PHP script file loads indefinitely on browser when flock (LOCK_SH/LOCK_EX)

Question

I came across this link while trying to learn how to lock files to prevent a script reading from a file as another is writing, or two scripts writing to the same file simultaneously.

I created two scripts, readandwritelock.php and readlock.php, the first script to retrieve the file with file_get_contents, append it and then write back to the same file with file_put_contents($file, $data, LOCK_EX), and the second that just retrieves the file with file_get_contents after flock($file, LOCK_SH).

<?php
    //readandwritelock.php

    $myfile = fopen('15-11-2018.txt', 'r+');
    if (flock($myfile, LOCK_SH)) {

        echo "Gotten lock<br>";
        $current = file_get_contents('15-11-2018.txt');

        /*I commented this on my second test to see if file_put_contents will work.
        After uncommenting and third test, it does not work anymore.

        if (flock($myfile, LOCK_UN)) {

            echo "Unlocked<br>";
        }*/

        $current .= "appending";

        if (file_put_contents('15-11-2018.txt', $current, LOCK_EX)) {
            echo "Success";
        }
        else {
            echo "Failed";
            //browser loads indefinitely so this does not run
        }

        fclose($myfile);
    }
?>

The problem I am facing is that the first try I was able to file_get_contents after getting the lock, and then releasing the lock and proceed to append and file_put_contents($file, $data, LOCK_EX). However on the second try I decided to comment the releasing of the LOCK_SH lock to test and see what would happen. The script file loads indefinitely (Waiting for localhost...) on my browser, so I reverted back the changes for my third try, but this time the script file still loads indefinitely. It's as if the LOCK_SH was never released.

I must be doing something wrong, but I do not know what exactly it is. Could someone explain?

This was tested on XAMPP and macOS High Sierra and Chrome.

<?php
    //readlock.php
    //works as normal
    $myfile = fopen('15-11-2018.txt', 'r');

    if (flock($myfile, LOCK_SH)) {

        echo "Gotten lock<br>";
        $current = file_get_contents('15-11-2018.txt');
        echo $current;

        if (flock($myfile, LOCK_UN)) {
            echo "<br>Unlocked";
        }
        fclose($myfile);
    }
?>

Answer 1

The reason why your browser seems to load indefinitely is because your PHP file never finishes.

First you get a LOCK_SH (a shared or read lock) for your file, which is fine while you are reading the content. The problem is that you also try to get a LOCK_EX (an exclusive lock) on the same file in the file_put_contents function. Therefore the file_put_contents functions blocks until all other locks (shared AND exclusive ones) are unlocked, which can't work (this is a deadlock).

For your code to work properly, you can either try to get an exlusive lock in the first place

if( flock($myfile, LOCK_EX) ) {
    // ...

or you unlock the shared lock before you write

flock($myfile, LOCK_UN);
if ( file_put_contents('15-11-2018.txt', $current, LOCK_EX) ) {
    // ...

In general it is a good idea to keep a locks life as short as possible. If you plan to make extensive manipulations to your data between reading and writing, I would recommend to unlock the file right after reading and lock it again right for writing.