determine if file is complete

I am trying to write a video ruby transformer scrip开发者_开发百科t (using ffmpeg) that depends on mov files being ftped to a server.

The problem I've run into is that when a large file is uploaded by a user, the watch script (using rb-inotify) attempts to execute (and run the transcoder) before the mov is completely uploaded.

I'm a complete noob. But I'm trying to discover if there is a way for me to be able to ensure my watch script doesn't run until the file(s) is/are completely uploaded.

My watch script is here:

watch_me = INotify::Notifier.new
watch_me.watch("/directory_to_my/videos", :close_write) do |directories|
  load '/directory_to_my/videos/.transcoder.rb'
end
watch_me.run

Thank you for any help you can provide.

Just relying on inotify(7) to tell you when a file has been updated isn't a great fit for telling when an upload is 'complete' -- an FTP session might time out and be re-started, for example, allowing a user to upload a file in chunks over several days as connectivity is cheap or reliable or available. inotify(7) only ever sees file open, close, rename, and access, but never the higher-level event "I'm done modifying this file", as the user would understand it.

There are two mechanisms I can think of: one is to have uploads go initially into one directory and ask the user to move the file into another directory when the upload is complete. The other creates some file meta-data on the client and uses that to "know" when the upload is complete.

Move completed files manually

If your users upload into the directory ftp/incoming/temporary/, they can upload the file in as many connections is required. Once the file is "complete", they can rename the file (rename ftp/incoming/temporary/hello.mov ftp/incoming/complete/hello.mov) and your rb-inotify interface looks for file renames in the ftp/incoming/complete/ directory, and starts the ffmpeg(1) command.

Generate metadata

For a transfer to be "complete", you're really looking for two things:

• The file is the same size on both systems.
• The file is identical on both systems.

Since "identical" is otherwise difficult to check, most people content themselves with checking if the contents of the file, when run through a cryptographic hash function such as MD5 or SHA-1 (or better, SHA-224, SHA-256, SHA-384, or SHA-512) functions. MD5 is quite fine if you're guarding against incomplete transmission but if you intend on using the output of the function for other means, using a stronger function would be wise.

MD5 is really tempting though, since tools to create and validate MD5 hashes are very widespread: md5sum(1) on most Linux systems, md5(1) on most BSD systems (including OS X).

$ md5sum /etc/passwd 
c271aa0e11f560af419557ef49a27ac8  /etc/passwd
$ md5sum /etc/passwd > /tmp/sums
$ md5sum -c /tmp/sums
/etc/passwd: OK

The md5sum -c command asks the md5sum(1) program to check the file of hashes and filenames for correctness. It looks a little silly when used on just a single file, but when you've got dozens or hundreds of files, it's nice to let the software do the checking for you. For example: http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.0.19-real-real/MD5SUMS -- Mozilla has published such files with 860 entries -- checking them by hand would get tiring.

Because checking hashes can take a long time (five minutes on my system to check a high-definition hour-long video that wasn't recently used), it'd be a good idea to only check the hashes when the filesizes match. Modify your upload tool to send along some metadata about how long the file is and what its cryptographic hash is. When your rb-inotify script sees file close requests, check the file size, and if the sizes match, check the cryptographic hash. If the hashes match, then start your ffmpeg(1) command.

It seems easier to upload the file to a temporal directory on the server and move it to the location your script is watching once the transfer is completed.