如果两个人追加到file_put_contents的文件是合并的字符串？

Question

I have the following function to write to a log file:

function writelog($type, $text){
        global $root;
        $userid=$_SESSION['userid'];
        $tdate=date('d/m/Y H:i:s');
        $file = $root.'/logs/log.txt';
        $input=$userid."^".$type."^".$tdate."^".$text."*";  
        file_put_contents($file, $input, FILE_APPEND);
}

This function is used to add entries to a log whenever an admin of my database does a certain activity.

What would happen if two admins perform an action at exactly the same time which writes to the log file? Is it possible that the strings appended to the file would then be jumbled or merged together or would it always be inserted sequentially?

Answer 1

This can cause the file to contain corrupted data due to race condition. It's not safe to rely on the fact that it only tries to append the data.

Instead, you should ensure that there is no concurrent access to the file. Fortunately enough file_put_contents has a special flag for this named LOCK_EX:

Acquire an exclusive lock on the file while proceeding to the writing.

You can use it like this:

file_put_contents($file, $input, FILE_APPEND | LOCK_EX);

Just make sure that your filesystem supports proper locking mechanisms. For example, I had trouble with my hosting company at some point due to the fact that they used NFS, whose support for exclusive locks is lacking.