PDO / MySQL / PHP / OS X：MySQL服务器在后续查询中消失了吗？

Question

I think this is an issue in code somewhere, but the code's so simple I'm not sure what it could be.

I've verified wait_timeout is high enough and have gone through everything here: http://dev.mysql.com/doc/refman/5.1/en/gone-away.html without any success.

This happens reproducibly on the second query executed in one script run so I'm sure it's a coding error.

I created a really simple wrapper around the PDO class to have a singleton database handle:

<?php

class PDOWrapper
{
    protected static $instance;
    protected $dbh;

    function __construct()
    {
        if ( is_null(static::$instance) )
        {
            static::$instance = $this;
            $this->connect_to_db();
        }
    }

    static function instance()
    {
        if ( is_null(static::$instance) )
        {
            new static;
        }

        return static::$instance;
    }

    private function connect_to_db()
    {
        $db_info = array(
            0 => array(
                'hostname' => "Host",
                'username' => "User",
                'password' => "Pass",
                'db' => "DB",
            )
        );

        //Try to connect to the database
        try 
        {
            $dbh = new PDO('mysql:host=' . $db_info[0]['hostname'] . ';dbname=' . $db_info[0]['db'], $db_info[0]['username'], $db_info[0]['password'], array( PDO::ATTR_PERSISTENT => true, PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, PDO::ATTR_EMULATE_PREPARES => true ));
        }
        catch (PDOException $e)
        {
            log_message("Error connecting to DB!: " . $e->getMessage(), LOG_LEVEL_CRITICAL );
            return false;
        }

        $this->dbh = $dbh;
    }

    public static function get_dbh()
    {
        if ( is_null(static::$instance) )
        {
            new static;
        }

        return static::$instance->dbh;
    }
}

I then use the wrapper like so:

function somefunc(){
    $dbh = PDOWrapper::get_dbh();
    $future_sth = $dbh->prepare("SELECT * FROM some_table");
    $future_sth->execute();
    $ret = $future_sth->fetchAll(PDO::FETCH_ASSOC);
    print_r($ret);
    $future_sth->closeCursor();
    return $ret;
}

I call this function repeatedly as part of an event loop. The first time it calls it, the print_r runs fine and it prints out the rows I expect to see.

After the function has been executed once, however, I get the following:

Warning: Error while sending QUERY packet. PID=92871
Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away'

I don't know why it'd have "gone away". My my.cnf looks ok. Wait timeout is massive and this happens immediately as soon as I run the second query anyway. Any ideas?

It doesn't look like anything is obviously wrong in the MySQL error log:

120925 12:48:46 mysqld_safe Starting mysqld daemon with databases from /usr/local/var/mysql
120925 12:48:46 [Warning] The syntax '--log' is deprecated and will be removed in a future release. Please use '--general-log'/'--general-log-file' instead.
120925 12:48:46 [Warning] Setting lower_case_table_names=2 because file system for /usr/local/var/mysql/ is case insensitive
120925 12:48:46 InnoDB: The InnoDB memory heap is disabled
120925 12:48:46 InnoDB: Mutexes and rw_locks use GCC atomic builtins
120925 12:48:46 InnoDB: Compressed tables use zlib 1.2.5
120925 12:48:46 InnoDB: Initializing buffer pool, size = 128.0M
120925 12:48:46 InnoDB: Completed initialization of buffer pool
120925 12:48:46 InnoDB: highest supported file format is Barracuda.
120925 12:48:46  InnoDB: Waiting for the background threads to start
120925 12:48:47 InnoDB: 1.1.8 started; log sequence number 2273680401
120925 12:48:47 [Note] Server hostname (bind-address): '0.0.0.0'; port: 3306
120925 12:48:47 [Note]   - '0.0.0.0' resolves to '0.0.0.0';
120925 12:48:47 [Note] Server socket created on IP: '0.0.0.0'.
120925 12:48:47 [Note] Event Scheduler: Loaded 0 events
120925 12:48:47 [Note] /usr/local/Cellar/mysql/5.5.25a/bin/mysqld: ready for connections.
Version: '5.5.25a-log'  socket: '/tmp/mysql.sock'  port: 3306  Source distribution