MySQL - Locking a row using UDPATE&SELECT - PDO in php

Question

I have the following query:

$query = "UPDATE list SET locked=1, @id:=id WHERE worked=0 and is404=0 AND error=0 AND locked=0 LIMIT 1;SELECT * FROM list WHERE id=@id;";
$stmt  = $pdo->prepare($query);

if ($stmt->execute()) {
    $model = $stmt->fetch(PDO::FETCH_OBJ);
    if ( isset($model) && !isset($model->id) ) {
        echo json_encode(array('error' => -1, 'type' => 'No URL'));
        exit;
    }
}

What I'm trying to accomplish is to lock a row using one of its fields called 'locked' and thus avoiding other parallel process intefer among them.

If I run the query in console, it works fine and returns the locked row by the @id used in the UPDATE.

The problem comes when using PDO in php. the row get locked but the SELECT statement does not return anything.

What's wrong in my code?

Answer 1

This is not a single query but two queries

$pdo->query("UPDATE list SET locked=1, @id:=id WHERE worked=0 and is404=0 AND error=0 AND locked=0 LIMIT 1");
$stmt = $pdo->query("SELECT * FROM list WHERE id=@id");
$model = $stmt->fetch(PDO::FETCH_OBJ);

That said, the locking model you choose is unreliable, two parallel processes could return the same id