php处理多个进程操作同一文件的diff思想

285572001

　　本文件转自链接：http://www.bingx.com/html/lamp/2009/0914/159.html

If there is a file that´s excessively being rewritten by many different users, you´ll note that two almost-simultaneously accesses on that file could interfere with each other. For example if there´s a chat history containing only the last 25 chat lines. Now adding a line also means deleting the very first one. So while that whole writing is happening, another user might also add a line, reading the file, which, at this point, is incomplete, because it´s just being rewritten. The second user would then rewrite an incomplete file and add its line to it, meaning: you just got yourself some data loss!

If flock() was working at all, that might be the key to not let those interferences happen - but flock() mostly won´t work as expected (at least that´s my experience on any linux webserver I´ve tried), and writing own file-locking-functions comes with a lot of possible issues that would finally result in corrupted files. Even though it´s very unlikely, it´s not impossible and has happened to me already.

So I came up with another solution for the file-interference-problem:

1. A file that´s to be accessed will first be copied to a temp-file directory and its last filemtime() is being stored in a PHP-variable. The temp-file gets a random filename, ensuring no other process is able to interfere with this particular temp-file.
2. When the temp-file has been changed/rewritten/whatever, there´ll be a check whether the filemtime() of the original file has been changed since we copied it into our temp-directory.
2.1. If filemtime() is still the same, the temp-file will just be renamed/moved to the original filename, ensuring the original file is never in a temporary state - only the complete previous state or the complete new state.
2.2. But if filemtime() has been changed while our PHP-process wanted to change its file, the temp-file will just be deleted and our new PHP-fileclose-function will return a FALSE, enabling whatever called that function to do it again (ie. upto 5 times, until it returns TRUE).

These are the functions I´ve written for that purpose:

<?php
$dir_fileopen = "../AN/INTERNAL/DIRECTORY/fileopen";

function randomid() {
    return time().substr(md5(microtime()), 0, rand(5, 12));
}

function cfopen($filename, $mode, $overwriteanyway = false) {
    global $dir_fileopen;
    clearstatcache();
    do {
      $id = md5(randomid(rand(), TRUE));
      $tempfilename = $dir_fileopen."/".$id.md5($filename);
    } while(file_exists($tempfilename));
    if (file_exists($filename)) {
      $newfile = false;
      copy($filename, $tempfilename);
    }else{
      $newfile = true;
    }
    $fp = fopen($tempfilename, $mode);
    return $fp ? array($fp, $filename, $id, @filemtime($filename), $newfile, $overwriteanyway) : false;
}

function cfwrite($fp,$string) { return fwrite($fp[0], $string); }

function cfclose($fp, $debug = "off") {
    global $dir_fileopen;
    $success = fclose($fp[0]);
    clearstatcache();
    $tempfilename = $dir_fileopen."/".$fp[2].md5($fp[1]);
    if ((@filemtime($fp[1]) == $fp[3]) or ($fp[4]==true and !file_exists($fp[1])) or $fp[5]==true) {
      rename($tempfilename, $fp[1]);
    }else{
      unlink($tempfilename);
      if ($debug != "off") echo "While writing, another process accessed $fp[1]. To ensure file-integrity, your changes were rejected.";
      $success = false;
    }
    return $success;
}
?>

$overwriteanyway, one of the parameters for cfopen(), means: If cfclose() is used and the original file has changed, this script won´t care and still overwrite the original file with the new temp file. Anyway there won´t be any writing-interference between two PHP processes, assuming there can be no absolute simultaneousness between two (or more) processes.