Serving file via PHP script and safe replacing this file on-the-fly

Question

My PHP script serves files using readfile() and manually added HTTP headers. Files are about ~1 MB big, so no partial-reading is necessary.

What is the best way to replace this file with a updated one, but ensure already started downloads won't be broken? I'd like pending downloads to be finished with fetched old file version.

Platform: Linux, PHP 5, Apache 2.

Answer 1

You could use flock to get a lock on the file handle - this will mean you writer process will block until it is able to get an exclusive lock.

So, your readers would obtain a LOCK_SH shared lock, and then simply use fpassthru to output the file, and finally release the lock.

The write would obtain a LOCK_EX exclusive lock, perform the write in safety, and then release the lock allowing any blocked readers to start fetching the new file.

Answer 2

Use version numbering in the filename:

$candidates=sort(glob($dir . $prefix . '*'));
$most_recent=array_pop($candidates);

e.g. file0001.pdf file0002.pdf file0003.pdf

C.