Binary download.coffee

##
# Problem: sending a binary download in ExpressJS.
#
# I was trying to proxy download a binary file from S3 and serve it to the user with a different name.
# Using the `request` module didn't work; text files came back fine but binary files would be corrupted
# in strange ways (see attached diff).
#
# I ended up using the built-in `https` module and doing it the hard way.
#
# My theory: the request module expects to only work with text and was performing some UTF-8 coercion
# of the binary data.
##

https = require('https')
urllib = require('url')

download = (req, res) ->
  s3url = "https://something-s3.amazonaws.com/file/sdsdf09sd7f90s8d7f.gif"
  origName = "filename.gif"

  # Proxy the download to the user, but give it the new name.
  https.get urllib.parse(s3url), (response) ->
    # The response is given to us one chunk at a time, in Buffer objects.
    data = []
    response.on("data", (chunk) ->
      data.push chunk
    ).on("end", ->
      # data is an array of Buffers, so we take each octet of each buffer
      # and combine them into one big octet array to pass to a new Buffer
      # instance for sending to the browser.
      # See: http://chad.pantherdev.com/node-js-binary-http-streams/
      buffer = new Buffer(data.reduce (prev, current) ->
        return prev.concat(Array.prototype.slice.call(current))
      , [])
      res.setHeader "Content-Disposition", "attachment; filename=#{origName}"
      res.setHeader "Content-Type", "application/octet-stream"
      res.end buffer, 'binary'
    )

  ##
  # The following DID NOT WORK. It would corrupt the binary files, see
  # the attached diff. Un-commented out for syntax highlighting.
  ##
  # Proxy the download to the user, but give it the new name.
  request s3url, (error, response, body) ->
    if !error and response.statusCode is 200
      res.setHeader "Content-Disposition", "attachment; filename=#{origName}"
      res.setHeader "Content-Type", response.headers["content-type"]
      #res.send(new Buffer(body, 'binary')) # this does the same as res.end below
      res.end body, 'binary'
    else
      res.send 500, message: "Failed to download file."

Diff of a gif.diff

-00000000  47 49 46 38 39 61 10 00  10 00 c2 00 00 00 00 00  |GIF89a..........|
+00000000  47 49 46 38 39 61 10 00  10 00 fd 00 00 00 00 00  |GIF89a..........|

-00000010  00 98 fd 88 88 88 ff ff  ff 00 ba 00 44 44 44 22  |............DDD"|
+00000010  00 fd fd fd fd fd fd fd  fd 00 fd 00 44 44 44 22  |............DDD"|

-00000020  22 22 00 00 00 21 f9 04  01 00 00 03 00 2c 00 00  |""...!.......,..|
+00000020  22 22 00 00 00 21 fd 04  01 00 00 03 00 2c 00 00  |""...!.......,..|

-00000030  00 00 10 00 10 00 00 03  35 38 ba dc fe 70 80 39  |........58...p.9|
+00000030  00 00 10 00 10 00 00 03  35 38 fd fd fd 70 fd 39  |........58...p.9|

-00000040  a3 34 25 1b e0 a8 ff 9c  02 08 64 69 0a a1 64 52  |.4%.......di..dR|
+00000040  fd 34 25 1b fd fd fd fd  02 08 64 69 0a fd 64 52  |.4%.......di..dR|

-00000050  eb 32 92 5c 10 bc a8 5b  02 f2 7c db 30 e1 d3 29  |.2.\...[..|.0..)|
+00000050  fd 32 fd 5c 10 fd fd 5b  02 fd 7c fd 30 fd fd 29  |.2.\...[..|.0..)|

-00000060  1a 6a d2 12 9d 8e 41 10  c8 c2 6c 3a 19 09 00 3b  |.j....A...l:...;|
+00000060  1a 6a fd 12 fd fd 41 10  fd fd 6c 3a 19 09 00 3b  |.j....A...l:...;|