检查文件是否被并发进程锁定

Question

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文件，和防止在文件仍在写入时尝试读取它。

由于并发性质，我很难正确测试它，而且我不确定如何处理这个问题。到目前为止我已经知道了：

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：

有没有办法等到文件不再被锁定，同时保留给这个时间限制的选项？
当有另一个进程锁定该文件时，fclose() 是否会自动解锁所有锁？

Answer 1

这里回答了第一个问题How to detect the finish with file_put_contents() in php? and beacuse PHP is single-threaded, only solution is to use extension of core PHP using PTHREADS and one good simple article about it is https://www.mullie.eu/parallel-processing-multi-tasking-php/

这里回答第二个问题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，它将关闭文件并释放锁。

Answer 2

我经常使用一个小的class...它安全且快速，基本上只有当您获得文件的独占锁时才必须写入，否则您应该等到被锁定...

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关卡，你可以使用另一个技巧...使用 while (1) 初始化一个无限循环，该循环仅在获得独占锁时中断，不建议这样做，因为它会阻塞你的服务器一段时间未定义时间...

  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，可以直接写入文件，但正如您所说的那样有限制... 竞争条件 存在并且即使您尝试使用也可能发生LOCK_EX。我认为 php class 更灵活和可用...

看到这个处理类似问题的线程：php flock behaviour when file is locked by one process

Answer 3

我写了一个使用 sleep() 的小测试，这样我就可以通过一个简单的 AJAX 调用来模拟并发 read/write 进程。这似乎回答了两个问题：

当文件被锁定时，接近估计写入持续时间的休眠和随后的锁定检查允许等待。这甚至可以放在一个有间隔的 while 循环中。
fclose() 确实没有从已经运行的进程中删除锁，正如一些答案所确认的那样。

根据文档，

PHP5.5 和 windows 下的版本不支持 $wouldblock 参数，我能够在 Windows + PHP5.3 上对此进行测试，并得出结论，我测试的 file_is_locked() 在这种情况下仍然有效： flock() 仍然 return false 只是没有 $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;
    }
}

Answer 4

这是@Alessandro 回答的修复程序，可以正常工作并且不会永远锁定文件 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 ;
      }
      
      // Should some other permission value be necessary
      if (isset($permission)) $this->permission = $permission;
      if($this->timeLock()){
          $this->releaseLock();
      }

    }
    /* 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);
    }
    private function timeLock() {
      // Retrieve the contents of the lock file
      $timeout = @file_get_contents($this->filename);
      // If no contents retrieved return true
      if ($timeout === false) return true;
      // Return the timeout value
      return (intval($timeout) < time());
    }
  }

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);
  }

希望节省一些时间

检查文件是否被并发进程锁定

Check if file is locked by concurrent process

php

fopen

fclose

flock