我有一个使用file_put_contents()
写入文件的进程:
file_put_contents ( $file, $data, LOCK_EX );
我添加了LOCK_EX
参数以防止并发进程写入同一个文件,并防止在仍然被写入时尝试读取它。
由于并发性,我在测试这个问题时遇到了困难,而且我不确定如何处理这个问题。到目前为止我有这个:
if (file_exists($file)) {
$fp = fopen($file, 'r+');
if (!flock($fp, LOCK_EX|LOCK_NB, $wouldblock)) {
if ($wouldblock) {
// how can I wait until the file is unlocked?
} else {
// what other reasons could there be for not being able to lock?
}
}
// does calling fclose automatically close all locks even is a flock was not obtained above?
fclose($file);
}
问题是:
fclose()
会自动解锁所有锁吗?我经常使用一个小类...这是安全和快速的,基本上你只有在你获得文件的独占锁定时才能写,否则你应该等到被锁定...
lock_file.php
<?php
/*
Reference Material
http://en.wikipedia.org/wiki/ACID
*/
class Exclusive_Lock {
/* Private variables */
public $filename; // The file to be locked
public $timeout = 30; // The timeout value of the lock
public $permission = 0755; // The permission value of the locked file
/* Constructor */
public function __construct($filename, $timeout = 1, $permission = null, $override = false) {
// Append '.lck' extension to filename for the locking mechanism
$this->filename = $filename . '.lck';
// Timeout should be some factor greater than the maximum script execution time
$temp = @get_cfg_var('max_execution_time');
if ($temp === false || $override === true) {
if ($timeout >= 1) $this->timeout = $timeout;
set_time_limit($this->timeout);
} else {
if ($timeout < 1) $this->timeout = $temp;
else $this->timeout = $timeout * $temp;
}
// Should some other permission value be necessary
if (isset($permission)) $this->permission = $permission;
}
/* Methods */
public function acquireLock() {
// Create the locked file, the 'x' parameter is used to detect a preexisting lock
$fp = @fopen($this->filename, 'x');
// If an error occurs fail lock
if ($fp === false) return false;
// If the permission set is unsuccessful fail lock
if (!@chmod($this->filename, $this->permission)) return false;
// If unable to write the timeout value fail lock
if (false === @fwrite($fp, time() + intval($this->timeout))) return false;
// If lock is successfully closed validate lock
return fclose($fp);
}
public function releaseLock() {
// Delete the file with the extension '.lck'
return @unlink($this->filename);
}
public function timeLock() {
// Retrieve the contents of the lock file
$timeout = @file_get_contents($this->filename);
// If no contents retrieved return error
if ($timeout === false) return false;
// Return the timeout value
return intval($timeout);
}
}
?>
简单使用如下:
include("lock_file.php");
$file = new Exclusive_Lock("my_file.dat", 2);
if ($file->acquireLock()) {
$data = fopen("my_file.dat", "w+");
$read = "READ: YES";
fwrite($data, $read);
fclose($data);
$file->releaseLock();
chmod("my_file.dat", 0755);
unset($data);
unset($read);
}
如果你想添加更复杂的级别,你可以使用另一个技巧...使用while (1)
初始化一个无限循环,只有在获得独占锁时才会中断,不建议因为可以阻止你的服务器一个未定义的时间......
include("lock_file.php");
$file = new Exclusive_Lock("my_file.dat", 2);
while (1) {
if ($file->acquireLock()) {
$data = fopen("my_file.dat", "w+");
$read = "READ: YES";
fwrite($data, $read);
fclose($data);
$file->releaseLock();
chmod("my_file.dat", 0755);
unset($data);
unset($read);
break;
}
}
file_put_contents()
非常快,直接写入文件,但正如你所说有一个限制...竞争条件存在,即使你试图使用LOCK_EX
可能会发生。我认为php类更灵活,更实用......
看到这个处理类似问题的线程:php flock behaviour when file is locked by one process
第一个问题在这里回答How to detect the finish with file_put_contents() in php?和beacuse PHP是单线程的,唯一的解决方案是使用PTHREADS使用核心PHP的扩展,一个关于它的简单文章是https://www.mullie.eu/parallel-processing-multi-tasking-php/
第二个问题在这里回答Will flock'ed file be unlocked when the process die unexpectedly?
fclose()将仅解锁使用fopen()或fsockopen()打开的有效句柄,因此如果句柄仍然有效,则是它将关闭文件并释放锁定。
我编写了一个使用sleep()
的小测试,以便我可以通过简单的AJAX调用来模拟并发读/写过程。这似乎回答了这两个问题:
fclose()
确实没有从已经运行的进程中删除锁定,如某些答案中所确认的那样。Windows上的PHP5.5及更低版本根据文档不支持$wouldblock
参数,我能够在Windows + PHP5.3上测试这个并得出结论,我的测试中的file_is_locked()
仍然在这种情况下工作:flock()
仍然会返回false没有$wouldblock
参数但它仍然会在我的else
检查中被捕获。
if (isset($_POST['action'])) {
$file = 'file.txt';
$fp = fopen($file, 'r+');
if ($wouldblock = file_is_locked($fp)) {
// wait and then try again;
sleep(5);
$wouldblock = file_is_locked($fp);
}
switch ($_POST['action']) {
case 'write':
if ($wouldblock) {
echo 'already writing';
} else {
flock($fp, LOCK_EX);
fwrite($fp, 'yadayada');
sleep(5);
echo 'done writing';
}
break;
case 'read':
if ($wouldblock) {
echo 'cant read, already writing';
} else {
echo fread($fp, filesize($file));
}
break;
}
fclose($fp);
die();
}
function file_is_locked( $fp ) {
if (!flock($fp, LOCK_EX|LOCK_NB, $wouldblock)) {
if ($wouldblock) {
return 'locked'; // file is locked
} else {
return 'no idea'; // can't lock for whatever reason (for example being locked in Windows + PHP5.3)
}
} else {
return false;
}
}