Last modified: 2008-09-27 23:40:39 UTC

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Bug 12342 - get_sock() in memcached-client.php calculates hash in wrong way
get_sock() in memcached-client.php calculates hash in wrong way
Status: RESOLVED FIXED
Product: MediaWiki
Classification: Unclassified
General/Unknown (Other open bugs)
1.12.x
All All
: Normal normal (vote)
: ---
Assigned To: Nobody - You can work on this!
: patch-need-review
Depends on:
Blocks:
  Show dependency treegraph
 
Reported: 2007-12-18 17:58 UTC by Marooned
Modified: 2008-09-27 23:40 UTC (History)
1 user (show)

See Also:
Web browser: ---
Mobile Platform: ---
Assignee Huggle Beta Tester: ---


Attachments

Description Marooned 2007-12-18 17:58:46 UTC
Function get_sock uses loop for for searching for memcache server.
This is:

<pre>      for ($tries = 0; $tries<20; $tries++)
      {
         $host = $this->_buckets[$hv % $this->_bucketcount];
         $sock = $this->sock_to_host($host);
         if (is_resource($sock)) {
            $this->_flush_read_buffer($sock);
            return $sock;
         }
         $hv += $this->_hashfunc($tries . $realkey);
      }</pre>

If there are servers in _buckets then the if(is_resource..) is true and loop exits.

However, if there is no good server then the line:
<pre>$hv += $this->_hashfunc($tries . $realkey);</pre>
is reached and the $hv is increased. In the next loop passes <pre>$hv % $this->_bucketcount</pre> will return negative numbers as the $hv will pass beyond the bounds of the integer type.

My idea to fix it is to change
<pre>$hv += $this->_hashfunc($tries . $realkey);</pre>
to:
<pre>$hv ^= $this->_hashfunc($tries . $realkey);</pre>
or:
<pre>$hv += $this->_hashfunc($tries . $realkey);
$hv ^= 0x7fffffff;</pre>

One should performe a test of distribution of this variable (a histogram) after making this changes.
Comment 1 Marooned 2007-12-18 18:02:58 UTC
Ech.. there is no "preview" or "edit" option [why?] nor the help about tags that can be used to format this text.

If someone who can edit will see it - please - remove/change the <pre> above to something that is accepted here to format a code *and* delete this comment.

Thanks
Comment 2 Marooned 2007-12-19 16:40:54 UTC
Ah! What a terrible mistake!
The last suggested version should be:

$hv += $this->_hashfunc($tries . $realkey);
$hv &= 0x7fffffff;

That is '&' instead of '^' of course! I'm sorry for that typo.
[and this is a version which I use on my local test MW]
Comment 3 Aaron Schulz 2008-09-27 23:21:33 UTC
This would explain the mysterious negative indexes that popped up on a maintenance script
Comment 4 Aaron Schulz 2008-09-27 23:36:41 UTC
Fixed in r41326
Comment 5 Marooned 2008-09-27 23:40:39 UTC
Thanks for applying this fix.

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