PHP PDO在SWOOLE模式下關閉數據庫鏈接一些注意點

最近在swoole中使用php pdo擴展訪問數據庫的時候，發現了一個頗有意思的事情。

我測試用的版本是 PHP 7.1.13

官方手冊明確到告訴咱們，使用PDO是這樣關閉數據庫鏈接的，只須要將PDO對象置爲null便可，這段代碼就是手冊裏的

<?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();
}

我實際測下來也的確如此。 這是query執行前的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)

這是query運行後，PDO 對象置爲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 對象置爲null後，數據庫鏈接的的確確關閉了，看來官方手冊沒錯

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)

可是接下來有意思的事情就要發生了，看這段代碼

<?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對象被置爲null以後，MySQL的鏈接並無被真的釋放

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)

是否是很詭異？不要緊，咱們在改下這段代碼

<?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$s也被置爲null以後將$pdo置爲null，pdo申請的數據庫鏈接就被真真正正的釋放了

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)

這是爲何？看下面這段代碼

$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);

輸出

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

你會發現，當$pdo對象第一次打印的時候他的引用計數是2（爲何是2，而不是1呢？傳送門 http://php.net/manual/zh/function.debug-zval-dump.php ）在$pdo->prepare以後引用計數變成了3，也就是說，即便$pdo被置爲null，它依然仍是會被$s引用，並不會被PHP的GC給回收掉，因此它建立的數據庫鏈接也不會被釋放。

在FPM模式下的PHP，這個問題並不會引發什麼問題，可是在SWOOLE這種運行環境下，假如你的PDO對象在被一些全局變量（傳送門 https://wiki.swoole.com/wiki/page/p-zend_mm.html ） 引用，即便你將PDO對象置爲null，你的數據庫鏈接依然不會被釋放。

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