Description:
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From manual page: http://www.php.net/function.session-set-save-handler
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Session locking is not described at all. The callbacks for "open" and "close" must aquire a write lock on the ressource that stores the session data. If this is not done, race conditions will destroy the data.

The internal files handler gets it right.

The Example #2, which reimplements this handler, gets it wrong!

I don't know about any other internal save handler, like sqlite or memcached.

This one is a duplicate of Bug #55624, but with better explanation and more code. :)

Test script:
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Please see http://pastebin.com/EbqigxmY



Expected result:
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When running the script from http://pastebin.com/EbqigxmY the expected behaviour is that all three iframes get filled one after another, with 1 second break between each.

The last response should list all three subrequests and the master request in the var_dump().

Actual result:
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Using the example #2 code, all iframes get filled after 1 second, and all of them only report the master request and its own subrequest in the dump.