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As mentioned in #1436, it would be nice if the update function (Piwik_Tracker_Visit::handleKnownVisit) would be hookable. Here is a patch to do so.
Attachment: patch for /core/Tracker/Visit.php
(In ) Fixes #1440 Adding hook to update the visitor row in Tracker
Thanks for the patch - I had to make 2 changes to the patch as it was throwing php NOTICE ($sqlQuery was not defined, and $sqlActionUpdate was not defined)