Only grab completed files

I'm making a real simple "backend" (PHP5) for two flash/air-applications. One of them will upload a photo, the backend will save it to a folder, and the second app will poll the backend for new photo's and show them.

I don't got any access to a database, so the backend has to be pure PHP5 and nothing more. That's why I chose to save the images to a folder (with a timestamp in their names) and use readdir() to get them back.

This all works like a charm. Nevertheless, I would really like to make sure the backend only returns photo's that are completely uploaded, preventing the second app to 开发者_如何学Pythontry to load an unfinished image. Are there any methods/tricks that I can use to validate a file?

You could check the filesize a couple hundred milliseconds apart and see if it changes:

$first = filesize($file);
// wait 100ms
usleep(100000);
$second = filesize($file);

if($first == $second) {
  // file is no longer being actively uploaded
}

The usual trick for atomic filesystem operations is to write into a temporary file that is not matched by the reader (e.g. XXX.jpg.tmp) and once it's completely uploaded, rename it to it's target name. Renames on the same volume are atomic, so there is no point where the file is either uncomplete or unavailable.

A really easy and common way to do so would be to create a trigger file based on the files name, so that you get something like

123.jpg
123.rdy

or

123.jpg
123.jpg.rdy

You create that file (just an empty stub) as soon as the upload is complete. The application that grabs files to load only cares about files with a trigger file and then processes those. Alternatively, you could also save the uploaded file as ie. 123.bsy or 123.jpg.bsy while it is still being uploaded and then rename it to the finale name 123.jpg after the upload is done. Since renames in the same directory are usually really cheap operations in term of processing time, the chances of running in a race condition should be pretty low. (This might or might now depend on the OS used, though ...)

If you need to keep the files in that place, you could, of course, use a database where you add a record for each file, as the upload is complete. The other app could then just provide files with a matching database record.

After writing this all down I figgered it out myself. What I did was adding the exact amount of bytes in the filename as well and validate that while outputting the list of images. The .tmp/.bsy-sollution is nice also, but I read it a bit to late :) Upside to my solution is that no more renaming is required after the upload is done. Thanks everybody for your fast answers!