Create file in a thread-safe manner - php

Question

I have an array of filenames and each process need to create and write only to a single file.
This is what I came to:
foreach ($filenames as $VMidFile) {
if (file_exists($VMidFile)) { // A
continue;
}
$fp = fopen($VMidFile, 'c'); // B
if (!flock($fp, LOCK_EX | LOCK_NB)) { // C
continue;
}
if (!filesize($VMidFile)) { // D
// write to the file;
flock($fp, LOCK_UN);
fclose($fp);
break;
}
flock($fp, LOCK_UN);
fclose($fp); // E
}
But I don't like that I'm relying on the filesize.
Any proposals to do it in another (better) way?
UPD: added the labels to discuss easily
UPD 2: I'm using filesize because I don't see any other reliable way to check if the current thread created the file (thus it's empty yet)
UPD 3: the solution should be condition race free.

Answer 1

A possible, slightly ugly solution would be to lock on a lock file and then testing if the file exists:
$lock = fopen("/tmp/".$filename."LOCK", "w"); // A
if (!flock($lock, LOCK_EX)) { // B
continue;
}
if(!file_exists($filename)){ // C
//File doesn't exist so we know that this thread will create it
//Do stuff to $filename
flock($lock, LOCK_UN); // D
fclose($lock);
}else{
//File exists. This thread didn't create it (at least in this iteration).
flock($lock, LOCK_UN);
fclose($lock);
}
This should allow exclusive access to the file and also allows deciding whether the call to fopen($VMidFile, 'c'); will create the file.

Answer 2

Rather than creating a file and hoping that it's not interfered with:
create a temporary file
do all necessary file operations on it
rename it to the new location if location doesn't exist.
Technically, since rename will overwrite the destination there is a chance that concurrent threads will still clash. That's very unlikely if you have:
if(!file_exists($lcoation) { rename(...
You could use md5_file to verify the file contents is correct after this block.

Answer 3

You can secure exclusive access using semaphores (UNIX only, and provided the sysvsem extension is installed):
$s = sem_get(ftok($filename), 'foo');
sem_acquire($s);
// Do some critical work...
sem_release($s);
Otherwise you can also use flock. It does not require any special extensions, but according to comments on PHP.net is a bit slower than using semaphores:
$a = fopen($file, 'w');
flock($a, LOCK_EX);
// Critical stuff, again
flock($a, LOCK_UN);

Answer 4

Use mode 'x' instead of 'c' in your fopen call. And check the resulting $fp, if it's false, the file wasn't created by the current thread, and you should continue to the next filename.
Also, depending your PHP's installation settings, you may want to put an # in front of the fopen call to suppress any warnings if fopen($VMidFile, 'x') is unable to create the file because it already existed.
This should work even without flock.