PHP PDO在SWOOLE模式下关闭数据库链接一些注意点

最近在swoole中使用php pdo扩展访问数据库的时候，发现了一个颇有意思的事情。

我测试用的版本是 PHP 7.1.13

官方手册明确到告诉咱们，使用PDO是这样关闭数据库链接的，只须要将PDO对象置为null便可，这段代码就是手册里的

<?php
try {
    $dbh = new PDO('mysql:host=127.0.0.1;dbname=test', 'root', '');
    foreach($dbh->query('SELECT * from test') as $row) {
        print_r($row);
    }
    sleep(100);	
    $dbh = null;
} catch (PDOException $e) {
    print "Error!: " . $e->getMessage() . "<br/>";
    die();
}

我实际测下来也的确如此。 这是query执行前的MySQL的PROCESSLIST

mysql> SHOW PROCESSLIST;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 10 | root | localhost:56477 | NULL | Query   |    0 | starting | SHOW PROCESSLIST |
+----+------+-----------------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)

这是query运行后，PDO 对象置为null前的MySQL的PROCESSLIST

mysql> SHOW PROCESSLIST;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 10 | root | localhost:56477 | NULL | Query   |    0 | starting | SHOW PROCESSLIST |
| 14 | root | localhost:56572 | test | Sleep   |    2 |          | NULL             |
+----+------+-----------------+------+---------+------+----------+------------------+
2 rows in set (0.00 sec)

PDO 对象置为null后，数据库链接的的确确关闭了，看来官方手册没错

mysql> SHOW PROCESSLIST;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 10 | root | localhost:56477 | NULL | Query   |    0 | starting | SHOW PROCESSLIST |
+----+------+-----------------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)

可是接下来有意思的事情就要发生了，看这段代码

<?php
$pdo = new PDO(
    'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
    [
        PDO::ATTR_TIMEOUT => 10,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_PERSISTENT => false,
        PDO::ATTR_EMULATE_PREPARES => false,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
    ]
);

$s = $pdo->prepare("select * from test WHERE id=:id");
$s->execute(['id' => 1]);
$s->fetchAll();

$pdo = null;
sleep(100);

在PDO对象被置为null以后，MySQL的链接并无被真的释放

mysql> SHOW PROCESSLIST;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 10 | root | localhost:56477 | NULL | Query   |    0 | starting | SHOW PROCESSLIST |
| 18 | root | localhost:56606 | test | Sleep   |    3 |          | NULL             |
+----+------+-----------------+------+---------+------+----------+------------------+
2 rows in set (0.00 sec)

是否是很诡异？不要紧，咱们在改下这段代码

<?php
$pdo = new PDO(
    'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
    [
        PDO::ATTR_TIMEOUT => 10,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_PERSISTENT => false,
        PDO::ATTR_EMULATE_PREPARES => false,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
    ]
);

$s = $pdo->prepare("select * from test WHERE id=:id");
$s->execute(['id' => 1]);
$s->fetchAll();

$s=null;
$pdo = null;
sleep(100);

在$s也被置为null以后将$pdo置为null，pdo申请的数据库链接就被真真正正的释放了

mysql> SHOW PROCESSLIST;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 10 | root | localhost:56477 | NULL | Query   |    0 | starting | SHOW PROCESSLIST |
+----+------+-----------------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)

这是为何？看下面这段代码

$pdo = new PDO(
    'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
    [
        PDO::ATTR_TIMEOUT => 10,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_PERSISTENT => false,
        PDO::ATTR_EMULATE_PREPARES => false,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
    ]
);

debug_zval_dump($pdo);

$s = $pdo->prepare("select * from test WHERE id=:id");

debug_zval_dump($pdo);

$s->execute(['id' => 1]);
$s->fetchAll();

$s=null;

debug_zval_dump($pdo);

$pdo = null;
sleep(100);

输出

object(PDO)#1 (0) refcount(2){
}
object(PDO)#1 (0) refcount(3){
}
object(PDO)#1 (0) refcount(2){
}

你会发现，当$pdo对象第一次打印的时候他的引用计数是2（为何是2，而不是1呢？传送门 http://php.net/manual/zh/function.debug-zval-dump.php ）在$pdo->prepare以后引用计数变成了3，也就是说，即便$pdo被置为null，它依然仍是会被$s引用，并不会被PHP的GC给回收掉，因此它建立的数据库链接也不会被释放。

在FPM模式下的PHP，这个问题并不会引发什么问题，可是在SWOOLE这种运行环境下，假如你的PDO对象在被一些全局变量（传送门 https://wiki.swoole.com/wiki/page/p-zend_mm.html ） 引用，即便你将PDO对象置为null，你的数据库链接依然不会被释放。

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