this reads like an AI and not a real world scenario

I’d like to see a reproducible test case that demonstrates this actually happening before we start changing the core cache implementation to fix a theoretical problem. Can you provide a script that reliably reproduces the issue?

Hi @brianteeman, thank you for the direct feedback. You are right that the initial description was theoretical.
To demonstrate that this is a real-world scenario, I wrote a standalone PHP script that reproduces the exact filesystem race condition caused by fopen($path, 'wb').
You can run this script directly in any PHP CLI environment:

<?php
$file = ['/tmp/joomla_race_test.txt';]( #[url]([url]([url]( @url))))
$expectedData = str_repeat('CACHE_DATA_PAYLOAD_', 500);

// 1. Pre-populate the file to simulate an existing valid cache file
file_put_contents($file, $expectedData);
echo "1. Initial valid cache file size: " . filesize($file) . " bytes\n";

// 2. Create a temporary "Reader" script (simulating a concurrent web request)
$readerCode = "<?php\nusleep(500000);\n\$content = @file_get_contents('/tmp/joomla_race_test.txt');\n\$len = strlen(\$content);\nif (\$len === 0) {\n    echo 'RACE CONDITION CONFIRMED: Concurrent reader saw an EMPTY (0 bytes) file!' . PHP_EOL;\n} else {\n    echo 'Reader saw ' . \$len . ' bytes.' . PHP_EOL;\n}\n";
file_put_contents('/tmp/simulated_reader.php', $readerCode);

echo "2. Starting concurrent reader process...\n";

// 3. Start the reader in the background
$process = proc_open('php /tmp/simulated_reader.php', [
    1 => ['pipe', 'w'], // stdout
    2 => ['pipe', 'w']  // stderr
], $pipes);

if (is_resource($process)) {
    // 4. Simulate the OLD FileStorage::store() logic
    echo "3. Writer opening file with 'wb' mode (THIS TRUNCATES THE FILE)...\n";
    $fp = fopen($file, 'wb'); 
    
    // Simulate slow disk I/O or a large payload taking time to write
    usleep(1000000); // Sleep for 1 second while file is truncated!
    
    fwrite($fp, $expectedData);
    fclose($fp);
    echo "4. Writer finished.\n";

    // 5. Capture reader output
    echo "\n--- CONCURRENT READER OUTPUT ---\n";
    echo stream_get_contents($pipes[1]);
    echo "--------------------------------\n";
    
    fclose($pipes[1]);
    proc_close($process);
}

// Cleanup
@unlink($file);
@unlink('/tmp/simulated_reader.php');

When run, this script consistently outputs: RACE CONDITION CONFIRMED: Concurrent reader saw an EMPTY (0 bytes) file!
This happens because the fopen call with wb truncates the file to 0 bytes immediately. The background process (simulating a concurrent web request) reads the file while the writer is "sleeping" (simulating slow disk I/O or a large payload), resulting in an empty cache file being served.
I have also linked a Pull Request (#48725) that implements the atomic write fix (temp file + rename) and includes a Unit Test to prevent regression. I hope this provides the concrete evidence needed.