res.headers.location, callback);
        default:
            return callback(new Error(res.statusCode));
        }
    })
    .on('error', callback);
}

function writeResults(destFile, res, callback) {
    'write response text to a file';

    assert(!res.buffered);
    console.error('streaming to', destFile);

    var stream = fs.createWriteStream(destFile),
        size = 0;

    // you can tell I'm getting annoyed
    function onError(err) {
        console.error('ERROR', err);
        callback(err);
    }

    res.on('error', onError);
    res.on('data', function(chunk) {
        size += chunk.length;
    });
    stream.on('error', onError);
    stream.on('close', function() { // also fails for finish
        callback(null, size);
    });
    res.pipe(stream);
    res.resume();
}

function download(details, callback) {
    'download a file if it is absent or empty, making its parent directory.';

    function afterWriting(err, size) {
        if (err) {
            return callback(err);
        } else if (size === details.size || !details.size) {
            return callback(null);
        } else {
            return callback(new Error('unexpected size: ' + size));
        }
    }

    function afterRequesting(err, res) {
        if (err) {
            return callback(err);
        } else {
            res.pause(); // we don't want to miss writes
            return setImmediate(writeResults, details.as, res, afterWriting);
        }
    }

    fs.existsSync(details.as) && fs.truncateSync(details.as, 0);
    fetch(details.url, afterRequesting);
}

module.exports = download;

if (!module.parent) {
    download({
        url: 'https://github.com/twbs/bootstrap/archive/v3.1.1.zip',
        as: path.join(__dirname, 'v3.1.1.zip'),
        size: 2357836
    }, console.error);
}

package.json

{
  "dependencies": {},
  "version": "0.4.0"
}